LTC3621HMS8E-25#TRPBF - LINEAR TECHNOLOGY

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

Typical applicaTion

FeaTures DescripTion

17V, 1A Synchronous

Step-Down Regulator with

3.5µA Quiescent Current

The LT C

3621/LTC3621-2 is a high efficiency 17V, 1A

synchronous monolithic step-down regulator. The switch-

ing frequency is fixed to 1MHz or 2.25MHz with a ±40%

synchronizing range. The regulator features ultralow quies-

cent current and high efficiencies over a wide VOUT range.

The step-down regulator operates from an input voltage

range of 2.7V to 17V and provides an adjustable output

range from 0.6V to VIN while delivering up to 1A of output

current. A user-selectable mode input is provided to allow

the user to trade off ripple noise for light load efficiency;

Burst Mode operation provides the highest efficiency at

light loads, while pulse-skipping mode provides the lowest

voltage ripple. The MODE pin can also be used to allow the

user to sync the switching frequency to an external clock.

LTC3621 Options

PART NAME FREQUENCY VOUT

LTC3621 1.00MHz Adjustable

LTC3621-3.3 1.00MHz 3.3V

LTC3621-5 1.00MHz 5V

LTC3621-2 2.25MHz Adjustable

LTC3621-23.3 2.25MHz 3.3V

LTC3621-25 2.25MHz 5V

Efficiency and Power Loss vs Load at 1MHz

2.5V VOUT with 400mA Burst Clamp, fSW = 1MHz

applicaTions

n Wide VIN Range: 2.7V to 17V

n Wide VOUT Range: 0.6V to VIN

n 95% Max Efficiency

n Low IQ < 3.5µA, Zero-Current Shutdown

n Constant Frequency (1MHz/2.25MHz)

n Full Dropout Operation with Low IQ

n 1A Rated Output Current

n ±1% Output Voltage Accuracy

n Current Mode Operation for Excellent Line and Load

Transient Response

n Synchronizable to External Clock

n Pulse-Skipping, Forced Continuous, Burst Mode

Operation

n Internal Compensation and Soft-Start

n Overtemperature Protection

n Compact 6-Lead DFN (2mm × 3mm) Package or

Thermally-Enhanced MS8E Package with Power

Good Output and Independent SGND Pin

n Portable-Handheld Scanners

n Industrial and Embedded Computing

n Automotive Applications

n Emergency Radio L, LT, LTC, LTM, Burst Mode, Linear Technology, the Linear logo and LTSpice are registered

trademarks and Hot Swap and LTpowerCAD are trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Protected by U.S. Patents,

including 5481178, 6580258, 6498466, 6611131, 6177787, 5705919, 5847554.

604k 22pF

1µF

22µF

3621 TA01a

4.7µH

OUT

2.5V

191k

VIN

RUN

VIN

2.7V TO 17V

LTC3621

GND

MODE/SYNC

INTVCC

10µF

LOAD CURRENT (A)

EFFICIENCY (%)

POWER LOSS (W)

100

0.2 0.4 0.6

3621 TA01b

0.1

0.6

0.4

0.5

0.3

0.2

0.8 1

EFFICIENCY

POWER LOSS

VOUT = 5V

VOUT = 3.3V

VOUT = 2.5V

VIN = 12V

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

absoluTe MaxiMuM raTings

VIN Voltage ................................................. 17V to –0.3V

RUN Voltage................................................ VIN to –0.3V

MODE/SYNC, FB Voltages ............................ 6V to –0.3V

PGOOD Voltages .......................................... 6V to –0.3V

(Note 1)

TOP VIEW

MODE/SYNC

INTVCC

VIN

RUN

DCB PACKAGE

6-LEAD (2mm

3mm) PLASTIC DFN

GND

TJMAX = 125°C, θJA = 64°C/W, θJC = 9.6°C/W

EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB

VIN

RUN

PGOOD

SGND

MODE/SYNC

INTVCC

TOP VIEW

GND

MS8E PACKAGE

8-LEAD PLASTIC MSOP

TJMAX = 150°C, θJA = 40°C/W, θJC = 10°C/W

EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB

pin conFiguraTion

orDer inForMaTion

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

LTC3621EDCB#PBF LTC3621EDCB#TRPBF LGDG 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621IDCB#PBF LTC3621IDCB#TRPBF LGDG 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621EDCB-3.3#PBF LTC3621EDCB-3.3#TRPBF LGQF 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621IDCB-3.3#PBF LTC3621IDCB-3.3#TRPBF LGQF 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621EDCB-5#PBF LTC3621EDCB-5#TRPBF LGQC 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621IDCB-5#PBF LTC3621IDCB-5#TRPBF LGQC 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621EMS8E#PBF LTC3621EMS8E#TRPBF LTGDH 8-Lead Plastic MSOP –40°C to 125°C

