LTC3614HUDD#PBF - LINEAR TECHNOLOGY

LTC3614

3614fc

For more information www.linear.com/LTC3614

Typical applicaTion

FeaTures

applicaTions

DescripTion

4A, 4MHz Monolithic

Synchronous Step-Down

DC/DC Converter

The LT C

3614 is a low quiescent current monolithic syn-

chronous buck regulator using a current mode, constant

frequency architecture. The no-load DC supply current

in sleep mode is only 75µA while maintaining the output

voltage (Burst Mode operation) at no load, dropping to

zero current in shutdown. The 2.25V to 5.5V input supply

voltage range makes the LTC3614 ideally suited for single

Li-Ion as well as fixed low voltage input applications. 100%

duty cycle capability provides low dropout operation,

extending the operating time in battery-powered systems.

The operating frequency is externally programmable up to

4MHz, allowing the use of small surface mount inductors.

For switching-noise-sensitive applications, the LTC3614

can be synchronized to an external clock at up to 4MHz.

Forced continuous mode operation in the LTC3614 reduces

noise and RF interference. Adjustable compensation allows

the transient response to be optimized over a wide range

of loads and output capacitors.

The internal synchronous switch increases efficiency and

eliminates the need for an external catch diode, saving

external components and board space. The LTC3614

is offered in a leadless 24-pin 3mm × 5mm thermally

enhanced QFN package.

L, LT , LT C , LT M , Linear Technology, the Linear logo and Burst Mode are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective owners.

Protected by U.S. Patents, including 6580258, 5481178, 5994885, 6304066, 6498466, 6611131.

Efficiency and Power Loss

vs Load Current

n 4A Output Current

n 2.25V to 5.5V Input Voltage Range

n Low Output Ripple Burst Mode

Operation: IQ = 75µA

n ±1% Output Voltage Accuracy

n Output Voltage Down to 0.6V

n High Efficiency: Up to 95%

n Low Dropout Operation: 100% Duty Cycle

n Programmable Slew Rate on SW Node Reduces

Noise and EMI

n Adjustable Switching Frequency: Up to 4MHz

n Optional Active Voltage Positioning (AVP) with

Internal Compensation

n Selectable Pulse-Skipping/Forced Continuous/Burst

Mode Operation with Adjustable Burst Clamp

n Programmable Soft-Start

n Inputs for Start-Up Tracking or External Reference

n DDR Memory Mode, IOUT = ±3A

n Available in a 24-Pin 3mm × 5mm QFN

Thermally Enhanced Package

n Point-of-Load Supplies

n Distributed Power Supplies

n Portable Computer Systems

n DDR Memory Termination

n Handheld Devices

OUTPUT CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (W)

100

1 100 1000 10000

3614 TA01b

0.1

0.01

VIN = 2.8V

VIN = 3.3V

VIN = 5V

VOUT = 2.5V

RUN

TRACK/SS

RT/SYNC

PGOOD

ITH

SGND

PGND

VIN

2.7V TO 5.5V

SRLIM/DDR

SVIN

LTC3614 SW

PVIN

330nH

665k

210k

3614 TA01a

10µF

×4

MODE VFB

47µF

×2

VOUT

2.5V

LTC3614

3614fc

For more information www.linear.com/LTC3614

absoluTe MaxiMuM raTings

PVIN, SVIN Voltages ..................................... –0.3V to 6V

SW Voltage ................................. –0.3V to (PVIN + 0.3V)

ITH, RT/SYNC Voltages ............... –0.3V to (SVIN + 0.3V)

SRLIM, TRACK/SS Voltages ....... –0.3V to (SVIN + 0.3V)

MODE, RUN, VFB Voltages .......... –0.3V to (SVIN + 0.3V)

PGOOD Voltage ............................................ –0.3V to 6V

Operating Junction Temperature Range

(Notes 2, 11) .......................................... –55°C to 150°C

Storage Temperature.............................. –65°C to 150°C

(Note 1)

TOP VIEW

PGND

UDD PACKAGE

24-LEAD (3mm × 5mm) PLASTIC QFN

SRLIM/DDR

RT/SYNC

SGND

PVIN

PGOOD

RUN

SVIN

PVIN

TRACK/SS

ITH

VFB

MODE

9 10 11 12

24 23 22 21

TJMAX = 150°C, θJA = 38°C/W

EXPOSED PAD (PIN 25) IS PGND, MUST BE SOLDERED TO PCB

pin conFiguraTion

orDer inForMaTion

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC3614EUDD#PBF LTC3614EUDD#TRPBF LFVM 24-Lead (3mm × 5mm) Plastic QFN –40°C to 125°C

LTC3614IUDD#PBF LTC3614IUDD#TRPBF LFVM 24-Lead (3mm × 5mm) Plastic QFN –40°C to 125°C

LTC3614HUDD#PBF LTC3614HUDD#TRPBF LFVM 24-Lead (3mm × 5mm) Plastic QFN –40°C to 150°C

LTC3614MPUDD#PBF LTC3614MPUDD#TRPBF LFVM 24-Lead (3mm × 5mm) Plastic QFN –55°C to 150°C

Consult LT C Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

Consult LT C Marketing for information on non-standard lead based finish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

LTC3614

3614fc

For more information www.linear.com/LTC3614

elecTrical characTerisTics

The l denotes the specifications which apply over the specified operating

junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 3.3V, RT/SYNC = SVIN unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Operating Voltage Range l2.25 5.5 V

VUVLO Undervoltage Lockout Threshold SVIN Ramping Down

SVIN Ramping Up

1.7

2.25

VFB Feedback Voltage Internal Reference (Note 3) VTRACK = SVIN, VDDR = 0V

0°C < TJ < 85°C

–40°C < TJ < 125°C

–55°C < TJ < 150°C

0.594

0.591

0.589

0.6

0.606

0.609

Feedback Voltage External Reference

(Note 7)

(Note 3) VTRACK = 0.3V, VDDR = SVIN 0.288 0.300 0.312 V

(Note 3) VTRACK = 0.5V, VDDR = SVIN 0.488 0.500 0.512 V

IFB Feedback Input Current VFB = 0.6V l±30 nA

∆VLINEREG Line Regulation SVIN = PVIN = 2.25V to 5.5V

(Notes 3, 4) TRACK/SS = SVIN

–40°C < TJ < 125°C

–55°C < TJ < 150°C

0.2

0.3

%/V

∆VLOADREG Load Regulation ITH from 0.5V to 0.8V (Notes 3, 4)

VITH = SVIN (Note 5)

0.25

2.6

ISActive Mode Supply Current VFB = 0.5V, VMODE = SVIN (Note 6) 1100 µA

Sleep Mode Supply Current VFB = 0.7V, VMODE = 0V, ITH = SVIN

(Note 5)

75 100 µA

VFB = 0.7V, VMODE = 0V (Note 4) 130 175 µA

Shutdown Current SVIN = PVIN = 5.5V, VRUN = 0V 0.1 1 µA

RDS(ON) Top Switch On-Resistance PVIN = 3.3V (Note 10) 35 mΩ

Bottom Switch On-Resistance PVIN = 3.3V (Note 10) 25 mΩ

ILIM Top Switch Current Limit Sourcing (Note 8), VFB = 0.5V

Duty Cycle <35%

Duty Cycle = 100%

7.5

10.5

Bottom Switch Current Limit Sinking (Note 8), VFB = 0.7V,

Forced Continuous Mode

–6 –8 –11 A

gm(EA) Error Amplifier Transconductance –5µA < IITH < 5µA (Note 4) 200 µS

IEAO Error Amplifier Maximum Output

Current

(Note 4) ±30 µA

tSS Internal Soft-Start Time VFB from 0.06V to 0.54V,

TRACK/SS = SVIN

0.65 1.2 1.9 ms

VTRACK/SS Enable Internal Soft-Start (Note 7 ) 0.62 V

tTRACK/SS_DIS Soft-Start Discharge Time at Start-Up 60 µs

RON(TRACK/SS_DIS) TRACK/SS Pull-Down Resistor at

Start-Up

200 Ω

fOSC Oscillator Frequency RT/SYNC = 370k l0.8 1 1.2 MHz

Internal Oscillator Frequency VRT/SYNC = SVIN l1.8 2.25 2.7 MHz

fSYNC Synchronization Frequency Range 0.3 4 MHz

VRT/SYNC SYNC Input Threshold High 1.2 V

SYNC Input Threshold Low . 0.3 V

ISW(LKG) Switch Leakage Current SVIN = PVIN = 5.5V, VRUN = 0V 0.1 1 µA

VDDR DDR Option Enable Voltage SVIN – 0.3 V

LTC3614

3614fc

For more information www.linear.com/LTC3614

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 2: The LTC3614 is tested under pulsed load conditions such that TJ ≈

TA. The LTC3614E is guaranteed to meet specifications from 0°C to 85°C

junction temperature. Specifications over the –40°C to 125°C operating

junction temperature range are assured by design, characterization and

correlation with statistical process controls. The LTC3614I is guaranteed

to meet specifications over the –40°C to 125°C operating junction

temperature, the LTC3614H is guaranteed to meet specifications over the

–40°C to 150°C operating junction temperature range and the LTC3614MP

is guaranteed and tested to meet specifications over the full –55°C to

150°C operating junction temperature range. High junction temperatures

degrade operating lifetimes; operating lifetime is derated for temperature

greater than 125°C. Note that the maximum ambient temperature

consistent with these specifications is determined by specific operating

conditions in conjunction with board layout, the rated package thermal

impedance and other environmental factors.