LTC3621IMS8E#PBF LTC3621IMS8E#TRPBF LTGDH 8-Lead Plastic MSOP –40°C to 125°C

LTC3621HMS8E#PBF LTC3621HMS8E#TRPBF LTGDH 8-Lead Plastic MSOP –40°C to 150°C

LTC3621EMS8E-3.3#PBF LTC3621EMS8E-3.3#TRPBF LTGNY 8-Lead Plastic MSOP –40°C to 125°C

LTC3621IMS8E-3.3#PBF LTC3621IMS8E-3.3#TRPBF LTGNY 8-Lead Plastic MSOP –40°C to 125°C

LTC3621HMS8E-3.3#PBF LTC3621HMS8E-3.3#TRPBF LTGNY 8-Lead Plastic MSOP –40°C to 150°C

LTC3621EMS8E-5#PBF LTC3621EMS8E-5#TRPBF LTGNX 8-Lead Plastic MSOP –40°C to 125°C

LTC3621IMS8E-5#PBF LTC3621IMS8E-5#TRPBF LTGNX 8-Lead Plastic MSOP –40°C to 125°C

LTC3621HMS8E-5#PBF LTC3621HMS8E-5#TRPBF LTGNX 8-Lead Plastic MSOP –40°C to 150°C

LTC3621EDCB-2#PBF LTC3621EDCB-2#TRPBF LGHY 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621IDCB-2#PBF LTC3621IDCB-2#TRPBF LGHY 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621EDCB-23.3#PBF LTC3621EDCB-23.3#TRPBF LGQG 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621IDCB-23.3#PBF LTC3621IDCB-23.3#TRPBF LGQG 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621EDCB-25#PBF LTC3621EDCB-25#TRPBF LGQD 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LTC3621IDCB-25#PBF LTC3621IDCB-25#TRPBF LGQD 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

Operating Junction Temperature Range (Notes 3, 6, 7)

LTC3621E, LTC3621I .......................... –40°C to 125°C

LTC3621H .......................................... –40°C to 150°C

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

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

elecTrical characTerisTics

The l denotes the specifications which apply over the specified operating

junction temperature range, otherwise specifications are at TJ = 25°C. (Note 3) VIN = 12V, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Operating Voltage 2.7 17 V

VOUT Operating Voltage 0.6 VIN V

IVIN Input Quiescent Current Shutdown Mode, VRUN = 0V

Burst Mode Operation

Forced Continuous Mode (Note4), VFB

< 0.6V

0.1

3.5

1.5

1.0

µA

VFB Regulated Feedback Voltage LTC3621/LTC3621-2

0.594

0.591

0.6

0.606

0.609

IFB FB Input Current LTC3621/LTC3621-2 10 nA

VOUT Regulated Fixed Output Voltage LTC3621-3.3/LTC3621-23.3

3.267

3.250

3.3

3.333

3.350

LTC3621-5/L

TC3621-25

4.950

4.925

5.0

5.050

5.075

IFB(VOUT) Feedback Input Leakage Current Fixed Output Versions 2 10 µA

ΔVLINE(REG) Reference Voltage Line Regulation VIN = 2.7V to 17V (Note 5) 0.01 0.015 %/V

ΔVLOAD(REG) Output Voltage Load Regulation (Note 5) 0.1 %

ILSW NMOS Switch Leakage

PMOS Switch Leakage

0.1

µA

RDS(ON) NMOS On-Resistance (Bottom FET) VIN = 5V 0.15 Ω

PMOS On-Resistance (Top FET) 0.37 Ω

DMAX Maximum Duty Cycle VFB = 0.5V, VMODE/SYNC = 1.5V l100 %

tON(MIN) Minimum On-Time 60 ns

VRUN RUN Input High Threshold

RUN Input Low Threshold

0.3

1.0 V

IRUN RUN Input Current VRUN = 12V 0 20 nA

orDer inForMaTion

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

LTC3621EMS8E-2#PBF LTC3621EMS8E-2#TRPBF LTGHZ 8-Lead Plastic MSOP –40°C to 125°C

LTC3621IMS8E-2#PBF LTC3621IMS8E-2#TRPBF LTGHZ 8-Lead Plastic MSOP –40°C to 125°C

LTC3621HMS8E-2#PBF LTC3621HMS8E-2#TRPBF LTGHZ 8-Lead Plastic MSOP –40°C to 150°C

LTC3621EMS8E-23.3#PBF LTC3621EMS8E-23.3#TRPBF LTGNZ 8-Lead Plastic MSOP –40°C to 125°C

LTC3621IMS8E-23.3#PBF LTC3621IMS8E-23.3#TRPBF LTGNZ 8-Lead Plastic MSOP –40°C to 125°C

LTC3621HMS8E-23.3#PBF LTC3621HMS8E-23.3#TRPBF LTGNZ 8-Lead Plastic MSOP –40°C to 150°C

LTC3621EMS8E-25#PBF LTC3621EMS8E-25#TRPBF LTGQB 8-Lead Plastic MSOP –40°C to 125°C

LTC3621IMS8E-25#PBF LTC3621IMS8E-25#TRPBF LTGQB 8-Lead Plastic MSOP –40°C to 125°C

LTC3621HMS8E-25#PBF LTC3621HMS8E-25#TRPBF LTGQB 8-Lead Plastic MSOP –40°C to 150°C

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

Consult LTC 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/

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

elecTrical characTerisTics

The l denotes the specifications which apply over the specified operating

junction temperature range, otherwise specifications are at TJ = 25°C. (Note 3) VIN = 12V, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VMODE/SYNC Pulse-Skipping Mode

Burst Mode Operation

Forced Continuous Mode

VINTVCC – 0.4

1.0

0.3

VINTVCC – 1.2

IMODE/SYNC MODE/SYNC Input Current 0 20 nA

tSS Internal Soft-Start Time 0.8 ms

ILIM Peak Current Limit

(E/I-Grade)

(H-Grade)

1.44

1.30

1.2

1.60 1.76

1.80

VUVLO VINTVCC Undervoltage Lockout VIN Ramping Up 2.4 2.6 2.7 V

VUVLO(HYS) VINTVCC Undervoltage Lockout Hysteresis 250 mV

VOVLO VIN Overvoltage Lockout Rising l18 19 20 V

VOVLO(HYS) VIN Overvoltage Lockout Hysteresis 300 mV

fOSC Oscillator Frequency LTC3621/LTC3621-3.3/LTC3621-5

(E/I-Grade)

(H-Grade)

0.92

0.82

0.78

1.00 1.08

1.16

MHz

LTC3621-2/L

TC3621-23.3/LTC3621-25

(E/I-Grade)

(H-Grade)

2.05

1.8

1.7

2.25 2.45

2.6

MHz

fSYNC SYNC Capture Range 60 140 %

VINTVCC VINTVCC LDO Output Voltage VIN > 4V 3.6 V

ΔVPGOOD Power Good Range ±7.5 ±12.5 %

RPGOOD Power Good Resistance PGOOD RDS(ON) at 500µA 275 350 Ω

tPGOOD PGOOD Delay PGOOD Low to High

PGOOD High to Low

Cycles

IPGOOD PGOOD Leakage Current 100 nA

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: Transient absolute maximum voltages should not be applied for

more than 4% of the switching duty cycle.

Note 3: The LTC3621 is tested under pulsed load conditions such that

TJ ≈ TA. The LTC3621E 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

LTC3621I is guaranteed over the –40°C to 125°C operating junction

temperature range, and the LTC3621H is guaranteed over the –40°C to

150°C operating junction temperature range. High junction temperatures

degrade operating lifetimes; operating lifetime is derated for junction

temperatures 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.

Note 4: The quiescent current in forced continuous mode does not include

switching loss of the power FETs.