The junction temperature (TJ) is calculated from the ambient temperature

(TA) and power dissipation (PD) according to the formula: TJ = TA + (PD

• θJA°C/W), where θJA is the package thermal impedance. The maximum

ambient temperature is determined by specific operating conditions in

conjunction with board layout, the rated package thermal resistance and

other environmental factors.

elecTrical characTerisTics

The l denotes the specifications which apply over the specified operating

junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 3.3V, RT/SYNC = SVIN unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VMODE

(Note 9)

Internal Burst Mode Operation 0.3 V

Pulse-Skipping Mode SVIN – 0.3 V

Forced Continuous Mode 1.1 SVIN • 0.58 V

External Burst Mode Operation 0.45 0.8 V

PGOOD Power Good Voltage Windows TRACK/SS = SVIN, Entering Window

VFB Ramping Up

VFB Ramping Down

–3

–6

TRACK/SS = SVIN, Leaving Window

VFB Ramping Up

VFB Ramping Down

–9

–11

tPGOOD Power Good Blanking Time Entering and Leaving Window 70 105 140 µs

RPGOOD Power Good Pull-Down On-Resistance 8 17 33 Ω

VRUN RUN voltage Input High

Input Low

0.4

Note 3: This parameter is tested in a feedback loop which servos VFB to

the midpoint for the error amplifier (VITH = 0.75V).

Note 4: External compensation on ITH pin.

Note 5: Tying the ITH pin to SVIN enables the internal compensation and

AVP mode.

Note 6: Dynamic supply current is higher due to the internal gate charge

being delivered at the switching frequency.

Note 7: See description of the TRACK/SS pin in the Pin Functions section.

Note 8: In sourcing mode the average output current is flowing out of the

SW pin. In sinking mode the average output current is flowing into the SW

Pin.

Note 9: See description of the MODE pin in the Pin Functions section.

Note 10: Guaranteed by correlation and design to wafer level

measurements for QFN packages.

Note 11: This IC includes overtemperature protection that is intended

to protect the device during momentary overload conditions. Junction

temperature will exceed 150°C when overtemperature protection is active.

Continuous operation above the specified maximum operating junction

temperature may impair device reliability.

LTC3614

3614fc

For more information www.linear.com/LTC3614

Typical perForMance characTerisTics

Efficiency vs Input Voltage

Burst Mode Operation

(VMODE = 0V)

Efficiency vs Frequency

Burst Mode Operation

(VMODE = 0V), IOUT = 2A

Line Regulation Burst Mode Operation Pulse-Skipping Mode Operation

INPUT VOLTAGE (V)

EFFICIENCY (%)

100

2.5 3 43.5 4.5

3614 G04

5 5.5

VOUT = 1.8V

IOUT = 6mA

IOUT = 600mA

IOUT = 2A

INPUT VOLTAGE (V)

2.20

–0.3

VOUT ERROR (%)

–0.2

–0.1

0.1

0.3

2.75 3.30 3.85 4.40

3614 G07

4.95 5.50

0.2

VOUT

20mV/DIV

1A/DIV

20µs/DIV 3614 G08

VOUT = 1.8V

IOUT = 150mA

VMODE = 0V

VOUT

20mV/DIV

1A/DIV

20µs/DIV 3614 G09

VOUT = 1.8V

IOUT = 150mA

VMODE = 3.3V

VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted.

Efficiency vs Load Current

Burst Mode Operation (VMODE = 0V)

Efficiency vs Load Current

Burst Mode Operation (VMODE = 0V) Efficiency vs Load Current

Load Regulation

(VOUT = 1.8V)

OUTPUT CURRENT (mA)

–0.3

VOUT ERROR (%)

1.5

0.3

0.7

1.1

1.3

0.9

0.1

0.5

1000 2000 3000

3614 G06

4000

–0.1

FORCED CONTINUOUS MODE

PULSE-SKIPPING MODE

INTERNAL Burst Mode OPERATION

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

1 100 1000 10000

3614 G01

VIN = 2.5V

VIN = 3.3V

VIN = 5V

VOUT = 1.8V

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

1 100 1000 10000

3614 G02

VIN = 2.5V

VIN = 3.3V

VIN = 5V

VOUT = 1.2V

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

1 100 1000 10000

3614 G03

Burst Mode OPERATION

PULSE-SKIPPING

FORCED CONTINUOUS

VOUT = 1.8V

FREQUENCY (MHz)

0.5

EFFICIENCY (%)

1 1.5 2.52 3 3.5

3614 G05

4 4.5

VIN = 3.3V

VOUT = 1.8V

150nH

330nH

470nH

LTC3614

3614fc

For more information www.linear.com/LTC3614

Sinking Current

Internal Start-Up in Forced

Continuous Mode

Tracking Up/Down in

Forced Continuous Mode,

Non DDR Mode

Load Step Transient in Forced

Continuous Mode without AVP Mode

Load Step Transient in Forced

Continuous Mode with AVP Mode

Load Step Transient in Forced

Continuous Mode Sourcing and

Sinking Current

VOUT

1V/DIV

VTRACK/SS

500mV/DIV

PGOOD

2V/DIV

2ms/DIV 3614 G18

VOUT = 0V TO 1.8V

IOUT = 3A, VTRACK/SS = 0V TO 0.7V

VMODE = 1.5V, VSRLIM/DDR = 0V

Typical perForMance characTerisTics

VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted.

VOUT

100mV/DIV

ILOAD

2A/DIV

100µs/DIV 3614 G13

VOUT = 1.8V

ILOAD = 100mA TO 4A, VMODE = 1.5V

COMPENSATION FIGURE 1

VOUT

100mV/DIV

ILOAD

2A/DIV

100µs/DIV 3614 G14

VOUT = 1.8V

ILOAD = 100mA TO 4A, VMODE = 1.5V

VOUT

200mV/DIV

ILOAD

2A/DIV

100µs/DIV 3614 G15

VOUT = 1.8V

ILOAD = –3A TO 3A, VMODE = 1.5V

COMPENSATION FIGURE 1

VOUT

100mV/DIV

2V/DIV

2A/DIV

1µs/DIV 3614 G16

VOUT = 1.8V

IOUT = –3A, VMODE = 1.5V

2A/DIV

VOUT

500mV/DIV

PGOOD

10V/DIV

RUN

10V/DIV

500µs/DIV 3614 G17

VOUT = 1.8V

IOUT = 0A, VMODE = 1.5V

Forced Continuous Mode Operation

VOUT

20mV/DIV

500mA/DIV

1µs/DIV 3614 G10

VOUT = 1.8V

IOUT = 100mA

VMODE = 1.5V

Load Step Transient in

Pulse-Skipping Mode

Load Step Transient in

Burst Mode Operation

VOUT

100mV/DIV

ILOAD

2A/DIV

100µs/DIV 3614 G11

VOUT = 1.8V

ILOAD = 100mA TO 4A, VMODE = 3.3V

COMPENSATION FIGURE 1

VOUT

100mV/DIV

ILOAD

2A/DIV

100µs/DIV 3614 G12

VOUT = 1.8V

ILOAD = 100mA TO 4A, VMODE = 0V

COMPENSATION FIGURE 1

LTC3614

3614fc

For more information www.linear.com/LTC3614

Tracking Up/Down in Forced

Continuous Mode, DDR Pin Tied

to SVIN

Reference Voltage

vs Temperature

Switch On-Resistance

vs Input Voltage

Frequency vs Input Voltage

Switch Leakage vs Temperature,

Main Switch

Switch Leakage vs Temperature,

Synchronous Switch

Switch On-Resistance

vs Temperature

Frequency vs Resistor on

RT/SYNC Pin Frequency vs Temperature

TEMPERATURE (°C)

–60 –40

0.594

REFERENCE VOLTAGE (V)

0.596

0.600

0.602

0.604

040 60 160140

3614 G20

0.598

–20 20 80 100 120

0.606

INPUT VOLTAGE (V)

2.5

DS(0N)

(Ω)

0.05

4.5

3614 G21

0.04

0.02

0.03

0.01

03.0 3.5 4.0 5.0 5.5

MAIN SWITCH

SYNCHRONOUS SWITCH

RESISTOR ON RT/SYNC PIN (kΩ)

FREQUENCY (kHz)

500

1500

2000

2500

800

4500

3614 G23

1000

400

200 1000 1200

600 1400

3000

3500

4000

TEMPERATURE (°C)

–60 –40

–1.5

FREQUENCY VARIATION (%)

–1.0

1.0

040 60 160140

3614 G24

–0.5

0.5

–20 20 80 100 120

INPUT VOLTAGE (V)

2.25

–2.5

FREQUENCY VARIATION (%)

–2.0

–1.0

–0.5

1.0

3614 G25

–1.5

0.5

3.75

3.25 5.25

2.75 4.25 4.75

Typical perForMance characTerisTics

VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted.