Note 5: The LTC3621 is tested in a proprietary test mode that connects VFB

to the output of error amplifier.

Note 6: TJ is calculated from the ambient, TA, and power dissipation, PD,

according to the following formula:

TJ = TA + (PD • θJA)

Note 7: 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.

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

Typical perForMance characTerisTics

Burst Mode Operation Pulse-Skipping Mode Operation Load Step

Soft-Start Operation Efficiency vs Input Voltage

Oscillator Frequency

vs Temperature

Efficiency vs Load Current

(Burst Mode Operation)

VIN Supply Current

vs Input Voltage

Efficiency vs Load at Dropout

Operation

TJ = 25°C, unless otherwise noted.

INPUT VOLTAGE (V)

VIN SUPPLY CURRENT (µA)

3621 G02

02468 10 12 14 18 20

SLEEP

3621 G04

5V/DIV

4µs/DIV

VOUT

AC-COUPLED

50mV/DIV

500mA/DIV

VIN = 12V

VOUT = 3.3V

Burst Mode OPERATION

IOUT = 50mA

L = 2.2µH

3621 G07

RUN

5V/DIV

PGOOD

2V/DIV

VOUT

1V/DIV

0.5A/DIV

400µs/DIV

3621 G05

5V/DIV

VOUT

AC-COUPLED

50mV/DIV

500mA/DIV

VIN = 12V

VOUT = 3.3V

PULSE SKIP MODE

IOUT = 10mA

L = 2.2µH

4µs/DIV 3621 G06

ILOAD

500mA/DIV

VOUT

100mV/DIV

500mA/DIV

VIN = 12V

VOUT = 3.3V

ILOAD = 0.05A

40µs/DIV

LOAD CURRENT (A)

EFFICIENCY (%)

3621 G01

100

0.001 0.1

0.01

VIN = 12V

FREQUENCY = 2.25MHz

VOUT = 2.5V

VOUT = 3.3V

VOUT = 5V

LOAD CURRENT (A)

EFFICIENCY (%)

3621 G03

100

0.0001 0.001 0.1

0.01

VIN = 5V

FREQUENCY = 2.25MHz

FORCED

CONTINUOUS

MODE

Burst Mode

OPERATION

TEMPERATURE (°C)

–50

OSCILLATOR FREQUENCY (MHz)

2.30

2.35

2.45

2.50

2.40

25 75 100

3621 G09

2.25

2.20

–25 0 50 125 150

2.15

2.05

2.00

2.10

INPUT VOLTAGE (V)

EFFICIENCY (%)

3621 G08

10 15

VOUT = 2.5V

ILOAD = 10mA

ILOAD = 1A

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

Typical perForMance characTerisTics

RDS(ON) vs Temperature Load Regulation

Line Regulation

Efficiency vs Load at 1MHz

VIN Supply Current

vs Temperature

Switch Leakage

vs Temperature

Oscillator Frequency

vs Supply Voltage

Reference Voltage

vs Temperature

RDS(ON) vs Input Voltage

TJ = 25°C, unless otherwise noted.

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY (MHz)

3621 G10

712 17

2.30

2.35

2.45

2.50

2.40

2.25

2.20

2.15

2.05

2.00

2.10

INPUT VOLTAGE (V)

100

RDS(ON) (mΩ)

300

500

700

200

400

600

4 8 12 16

3621 G12

2020 6 10 14 18

TOP FET

BOTTOM FET

TEMPERATURE (°C)

–50

100

RDS(ON) (mΩ)

150

250

300

350

600

450

050 75 100 125

3621 G13

200

500

550

400

–25 25 150

TOP FET

BOTTOM FET

LOAD CURRENT (mA)

–5

∆V

OUT

(%)

500

3621 G14

1000

1500

–1

–2

–3

–4

VIN = 12V

VOUT = 3.3V

FORCED CONTINUOUS MODE

INPUT VOLTAGE (V)

–0.5

∆V

OUT

ERROR (%)

4321

0.5

3621 G15

8765 11109 12

16151413

0.3

0.1

–0.1

–0.3

TEMPERATURE (°C)

–50

VIN SUPPLY CURRENT (µA)