TEMPERATURE (°C)

–60 –40

RDS(ON) (Ω)

0.005

0.015

0.020

0.025

0.050

0.045

3614 G22

0.010

0120

–20 100

40 160140

0.030

0.035

0.040

SYNCHRONOUS SWITCH

MAIN SWITCH

TEMPERATURE (°C)

–60

SWITCH LEAKAGE (nA)

8000

10000

12000

140

3614 G27

6000

4000

090

–10 40

2000

14000 VIN = 2.25V

VIN = 3.3V

VIN = 5.5V

TEMPERATURE (°C)

–60

SWITCH LEAKAGE (nA)

8000

10000

12000

100

3614 G27

6000

4000

0–20 20 60

–40 160

040 80 140

120

2000

16000

14000

VIN = 2.25V

VIN = 3.3V

VIN = 5.5V

VOUT

500mV/DIV

VTRACK/SS

200mV/DIV

PGOOD

2V/DIV

2ms/DIV 3614 G19

VOUT = 0V TO 1.2V

IOUT = 3A, VTRACK/SS = 0V TO 0.4V

VMODE = 1.5V, VSRLIM/DDR = 3.3V

LTC3614

3614fc

For more information www.linear.com/LTC3614

Start-Up from Shutdown with

Prebiased Output (Overvoltage)

(Forced Continuous Mode)

Output Voltage During Sinking

vs Input Voltage (VOUT = 1.8V,

0.47µH Inductor)

Typical perForMance characTerisTics

VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted.

VOUT

500mV/DIV

PGOOD

5V/DIV

5A/DIV

50µs/DIV 3614 G31

PREBIASED VOUT = 2.2V

VOUT = 1.2V, IOUT = 0A

VMODE = 1.5V

INPUT VOLTAGE (V)

OUT

(V)

1.86

1.84

1.82

1.80

1.78

1.88

2.25 4

1.74

1.76

3.252.75 4.5 5.25

3614 G32

–3A, 2MHz, 120°C

–3A, 2MHz, 25°C

Dynamic Supply Current vs Input

Voltage without AVP Mode

VOUT Short to GND,

Forced Continuous Mode

Dynamic Supply Current vs

Temperature without AVP Mode

INPUT VOLTAGE (V)

0.1

DYNAMIC SUPPLY CURRENT (mA)

100

2.25 3.25 3.75 4.25 4.75

0.01

2.75 5.25

3614 G28

FORCED CONTINUOUS MODE

PULSE-SKIPPING MODE

Burst Mode OPERATION

TEMPERATURE (°C)

0.1

DYNAMIC SUPPLY CURRENT (mA)

100

–60 –40 40 80 120 160140

0.01

0 20 60 100–20

3614 G29

FORCED CONTINUOUS MODE

PULSE-SKIPPING MODE

Burst Mode OPERATION

VOUT

500mV/DIV

5A/DIV

100µs/DIV 3614 G30

VOUT = 1.8V

IOUT = 0A

VMODE = 1.5V

LTC3614

3614fc

For more information www.linear.com/LTC3614

pin FuncTions

SRLIM/DDR (Pin 1): Slew Rate Limit. Tying this pin to

ground selects maximum slew rate. Minimum slew rate

is selected when the pin is open. Connecting a resistor

from SRLIM/DDR to ground allows the slew rate to be

continuously adjusted. If SRLIM/DDR is tied to SVIN, DDR

mode is selected. In DDR mode the slew rate limit is set

to maximum.

RT/SYNC (Pin 2): Oscillator Frequency. This pin provides

three ways of setting the constant switching frequency:

1. Connecting a resistor from RT/SYNC to ground will set

the switching frequency based on the resistor value.

2. Driving the RT/SYNC pin with an external clock signal

will synchronize the LTC3614 to the applied frequency.

The slope compensation is automatically adapted to the

external clock frequency.

3. Tying the RT/SYNC pin to SVIN enables the internal

2.25MHz oscillator frequency.

SGND (Pin 3): Signal Ground. All small-signal and com-

pensation components should connect to this ground,

which in turn should connect to PGND at a single point.

PVIN (Pins 4, 10, 11, 17): Power Input Supply. PVIN

connects to the source of the internal P-channel power

MOSFET. This pin is independent of SVIN and may be con-

nected to the same voltage or to a lower voltage supply.

SW (Pins 5, 6, 7, 8, 13, 14, 15, 16): Switch Node. Con-

nection to the inductor. These pins connect to the drains

of the internal power MOSFET switches.

NC (Pins 9, 12): Can be connected to ground or left open.

SVIN (Pin 18): Signal Input Supply. This pin powers the

internal control circuitry and is monitored by the under-

voltage lockout comparator.

RUN (Pin 19): Enable Pin. Forcing this pin to ground shuts

down the LTC3614. In shutdown, all functions are disabled

and the chip draws <1µA of supply current.

PGOOD (Pin 20): Power Good. This open-drain output is

pulled down to SGND on start-up and while the FB voltage

is outside the power good voltage window. If the FB volt-

age increases and stays inside the power good window

for more than 100µs the PGOOD pin is released. If the

FB voltage leaves the power good window for more than

100µs the PGOOD pin is pulled down.

In DDR mode (DDR = VIN), the power good window moves

in relation to the actual TRACK/SS pin voltage. During

up/down tracking the PGOOD pin is always pulled down.

In shutdown the PGOOD output will actively pull down

and may be used to discharge the output capacitors via

an external resistor.

MODE (Pin 21): Mode Selection. Tying the MODE pin

to SVIN or SGND enables pulse-skipping mode or Burst

Mode operation (with an internal Burst Mode clamp),

respectively. If this pin is held at slightly higher than half

of SVIN, forced continuous mode is selected. Connecting

this pin to an external voltage between 0.45V and 0.8V

selects Burst Mode operation with the burst clamp set to

the pin voltage. See the Operation section for more details.

VFB (Pin 22): Voltage Feedback Input Pin. Senses the

feedback voltage from the external resistive divider across

the output.

ITH (Pin 23): Error Amplifier Compensation. The current

comparator’s threshold increases with this control volt-

age. Tying this pin to SVIN enables internal compensation

and AVP mode.

TRACK/SS (Pin 24): Track/External Soft-Start/External

Reference. Start-up behavior is programmable with the

TRACK/SS pin:

1. Tying this pin to SVIN selects the internal soft-start

circuit.

2. External soft-start timing can be programmed with a

capacitor to ground and a resistor to SVIN.

3. TRACK/SS can be used to force the LTC3614 to track

the start-up behavior of another supply.

The pin can also be used as external reference input. See

the Applications Information section for more information.

PGND (Exposed Pad Pin 25): Power Ground. This pin

connects to the source of the internal N-channel power

MOSFET. This pin should be connected close to the (–)

terminal of CIN and COUT.

LTC3614

3614fc

For more information www.linear.com/LTC3614

FuncTional block DiagraM

–

MODE

SLEEP

MODE

BURST

COMPARATOR

ITH SENSE

COMPARATOR

ERROR

AMPLIFIER

FOLDBACK

AMPLIFIER

0.6V

0.3V

0.555V

TRACK/SS

0.645V

SRLIM/DDR

EXPOSED PAD

3614 BD

SOFT-START

BANDGAP

AND

BIAS

–

VFB

RUN

SGND RT/SYNC ITH

SVIN – 0.3V

PVIN PVIN

SVIN

PGOOD

LOGIC

PGND

REVERSE

COMPARATOR

IREV

OSCILLATOR

–

INTERNAL

COMPENSATION

CURRENT

SENSE

SLOPE

COMPENSATION

PMOS CURRENT

COMPARATOR

ITH

LIMIT

PVIN PVIN

DRIVER

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Mode Selection

The MODE pin is used to select one of four different

operating modes:

operaTion

Main Control Loop

The LTC3614 is a monolithic, constant frequency, current

mode step-down DC/DC converter. During normal opera-

tion, the internal top power switch (P-channel MOSFET) is

turned on at the beginning of each clock cycle. Current in

the inductor increases until the current comparator trips

and turns off the top power switch. The peak inductor cur-

rent at which the current comparator trips is controlled by

the voltage on the ITH pin. The error amplifier adjusts the

voltage on the ITH pin by comparing the feedback signal

from a resistor divider on the VFB pin with an internal 0.6V

reference. When the load current increases, it causes a

reduction in the feedback voltage relative to the reference.

The error amplifier raises the ITH voltage until the average

inductor current matches the new load current. Typical

voltage range for the ITH pin is from 0.1V to 0.8V with

0.45V corresponding to zero current.

When the top power switch shuts off, the synchronous

power switch (N-channel MOSFET) turns on until either

the bottom current limit is reached or the next clock cycle

begins. The bottom current limit is typically set at –8A for

forced continuous mode and 0A for Burst Mode operation

and pulse-skipping mode.

The operating frequency defaults to 2.25MHz when

RT/SYNC is connected to SVIN, or can be set by an ex-

ternal resistor connected between the RT/SYNC pin and

ground, or by a clock signal applied to the RT/SYNC pin.

The switching frequency can be set from 300kHz to 4MHz.

Overvoltage and undervoltage comparators pull the

PGOOD output low if the output voltage varies more than

±7.5% (typical) from the set point.

PS PULSE-SKIPPING MODE ENABLE

FORCED CONTINUOUS MODE ENABLE

Burst Mode ENABLE—INTERNAL CLAMP

3614 OP01

Burst Mode ENABLE—EXTERNAL CLAMP,

CONTROLLED BY VOLTAGE APPLIED AT

MODE PIN

SVIN – 0.3V

SVIN • 0.58

1.1V

0.8V

0.45V

0.3V

SGND

EXT

Mode Selection Voltage

Burst Mode Operation—Internal Clamp

Connecting the MODE pin to SGND enables Burst Mode

operation with an internal clamp. In Burst Mode operation

the internal power switches operate intermittently at light

loads. This increases efficiency by minimizing switching

losses. During the intervals when the switches are idle,

the LTC3614 enters sleep state where many of the internal

circuits are disabled to save power. During Burst Mode

operation, the minimum peak inductor current is internally

clamped and the voltage on the ITH pin is monitored by

the burst comparator to determine when sleep mode is

enabled and disabled. When the average inductor current

is greater than the load current, the voltage on the ITH pin

drops. As the ITH voltage falls below the internal clamp,

the burst comparator trips and enables sleep mode. Dur-

ing sleep mode, both power MOSFETs are held off and

the load current is solely supplied by the output capacitor.

When the output voltage drops, the top power switch is

turned back on and the internal circuits are re-enabled.

This process repeats at a rate that is dependent on the

load current.

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operaTion

Burst Mode Operation—External Clamp

Connecting the MODE pin to a voltage in the range of 0.45V

to 0.8V enables Burst Mode operation with external clamp.

During this mode of operation the minimum voltage on

the ITH pin is externally set by the voltage on the MODE

pin. It is recommended to use Burst Mode operation with

internal burst clamp for temperatures above 85°C ambient.

Pulse-Skipping Mode Operation

Pulse-skipping mode is similar to Burst Mode operation,

but the LTC3614 does not disable power to the internal

circuitry during sleep mode. This improves output voltage

ripple but uses more quiescent current, compromising

light load efficiency.

Tying the MODE pin to SVIN enables pulse-skipping mode.

As the load current decreases, the peak inductor current

will be determined by the voltage on the ITH pin until the

ITH voltage drops below the voltage level corresponding to

0A. At this point, the peak inductor current is determined

by the minimum on-time of the current comparator. If

the load demand is less than the average of the minimum

on-time inductor current, switching cycles will be skipped

to keep the output voltage in regulation.

Forced Continuous Mode

In forced continuous mode the inductor current is con-

stantly cycled which creates a minimum output voltage

ripple at all output current levels.

Connecting the MODE pin to a voltage in the range of

1.1V to SVIN • 0.58 will enable forced continuous mode

operation.

At light loads, forced continuous mode operation is less

efficient than Burst Mode or pulse-skipping operation, but

may be desirable in some applications where it is neces-

sary to keep switching harmonics out of the signal band.

Forced continuous mode must be used if the output is

required to sink current.

Dropout Operation

As the input supply voltage approaches the output voltage,

the duty cycle increases toward the maximum on-time.

Further reduction of the supply voltage forces the main

switch to remain on for more than one cycle, eventually

reaching 100% duty cycle. The output voltage will then be

determined by the input voltage minus the voltage drop

across the internal P-channel MOSFET and the inductor.

Low Supply Operation

The LTC3614 is designed to operate down to an input

supply voltage of 2.25V. An important consideration at low

input supply voltages is that the RDS(ON) of the P-channel

and N-channel power switches increases. The user should

calculate the power dissipation when the LTC3614 is used

at 100% duty cycle with low input voltages to ensure that

thermal limits are not exceeded. See the Typical Perfor-

mance Characteristics graphs.

Short-Circuit Protection

The peak inductor current at which the current comparator

shuts off the top power switch is controlled by the voltage

on the ITH pin.

If the output current increases, the error amplifier raises the

ITH pin voltage until the average inductor current matches

the new load current. In normal operation the LTC3614

clamps the maximum ITH pin voltage at approximately 0.8V

which corresponds typically to 9A peak inductor current.

When the output is shorted to ground, the inductor current

decays very slowly during a single switching cycle. The

LTC3614 uses two techniques to prevent current runaway

from occurring.

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If the output voltage drops below 50% of its nominal value,

the clamp voltage at ITH pin is lowered causing the maxi-

mum peak inductor current to decrease gradually with the

output voltage. When the output voltage reaches 0V the

clamp voltage at the ITH pin drops to 40% of the clamp

voltage during normal operation. The short-circuit peak

inductor current is determined by the minimum on-time

of the LTC3614, the input voltage and the inductor value.

This foldback behavior helps in limiting the peak inductor

current when the output is shorted to ground. It is disabled

during internal or external soft-start and tracking up/down

operation (see the Applications Information section).

A secondary limit is also imposed on the valley inductor

current. If the inductor current measured through the

bottom MOSFET increases beyond 12A typical, the top

power MOSFET will be held off and switching cycles will

be skipped until the inductor current is reduced.

operaTion

The basic LTC3614 application circuit is shown in Figure 1.

Operating Frequency

Selection of the operating frequency is a trade-off between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low

output ripple voltage.

The operating frequency of the LTC3614 is determined

by an external resistor that is connected between the RT/

SYNC pin and ground. The value of the resistor sets the

RUN

TRACK/SS

RT/SYNC

PGOOD

ITH

SGND

PGND

2.25V TO 5.5V

SRLIM/DDR

SVIN

LTC3614 SW

PVIN

CIN1

10µF

×4

470pF

CSS

22nF

330nH

392k

196k

3614 F01

MODE VFB

COUT2

100µF

VOUT

1.8V

15k

130k

RSS

CC1

10pF

(OPT)

Figure 1. 1.8V, 4A Step-Down Regulator

ramp current that is used to charge and discharge an

internal timing capacitor within the oscillator and can be

calculated by using the following equation:

RT=3.82 •1011Hz

fOSC Hz

( )

Ω– 16kΩ

Although frequencies as high as 4MHz are possible, the

minimum on-time of the LTC3614 imposes a minimum

limit on the operating duty cycle. The minimum on-time

is typically 60ns; therefore, the minimum duty cycle is

equal to 60ns •fOSC(Hz)•100%.

Tying the RT/SYNC pin to SVIN sets the default internal

operating frequency to 2.25MHz ±20%.

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Frequency Synchronization

The LTC3614’s internal oscillator can be synchronized to

an external frequency by applying a square wave clock

signal to the RT/SYNC pin. During synchronization, the top

switch turn-on is locked to the falling edge of the external

frequency source. The synchronization frequency range

is 300kHz to 4MHz. During synchronization all operation

modes can be selected.

It is recommended that the regulator is powered down

(RUN pin to ground) before removing the clock signal on

the RT/SYNC pin in order to reduce inductor current ripple.