25 75

3521 G17

–25 0 50 100 125 150

SLEEP

SHUTDOWN

TEMPERATURE (°C)

–100 –50

REFERENCE VOLTAGE

599.5

600.0

600.5

3521 G11

599.0

598.5

050 100

150

598.0

597.5

TEMPERATURE (°C)

–50

–3

SW LEAKAGE (µA)

3621 G18

–25 75 100

25 150125

BOTTOM FET

TOP FET

LOAD CURRENT (A)

EFFICIENCY (%)

3621 G16

100

0.0001 0.001 0.1

0.01

VIN = 12V

VOUT = 2.5V

VOUT = 3.3V

VOUT = 5V

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

pin FuncTions

(DFN/MSOP)

block DiagraM

SW (Pin 1/Pin 1): Switch Node Connection to the Inductor

of the Step-Down Regulator.

VIN (Pin 2/Pin 2): Input Voltage of the Step-Down Regulator.

RUN (Pin 3/Pin 3): Logic Controlled RUN Input. Do not

leave this pin floating. Logic high activates the step-down

regulator.

FB (Pin 4/Pin 5): Feedback Input to the Error Amplifier

of the Step-Down Regulator. Connect a resistor divider

tap to this pin. The output voltage can be adjusted from

0.6V to VIN by:

VOUT = 0.6V • [1 + (R2/R1)]

For Fixed VOUT options, connect the FB pin directly to VOUT.

PGOOD (Pin 4, MSOP Package Only): VOUT within Regu-

lation Indicator.

INTVCC (Pin 5/Pin 6): Low Dropout Regulator. Bypass with

at least 1µF to Ground.

MODE/SYNC (Pin 6/Pin 7): Burst Mode Select and External

Clock Synchronization of the Step-Down Regulator. Tie

MODE/SYNC to INTVCC for Burst Mode operation with a

400mA peak current clamp, tie MODE/SYNC to GND for

pulse skipping operation, and tie MODE/SYNC to a volt-

age between 1V and VINTVCC – 1.2V for forced continuous

mode. Furthermore, connecting MODE/SYNC to an external

clock will sync the system clock to the external clock and

put the part in forced continuous mode.

GND (Exposed Pad Pin 7/Pin 9): Ground Backplane for

Power and Signal Ground. Must be soldered to PCB ground.

SGND (Pin 8, MSOP Package Only): Signal Ground.

–

ERROR

AMPLIFIER BURST

AMPLIFIER

MAIN

I-COMPARATOR

–

OVERCURRENT

COMPARATOR

REVERSE

COMPARATOR

0.6V

ITH

MODE/SYNC

RUN

PGOOD

INTVCC

CLK

VIN – 5V SW

GND

3621 BD

VIN

FIXED VOUT

INTVCC

OSCILLATOR

LDO

MS8E PACKAGE ONLY

BUCK

LOGIC

AND

GATE DRIVE

SLOPE

COMPENSATION

0.8ms

SOFT-START

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

operaTion

The LTC3621 uses a constant-frequency, peak current

mode architecture. It operates through a wide VIN range

and regulates with ultralow quiescent current. The opera-

tion frequency is set at either 2.25MHz or 1MHz and can

be synchronized to an external oscillator ±40% of the

inherent frequency. To suit a variety of applications, the

selectable MODE/SYNC pin allows the user to trade off

output ripple for efficiency.

The output voltage is set by an external divider returned to

the FB pin. An error amplifier compares the divided output

voltage with a reference voltage of 0.6V and adjusts the

peak inductor current accordingly. In the MS8E package,

overvoltage and undervoltage comparators will pull the

PGOOD output low if the output voltage is not within 7.5%

of the programmed value. The PGOOD output will go high

immediately after achieving regulation and will go low 32

clock cycles after falling out of regulation.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET) is turned on at the beginning of a clock cycle.

The inductor current is allowed to ramp up to a peak level.

Once that level is reached, the top power switch is turned

off and the bottom switch (N-channel MOSFET) is turned

on until the next clock cycle. The peak current level is con-

trolled by the internally compensated ITH voltage, which is

the output of the error amplifier. This amplifier compares

the FB voltage to the 0.6V internal reference. When the

load current increases, the FB voltage decreases slightly

below the reference, which causes the error amplifier to

increase the ITH voltage until the average inductor current

matches the new load current.