AC coupling should be used if the external clock generator

cannot provide a continuous clock signal throughout start-

up, operation and shutdown of the LTC3614. The size of

capacitor CSYNC depends on parasitic capacitance on the

RT/SYNC pin and is typically in the range of 10pF to 22pF.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current ∆IL increases with higher VIN and decreases

with higher inductance:

∆IL=VOUT

fSW •L











•1– VOUT

VIN













Having a lower ripple current reduces the core losses

in the inductor, the ESR losses in the output capacitors

and the output voltage ripple. A reasonable starting point

for selecting the ripple current is ∆IL = 0.3 • IOUT(MAX).

The largest ripple current occurs at the highest VIN. To

guarantee that the ripple current stays below a specified

maximum, the inductor value should be chosen according

to the following equation:

L=VOUT

fSW •∆IL(MAX)













•1– VOUT

VIN(MAX)













The inductor value will also have an effect on Burst Mode

operation. The transition to low current operation begins

when the peak inductor current falls below a level set by the

burst clamp. Lower inductor values result in higher ripple

current which causes this to occur at lower load currents.

This causes a dip in efficiency in the upper range of low

current operation. In Burst Mode operation, lower induc-

tance values will cause the burst frequency to increase.

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. Actual core loss is independent of core size for

fixed inductor value, but it is very dependent on the induc-

tance selected. As the inductance increases, core losses de-

crease. Unfortunately,

increased inductance requires more

turns of wire and therefore, copper losses will increase.

LTC3614

SVIN

RT/SYNC

LTC3614

SVIN

VIN

0.4V RT/SYNC

SGND

LTC3614

SVIN

fOSC

2.25MHz

fOSC

1/TP

fOSC ∝1/RT

VIN

RT/SYNC

SGND

1.2V

0.3V

LTC3614

SVIN fOSC

1/TP

VIN

CSYNC

RT/SYNC

SGND

3614 F02

Figure 2. Setting the Switching Frequency

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Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing satura-

tion. Ferrite core material saturates “hard,” meaning

that inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequently output voltage

ripple. Do not allow a ferrite core to saturate, and select

external inductors respecting the temperature range of

the application!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price versus size requirements

and any radiated field/EMI requirements. Table 1 shows

some typical surface mount inductors that work well in

LTC3614 applications.

Input Capacitor (CIN) Selection

In continuous mode, the source current of the top P-

channel MOSFET is a square wave of duty cycle VOUT/

VIN. To prevent large input voltage transients, a low ESR

capacitor sized for the maximum RMS current must be

used at VIN.

The maximum RMS capacitor current is given by:

IRMS =IOUT(MAX) •VOUT

VIN

•VIN

VOUT

– 1











This formula has a maximum at VIN = 2VOUT

, where IRMS =

IOUT/2. This simple worst-case condition is commonly used

for design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based on only 2000 hours of life

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required. Generally select the capacitors respecting the

temperature range of the application! Several capacitors

may also be paralleled to meet size or height requirements

in the design.

Table 1. Representative Surface Mount Inductors

INDUCTANCE

(μH)

DCR

(mΩ)

SATURATION

CURRENT (A)

DIMENSIONS

(mm)

HEIGHT

(mm)

Vishay IHLP-2525CZ-01

0.10 1.5 60 6.5 × 6.9 3

0.15 1.9 52 6.5 × 6.9 3

0.20 2.4 41 6.5 × 6.9 3

0.22 2.5 40 6.5 × 6.9 3

0.33 3.5 30 6.5 × 6.9 3

0.47 4 26 6.5 × 6.9 3

Sumida CDMC6D28 Series

0.2 2.5 21.7 7.25 × 4.4 3

0.3 3.2 15.4 7.25 × 4.4 3

0.47 4.2 13.6 7.25 × 4.4 3

Cooper HCP0703 Series

0.22 2.8 23 7 × 7.3 3.0

0.47 4.2 17 7 × 7.3 3.0

0.68 5.5 15 7 × 7.3 3.0

Würth Electronik WE-HC744312 Series

0.25 2.5 18 7 × 7.7 3.8

0.47 3.4 16 7 × 7.7 3.8

Coilcraft SLC7530 Series

0.100 0.123 20 7.5 × 6.7 3

0.188 0.100 21 7.5 × 6.7 3

0.272 0.100 14 7.5 × 6.7 3

0.350 0.100 11 7.5 × 6.7 3

0.400 0.100 8 7.5 × 6.7 3

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Output Capacitor (COUT

) Selection

The selection of COUT is typically driven by the required

ESR to minimize voltage ripple and load step transients

(low ESR ceramic capacitors are discussed in the next

section). Typically, once the ESR requirement is satisfied,

the capacitance is adequate for filtering. The output ripple

∆VOUT is determined by:

∆VOUT ≤ ∆IL•ESR+1

8•fSW •COUT













where fOSC = operating frequency, COUT = output capaci-

tance and ∆IL = ripple current in the inductor. The output

ripple is highest at maximum input voltage since ∆IL

increases with input voltage.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Aluminum

electrolytic, special polymer, ceramic and dry tantalum

capacitors are all available in surface mount packages.

Tantalum capacitors have the highest capacitance density,

but can have higher ESR and must be surge tested for

use in switching power supplies. Aluminum electrolytic

capacitors have significantly higher ESR, but can often

be used in extremely cost-sensitive applications provided

that consideration is given to ripple current ratings and

long-term reliability.

Ceramic Input and Output Capacitors

Ceramic capacitors have the lowest ESR and can be cost

effective, but also have the lowest capacitance density,

high voltage and temperature coefficients, and exhibit

audible piezoelectric effects. In addition, the high Q of

ceramic capacitors along with trace inductance can lead

to significant ringing.

They are attractive for switching regulator use because

of their very low ESR, but great care must be taken when

using only ceramic input and output capacitors.

Ceramic capacitors are prone to temperature effects

which require the designer to check loop stability over

the operating temperature range. To minimize their large

temperature and voltage coefficients, only X5R or X7R

ceramic capacitors should be used.

When a ceramic capacitor is used at the input and the power

is being supplied through long wires, such as from a wall

adapter, a load step at the output can induce ringing at

the VIN pin. At best, this ringing can couple to the output

and be mistaken as loop instability. At worst, the ringing

at the input can be large enough to damage the part.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement. During a load step, the output capacitor must

instantaneously supply the current until the feedback loop

raises the switch current enough to support the load. The

time required for the feedback loop to respond is dependent

on the compensation components and the output capaci-

tor size. Typically, 3 to 4 cycles are required to respond

to a load step, but only in the first cycle does the output

drop linearly. The output droop, VDROOP

, is usually about

2 to 4 times the linear drop of the first cycle; however,

this behavior can vary depending on the compensation

component values. Thus, a good place to start is with the

output capacitor size of approximately:

COUT ≈

3.5 •∆I

OUT

•V

DROOP

This is only an approximation; more capacitance may

be needed depending on the duty cycle and load step

requirements.

In most applications, the input capacitor is merely required

to supply high frequency bypassing, since the impedance

to the supply is very low.

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Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation:

VOUT =0.6 •1+R1











 V

The resistive divider allows pin VFB to sense a fraction of

the output voltage as shown in Figure 1.

Burst Clamp Programming

If the voltage on the MODE pin is less than 0.8V, Burst

Mode operation is enabled.

If the voltage on the MODE pin is less than 0.3V, the in-

ternal default burst clamp level is selected. The minimum

voltage on the ITH pin is typically 525mV (internal clamp).

If the voltage is between 0.45V and 0.8V, the voltage on

the MODE pin (VBURST) is equal to the minimum voltage

on the ITH pin (external clamp) and determines the burst

clamp level IBURST

(typically from 0A to 7A).

When the ITH voltage falls below the internal (or external)

clamp voltage, the sleep state is enabled.

As the output load current drops, the peak inductor current

decreases to keep the output voltage in regulation. When

the output load current demands a peak inductor current

that is less than IBURST

, the burst clamp will force the peak

inductor current to remain equal to IBURST regardless of

further reductions in the load current.

Since the average inductor current is greater than the out-

put load current, the voltage on the ITH pin will decrease.

When the ITH voltage drops, sleep mode is enabled in

which both power switches are shut off along with most

of the circuitry to minimize power consumption. All cir-

cuitry is turned back on and the power switches resume

operation when the output voltage drops out of regulation.

The value for IBURST is determined by the desired amount

of output voltage ripple. As the value of IBURST increases,

the sleep period between pulses and the output voltage

ripple increase. Note that for very high VBURST voltage

settings, the power good comparator may trip, since the

output ripple may get bigger than the power good window.

Pulse-skipping mode, which is a compromise between low

output voltage ripple and efficiency, can be implemented

by connecting MODE to SVIN. This sets IBURST to 0A. In

this condition, the peak inductor current is limited by the

minimum on-time of the current comparator. The low-

est output voltage ripple is achieved while still operating

discontinuously. During very light output loads, pulse-

skipping allows only a few switching cycles to skip while

maintaining the output voltage in regulation.

Internal and External Compensation

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC load current.

When a load step occurs, VOUT shifts by an amount equal

to ∆ILOAD(ESR), where ESR is the effective series resistance

of COUT

. ∆ILOAD also begins to charge or discharge COUT

generating the feedback error signal that forces the regula-

tor to adapt to the current change and return VOUT to its

steady-state value. During this recovery time VOUT can

be monitored for excessive overshoot or ringing, which

would indicate a stability problem. The availability of the

ITH pin allows the transient response to be optimized over

a wide range of output capacitance.

The ITH external components (RC and CC) shown in Fig-

ure 1 provide adequate compensation as a starting point

for most applications. The values can be modified slightly

to optimize transient response once the final PCB layout

is done and the particular output capacitor type and value

have been determined. The output capacitors need to be

selected because the various types and values determine

the loop gain and phase. The gain of the loop will be in-

creased by increasing RC and the bandwidth of the loop

will be increased by decreasing CC. If RC is increased by

the same factor that CC is decreased, the zero frequency

will be kept the same, thereby keeping the phase shift the

same in the most critical frequency range of the feedback

loop. The output voltage settling behavior is related to the

stability of the closed-loop system. The external capaci-

tor, CC1, (Figure 1) is not needed for loop stability, but it

helps filter out any high frequency noise that may couple

onto that node.

applicaTions inForMaTion

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The first circuit in the Typical Applications section uses

faster compensation to improve step response.

A second, more severe transient is caused by switching

in loads with large (>1μF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT

, causing a rapid drop in VOUT

. No regulator can

alter its delivery of current quickly enough to prevent this

sudden step change in output voltage if the load switch

resistance is low and it is driven quickly. More output

capacitance may be required depending on the duty cycle

and load step requirements.

AVP Mode

Fast load transient response, limited board space and low

cost are typical requirements of microprocessor power

supplies. A microprocessor will typically exhibit full load

steps with very fast slew rate. The voltage at the micro-

processor must be held to about ±0.1V of nominal in spite

of these load current steps. Since the control loop cannot

respond this fast, the output capacitors must supply the

load current until the control loop can respond.

Normally, several capacitors in parallel are required to

meet microprocessor transient requirements. Capacitor

ESR and ESL primarily determine the amount of droop or

overshoot in the output voltage.

applicaTions inForMaTion

Figure 4. Load Step Transient Forced

Continuous Mode with AVP Mode

Consider the LTC3614 without AVP with a bank of tantalum

output capacitors. If a load step with very fast slew rate

occurs, the voltage excursion will be seen in both direc-

tions, for full load to minimum load transient and for the

minimum load to full load transient.

If the ITH pin is tied to SVIN, the active voltage position-

ing (AVP) mode and internal compensation are selected.

AVP mode intentionally compromises load regulation by

reducing the gain of the feedback circuit, resulting in an

output voltage that slightly varies with load current. When

the load current suddenly increases, the output voltage

starts from a level slightly higher than nominal so the out-

put voltage can droop more and stay within the specified

voltage range. When the load current suddenly decreases

the output voltage starts at a level lower than nominal

so the output voltage can have more overshoot and stay

within the specified voltage range (see Figures 3 and 4).

The benefit is a lower peak-to-peak output voltage deviation

for a given load step without having to increase the output

filter capacitance. Alternatively, the output voltage filter

capacitance can be reduced while maintaining the same

peak to peak transient response. Due to the reduced loop

gain in AVP mode, no external compensation is required.

Figure 3. Load Step Transient Forced

Continuous Mode (AVP Inactive)

VOUT

200mV/DIV

1A/DIV

50µs/DIV 3614 F03

VIN = 3.3V

VOUT = 1.8V

ILOAD = 100mA TO 3A

VMODE = 1.5V

COMPENSATION FIGURE 1

VOUT

100mV/DIV

1A/DIV

50µs/DIV 3614 F04

VIN = 3.3V

VOUT = 1.8V

ILOAD = 100mA TO 3A

VMODE = 1.5V

VITH = 3.3V

OUTPUT CAPACITOR VALUE FIGURE 1

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DDR Mode

The LTC3614 can both source and sink current if the MODE

pin is configured to forced continuous mode.

Current sinking is typically limited to 3A for 1MHz frequency

and a 0.47µH inductor, but can be lower at higher frequen-

cies and low output voltages. If higher ripple current can

be tolerated, smaller inductor values can increase the sink

current limit. See the Typical Performance Characteristics

curves for more information.

In addition, by tying the SRLIM/DDR pin to SVIN, lower

external reference voltage and tracking output voltage are

possible. See the Output Voltage Tracking and External

Reference Input sections.

Soft-Start

The RUN pin provides a means to shut down the LTC3614.

Tying the RUN pin to SGND places the LTC3614 in a low

quiescent current shutdown state (IQ < 1µA).

The LTC3614 is enabled by pulling the RUN pin high.

However, the applied voltage must not exceed SVIN. In

some applications the RUN signal is generated within

another power domain and is driven high while the SVIN

and PVIN are still 0V. In this case, it’s required to limit

the current into the RUN pin by either adding a 1MΩ

resistor or a 100k resistor plus a Schottky diode to SVIN.

After pulling the RUN pin high the chip enters a soft start-up

state. The type of soft start-up behavior is set by the

TRACK/SS pin:

1. Tying TRACK/SS to SVIN selects the internal soft-start

circuit. This circuit ramps the output voltage to the final

value within 1ms.

2. If a longer soft-start period is desired, it can be set

externally with a resistor and capacitor on the TRACK/

SS pin as shown in Figure 1. The TRACK/SS pin reduces

the value of the internal reference at VFB until TRACK/

SS is pulled above 0.6V. The external soft-start duration

can be calculated by using the following formula:

tSS =RSS •CSS •ln SVIN

SVIN – 0.6V













3. The TRACK/SS pin can be used to track the output

voltage of another supply.

Each time the RUN pin is tied high and the LTC3614 is

turned on, the TRACK/SS pin is internally pulled down

for ten microseconds in order to discharge the external

capacitor. This discharging time is typically adequate

for capacitors up to about 33nF. If a larger capacitor is

required, connect the external soft-start resistor to the

RUN pin.

Figure 5. Slew Rate at SW Pin vs SRLIM/DDR Resistor: Open, 100k, 10k

2ns/DIV

SW PIN

10k

100k

OPEN

3614 F05

VIN = 3.3V

VOUT = 1.8V

fSW = 2.25MHz

2ns/DIV

VIN = 3.3V

VOUT = 1.8V

fSW = 2.25MHz

10k

100k

OPEN

LTC3614

3614fc

For more information www.linear.com/LTC3614

applicaTions inForMaTion

During either internal or external soft-start, the MODE pin

is ignored and soft-start will always be in pulse-skipping

mode. In addition, the PGOOD pin is kept low and foldback

of the switching frequency is disabled.

Programmable Switch Pin Slew Rate

As switching frequencies rise, it is desirable to minimize the

transition time required when switching to minimize power

losses and blanking time for the switch to settle. However,

fast slewing of the switch node results in relatively high

external radiated EMI and high on chip supply transients,

which can cause problems for some applications.

The LTC3614 allows the user to control the slew rate of

the switching node SW by using the SRLIM/DDR pin.

Tying this pin to ground selects the fastest slew rate. The

slowest slew rate is selected when the pin is open. Con-

necting a resistor (between 10k and 100k) from SRLIM

pin to ground adjusts the slew rate between the maximum

and minimum values. The reduced dV/dt of the switch

node results in a significant reduction of the supply and

ground ringing, as well as lower radiated EMI.

Figure 6. Two Different Modes of Output Voltage Tracking

Particular attention should be used with very high switching

frequencies. Using the slowest slew rate (SRLIM open)

can reduce the minimum duty cycle capability.

Output Voltage Tracking Input

If the DDR pin is not tied to SVIN, once VTRACK/SS exceeds

0.6V, the run state is entered and the MODE selection,

power good and current foldback circuits are enabled.

In the run state, the TRACK/SS pin can be used for track-

ing down/up the output voltage of another supply. If the

VTRACK/SS drops below 0.6V, the LTC3614 enters the

down tracking state and VOUT is referenced to the TRACK/

SS voltage. If the TRACK/SS pin drops below 0.2V, the

switching frequency is reduced to ensure that the mini-

mum duty cycle limit does not prevent the output from

following the TRACK/SS pin. The run state will resume if

VTRACK/SS again exceeds 0.6V and VOUT is referenced to

the internal precision reference (see Figure 8).

Through the TRACK/SS pin, the output voltage can be set

up for either coincident or ratiometric tracking, as shown

in Figure 6.

TIME

(6b) Ratiometric Tracking

VOUT1

VOUT2

OUTPUT VOLTAGE

TIME 3614 F06

(6a) Coincident Tracking

VOUT1

VOUT2

OUTPUT VOLTAGE

LTC3614

3614fc

For more information www.linear.com/LTC3614

Figure 7a. Setup for Coincident Tracking

Figure 7b. Setup for Ratiometric Tracking

VFB2

R4 ≤ R3

VOUT2

OUT1

LTC3614

TRACK/SS2

VFB1 VIN

LTC3614

LTC3614 CHANNEL 2

SLAVE

LTC3614 CHANNEL 1

MASTER

TRACK/SS1

3614 F07a

VFB2

R3 R1/R2 < R5/R6

VOUT2

VOUT1

LTC3614

TRACK/SS2

VFB1

VIN

3614 F07b

LTC3614

LTC3614 CHANNEL 2

SLAVE

LTC3614 CHANNEL 1

MASTER

TRACK/SS1

To implement the coincident tracking behavior in Fig-

ure 6a, connect an extra resistive divider to the output

of the master channel and connect its midpoint to the

TRACK/SS pin for the slave channel. The ratio of this

divider should be selected to be the same as that of the

slave channel’s feedback divider (Figure 7a).

In this track-

ing mode, the master channel’s output must be set higher

than slave channel’s output. To implement the ratiometric

tracking behavior in Figure 6b, different resistor divider

values must be used as specified in Figure 7b.

For coincident start-up, the voltage value at the TRACK/SS

pin for the slave channel needs to reach the final reference

value after the internal soft-start time (around 1ms). The

master start-up time needs to be adjusted with an external

capacitor and resistor to ensure this.

External Reference Input (DDR Mode)

If the DDR pin is tied to SVIN (DDR mode), the run state is

entered when VTRACK/SS exceeds 0.3V and tracking down

behavior is possible if the VTRACK/SS voltage is below 0.6V.

This allows TRACK/SS to be used as an external reference

between 0.3V and 0.6V if desired. During the run state in

DDR mode, the power good window moves in relation

to the actual TRACK/SS pin voltage if the voltage value

is between 0.3V and 0.6V. Note: if TRACK/SS voltage is

0.6V, either the tracking circuit or the internal reference

can be used.

During up/down tracking the output current foldback is

disabled and the PGOOD pin is always pulled down (see

Figure 9).

applicaTions inForMaTion

LTC3614

3614fc

For more information www.linear.com/LTC3614

applicaTions inForMaTion

Figure 8. DDR Pin Not Tied to SVIN

Figure 9. DDR Pin Tied to SVIN. Example DDR Application

SOFT-START

STATE

tSS > 1ms

SHUTDOWN

STATE

0.6V

0.2V

VIN

VFB PIN

VOLTAGE

TRACK/SS

PIN VOLTAGE

RUN PIN

VOLTAGE

SVIN PIN

VOLTAGE

RUN STATE RUN STATE

TIME

3614 F08

REDUCED

SWITCHING

FREQUENCY

DOWN

TRACKING

STATE

TRACKING

STATE

SOFT-START

STATE

tSS > 1ms

SHUTDOWN

STATE

0.3V

0.45V

0.3V

0.2V

VIN

VFB PIN

VOLTAGE

EXTERNAL

VOLTAGE

REFERENCE 0.45V

TRACK/SS

PIN VOLTAGE

RUN PIN

VOLTAGE

SVIN PIN

VOLTAGE

RUN STATE RUN STATE

TIME

3614 F09

REDUCED

SWITCHING

FREQUENCY

DOWN

TRACKING

STATE

TRACKING

STATE

LTC3614

3614fc

For more information www.linear.com/LTC3614

Efficiency Considerations

The efficiency of a switching regulator is equal to the output

power divided by the input power times 100%. It is often

useful to analyze individual losses to determine what is

limiting the efficiency and which change would produce

the most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of

the losses: VIN quiescent current and I2R losses. The VIN

quiescent current loss dominates the efficiency loss at

very low load currents whereas the I2R loss dominates

the efficiency loss at medium to high load currents. In a

typical efficiency plot, the efficiency curve at very low load

currents can be misleading since the actual power lost is

usually of no consequence.

1. The VIN quiescent current is due to two components: the

DC bias current as given in the Electrical Characteristics

and the internal main switch and synchronous switch

gate charge currents. The gate charge current results

from switching the gate capacitance of the internal power

MOSFET switches. Each time the gate is switched from

low to high to low again, a packet of charge dQ moves

from VIN to ground. The resulting dQ/dt is the current

out of VIN due to gate charge, and it is typically larger

than the DC bias current. Both the DC bias and gate

charge losses are proportional to VIN; thus, their effects

will be more pronounced at higher supply voltages.

2. I2R losses are calculated from the resistances of the

internal switches, RSW

, and external inductor, RL. In

continuous mode the average output current flowing

through inductor L is “chopped” between the main

switch and the synchronous switch. Thus, the series

resistance looking into the SW pin is a function of both

top and bottom MOSFET RDS(ON) and the duty cycle

(DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs can

be obtained from the Typical Performance Character-

istics curves. To obtain I2R losses, simply add RSW to

RL and multiply the result by the square of the average

output current.

Other losses including CIN and COUT ESR dissipative

losses and inductor core losses generally account for

less than 2% of the total loss.

applicaTions inForMaTion

LTC3614

3614fc

For more information www.linear.com/LTC3614

applicaTions inForMaTion

Thermal Considerations

In most applications, the LTC3614 does not dissipate

much heat due to its high efficiency.

However, in applications where the LTC3614 is running

at high ambient temperature with low supply voltage and

high duty cycles, such as in dropout, the heat dissipated

may exceed the maximum junction temperature of the part.

If the junction temperature reaches approximately 170°C,

both power switches will be turned off and the SW node

will become high impedance.

To prevent the LTC3614 from exceeding the maximum

junction temperature, some thermal analysis is required.

The temperature rise is given by:

TRISE = (PD)(θJA)

where PD is the power dissipated by the regulator and

θJA is the thermal resistance from the junction of the die

to the ambient temperature. The junction temperature,

TJ, is given by:

TJ = TA + TRISE

where TA is the ambient temperature.

As an example, consider the case when the LTC3614 is in

dropout at an input voltage of 3.3V with a load current of

4A at an ambient temperature of 85°C. From the Typical

Performance Characteristics graph of Switch Resistance,

the RDS(ON) resistance of the P-channel switch is 0.038Ω.

Therefore, power dissipated by the part is:

PD = (IOUT)2 • RDS(ON) = 0.61W

For the QFN package, the θJA is 38°C/W.

Therefore, the junction temperature of the regulator op-

erating at 85°C ambient temperature is approximately:

TJ = 0.61W • 38°C/W + 85°C = 108°C

We can safely assume that the actual junction temperature

will not exceed the absolute maximum junction tempera-

ture of 125°C.

Note that for very low input voltage, the junction tempera-

ture will be higher due to increased switch resistance,

RDS(ON). It is not recommended to use full load current

with high ambient temperature and low input voltage.

To maximize the thermal performance of the LTC3614 the

exposed pad should be soldered to a ground plane. See

the PCB Layout Board Checklist.

Design Example

As a design example, consider using the LTC3614 in an

application with the following specifications:

VIN = 2.25V to 5.5V, VOUT = 1.8V, IOUT(MAX) = 4A, IOUT(MIN)

= 200mA, f = 2.6MHz.

Efficiency is important at both high and low load current,

so Burst Mode operation will be utilized.

First, calculate the timing resistor:

RT=3.82

2.6MHz

k – 16k =130kΩ

Next, calculate the inductor value for about 33% ripple

current at maximum VIN:

L=1.8V

2.6MHz •1.3A











•1– 1.8V

5.5V











=0.35µH

Using a standard value of 0.33µH inductor results in a

maximum ripple current of:

∆IL=1.8V

2.6MHz •0.33µH











•1– 1.8V

5.5V









=1.41A

COUT will be selected based on the ESR that is required

to satisfy the output voltage ripple requirement and the

bulk capacitance needed for loop stability. For this design,

a 100µF ceramic capacitor is used with a X5R or X7R

dielectric.

LTC3614

3614fc

For more information www.linear.com/LTC3614

Assuming worst-case conditions of VIN = 2VOUT, CIN should

be selected for a maximum current rating of:

IRMS =4A •1.8V

3.6V •3.6V

1.8V – 1









=2ARMS

Decoupling PVIN with four 10µF to 22µF capacitors is

adequate for most applications.

If we set R2 = 196k, the value of R1 can now be determined

by solving the following equation.

R1 = 196k •1.8V

0.6V

−1













A value of 392k will be selected for R1.

Finally, define the soft start-up time choosing the proper

value for the capacitor and the resistor connected to

TRACK/SS. If we set minimum tSS = 5ms and a resistor

of 2MΩ, the following equation can be solved with the

maximum SVIN = 5.5V :

CSS =

5ms

2MΩ•In 5.5V

5.5V – 0.6V











=21.6nF

The standard value of 22nF guarantees the minimum soft-

start up time of 5ms.

Figure 1 shows the schematic for this design example.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3614:

1. A ground plane is recommended. If a ground plane layer

is not used, the signal and power grounds should be

segregated with all small-signal components returning

to the SGND pin at one point which is then connected

to the PGND pin close to the LTC3614.

2. Connect the (+) terminal of the input capacitor(s), CIN,

as close as possible to the PVIN pin, and the (–) terminal

as close as possible to the exposed pad, PGND. This

capacitor provides the AC current into the internal power

MOSFETs.

3. Keep the switching node, SW, away from all sensitive

small-signal nodes.

4. Flood all unused areas on all layers with copper. Flood-

ing with copper will reduce the temperature rise of

power components. Connect the copper areas to PGND

(exposed pad) for best performance.

5. Connect the VFB pin directly to the feedback resistors.

The resistor divider must be connected between VOUT

and SGND.

applicaTions inForMaTion

LTC3614

3614fc

For more information www.linear.com/LTC3614

Typical applicaTions

General Purpose Buck Regulator with Fast Compensation

and Improved Step Response, 2.25MHz

RUN

TRACK/SS

RT/SYNC

PGOOD

ITH

PGOOD SGND

PGND

2.25V TO 5.5V

SRLIM/DDR

SVIN

LTC3614

PVIN

1µF

24Ω

0.33µH

392k

22pF

196k

3614 TA02a

MODE VFB

CO2

100µF

CC1

10pF

10µF

×4

220pF

CSS

10nF VOUT

1.8V

100k

R5B

L1: VISHAY IHLP-2525CZ-01 330nH

R5A

43k

RSS

4.7M

Efficiency vs Output Current

Load Step Response in

Forced Continuous Mode

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

1 100 1000 10000

3614 TA02b

VIN = 2.5V

VIN = 3.3V

VIN = 4V

VIN = 5.5V

VOUT = 1.8V

VOUT

100mV/DIV

IOUT

2A/DIV

50µs/DIV 3614 TA02c

VIN = 3.3V

VOUT = 1.8V

IOUT = 100mA TO 4A

VMODE = 1.5V

LTC3614

3614fc

For more information www.linear.com/LTC3614

Typical applicaTions

Master and Slave for Coincident Tracking Outputs Using a 1MHz External Clock

RUN

TRACK/SS

RT/SYNC

PGOOD

ITH

PGOOD SGND

PGND

VIN

2.25V TO 5.5V

SRLIM/DDR

SVIN

LTC3614

PVIN

CF1

1µF

RF1

24Ω

0.68µH

CHANNEL 1

MASTER

CHANNEL 2

SLAVE

715k

22pF

357k

464k

3614 TA03a

MODE VFB

CO12

100µF

CC2

10pF

22µF

×4

CC1

470pF

VOUT1

1.8V

10nF

4.7M

100k

1MHz

CLOCK

RC1

15k

RUN

TRACK/SS

RT/SYNC

PGOOD

ITH

PGOOD SGND

PGND

SRLIM/DDR

SVIN

LTC3614

PVIN

CF2

1µF

RF2

24Ω

0.68µH

301k

22pF

301k

MODE VFB

CO22

100µF

CC4

10pF

L1, L2: VISHAY IHLP-2525CZ-01 680nH

22µF

×4

CC3

470pF

VOUT2

1.2V

100k

RC2

15k

Coincident Start-Up Coincident Tracking Up/Down

500mV/DIV

2ms/DIV 3614 TA03b

VOUT1

VOUT2

500mV/DIV

200ms/DIV 3614 TA03c

VOUT1

VOUT2

LTC3614

3614fc

For more information www.linear.com/LTC3614

package DescripTion

UDD Package

24-Lead Plastic QFN (3mm × 5mm)

(Reference LTC DWG # 05-08-1833)

3.00 ± 0.10 1.50 REF

5.00 ± 0.10

NOTE:

1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

PIN 1

TOP MARK

(NOTE 6)

0.40 ± 0.10

23 24

BOTTOM VIEW—EXPOSED PAD

3.50 REF

0.75 ± 0.05

R = 0.115

TYP

PIN 1 NOTCH

R = 0.20 OR 0.25

× 45° CHAMFER

0.25 ± 0.05

0.50 BSC

0.200 REF

0.00 – 0.05

(UDD24) QFN 0808 REV Ø

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

0.70 ±0.05

0.25 ±0.05

3.50 REF

4.10 ± 0.05

5.50 ± 0.05

1.50 REF

2.10 ± 0.05

3.50 ± 0.05

PACKAGE OUTLINE

R = 0.05 TYP

1.65 ± 0.10

3.65 ± 0.10

1.65 ± 0.05

UDD Package

24-Lead Plastic QFN (3mm × 5mm)

(Reference LTC DWG # 05-08-1833 Rev Ø)

3.65 ± 0.05

0.50 BSC

LTC3614

3614fc

For more information www.linear.com/LTC3614

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

revision hisTory

REV DATE DESCRIPTION PAGE NUMBER

A 11/10 Load Regulation ITH Voltage updated to the Electrical Characteristics table.

Note 2 updated to the Electrical Characteristics section.

Text updated to the Soft-Start section in the Applications Information section.

Related Parts table updated.

3, 11, 12

B 11/13 Add H and MP grades and applicable temerature range references.

Modified Note 2.

Modified Typical Performance Characteristics graphs.

Modified Inductor Core Selection section.

Modified Inout Capacitor Selection section.

Modified Thermal Considerations section.

Throughout

6, 7

14, 15

C 05/14 Change low spec for Top Switch Current Limit (Duty Cycle=100%) to 5A 3

LTC3614

3614fc

For more information www.linear.com/LTC3614

 LINEAR TECHNOLOGY CORPORATION 2010

LT 0514 REV C • PRINTED IN USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC3614

relaTeD parTs

Typical applicaTion

DDR Termination With Ratiometric Tracking of VDD, 1MHz Ratiometric Start-Up

PART NUMBER DESCRIPTION COMMENTS

LTC3616 5.5V, 6A (IOUT) 4MHz Synchronous Step-Down DC/DC

Converter

95% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.6V,

IQ = 70µA, ISD < 1µA, 3mm × 5mm QFN24 Package

LTC3612 5.5V, 3A (IOUT), 4MHz, Synchronous Step-Down DC/DC

Converter

95% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.6V,

IQ = 70µA, ISD <1µA, 3mm × 4mm QFN-20 TSSOP20E Package

LTC3418 5.5V, 8A (IOUT), 4MHz, Synchronous Step-Down DC/DC

Converter

95% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.8V,

IQ = 380µA, ISD <1µA, 5mm × 7mm QFN-38 Package

LTC3415 5.5V, 7A (IOUT), 1.5MHz, Synchronous Step-Down DC/DC

Converter

95% Efficiency, VIN(MIN) = 2.5V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.6V,

IQ = 450µA, ISD <1µA, 5mm × 7mm QFN-38 Package

LTC3416 5.5V, 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC

Converter

95% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.8V,

IQ = 64µA, ISD <1µA, TSSOP20E Package

LTC3413 5.5V, 3A (IOUT Sink/Source), 2MHz, Monolithic Synchronous

Regulator for DDR/QDR Memory Termination

90% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) =

VREF/2, IQ = 280µA, ISD <1µA, TSSOP16E Package

LTC3412A 5.5V, 3A (IOUT), 4MHz, Synchronous Step-Down DC/DC

Converter

95% Efficiency, VIN(MIN) = 2.5V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.8V,

IQ = 60µA, ISD <1µA, 4mm × 4mm QFN-16 TSSOP16E Package

RUN

TRACK/SS

RT/SYNC

PGOODPGOOD

SGND

PGND

VIN

3.3V

VDD

1.8V SRLIM/DDR

SVIN

LTC3614

PVIN

0.33µH

200k

22pF

200k

3614 TA04a

MODE VFB

100µF

47µF

CC1

10pF

22µF

×4

2.2nF

VTT

0.9V

±3A

100k R8

365k

1M L1: COILCRAFT DO3316T

187k

562k

ITH

500mV/DIV

500µs/DIV 3614 TA04b

VDD

VTT