The main control loop is shut down by pulling the RUN

pin to ground.

Low Current Operation

Two discontinuous-conduction modes (DCMs) are available

to control the operation of the LTC3621 at low currents.

Both modes, Burst Mode operation and pulse-skipping,

automatically switch from continuous operation to the

selected mode when the load current is low.

To optimize efficiency, Burst Mode operation can be selected

by tying the MODE/SYNC pin to INTVCC. In Burst Mode

operation, the peak inductor current is set to be at least

400mA, even if the output of the error amplifier demands

less. Thus, when the switcher is on at relatively light output

loads, FB voltage will rise and cause the ITH voltage to

drop. Once the ITH voltage goes below 0.2V, the switcher

goes into its sleep mode with both power switches off.

The switcher remains in this sleep state until the external

load pulls the output voltage below its regulation point.

During sleep mode, the part draws an ultralow 3.5µA of

quiescent current from VIN.

To minimize VOUT ripple, pulse-skipping mode can be se-

lected by grounding the MODE/SYNC pin. In the LTC3621,

pulse-skipping mode is implemented similarly to Burst

Mode operation with the peak inductor current set to be

at about 66mA. This results in lower output voltage ripple

than in Burst Mode operation with the trade-off being

slightly lower efficiency.

Forced Continuous Mode Operation

Aside from the two discontinuous-conduction modes,

the LTC3621 also has the ability to operate in the forced

continuous mode by setting the MODE/SYNC voltage

between 1V and VINTVCC – 1V. In forced continuous mode,

the switcher will switch cycle by cycle regardless of what

the output load current is. If forced continuous mode is

selected, the minimum peak current is set to be –133mA

in order to ensure that the part can operate continuously

at zero output load.

High Duty Cycle/Dropout Operation

When the input supply voltage decreases towards the output

voltage, the duty cycle increases and slope compensation

is required to maintain the fixed switching frequency. The

LTC3621 has internal circuitry to accurately maintain the

peak current limit (ILIM) of 1.6A even at high duty cycles.

As the duty cycle approaches 100%, the LTC3621 enters

dropout operation. During dropout, if force continuous

mode is selected, the top PMOS switch is turned on

continuously, and all active circuitry is kept alive. How-

ever, if Burst Mode operation or pulse-skipping mode is

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

operaTion

selected, the part will transition in and out of sleep mode

depending on the output load current. This significantly

reduces the quiescent current, thus prolonging the use

of the input supply.

VIN Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTC3621 constantly

monitors the VIN pin for an overvoltage condition. When

VIN rises above 19V, the regulator suspends operation by

shutting off both power MOSFETs. Once VIN drops below

18.7V, the regulator immediately resumes normal opera-

tion. The regulator executes its soft-start function when

exiting an overvoltage condition.

Output Voltage Programming

For non-fixed output voltage parts, the output voltage is

set by external resistive divider according to the following

equation:

VOUT = 0.6V •1+ R2







The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

IRMS ≅IOUT(MAX)

VOUT

VIN

VOUT

–1

This formula has a maximum at VIN = 2VOUT, where:

IRMS ≅

OUT

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. Several capacitors may also be paralleled to meet

size or height requirements in the design. For low input

voltage applications, sufficient bulk input capacitance is

needed to minimize transient effects during output load

changes.

Output Capacitor (COUT) Selection

The selection of COUT is determined by the effective series

resistance (ESR) that is required to minimize voltage ripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

Low Supply Operation

The LTC3621 incorporates an undervoltage lockout circuit

which shuts down the part when the input voltage drops

below 2.7V. As the input voltage rises slightly above the

undervoltage threshold, the switcher will begin its basic

operation. However, the RDS(ON) of the top and bottom

switch will be slightly higher than that specified in the

electrical characteristics due to lack of gate drive. Refer

to graph of RDS(ON) versus VIN for more details.

Soft-Start

The LTC3621 has an internal 800µs soft-start ramp. During

start-up soft-start operation, the switcher will operate in

pulse-skipping mode.

applicaTions inForMaTion

OUT

3621 F01

LTC3621

SGND

Figure 1. Setting the Output Voltage

Input Capacitor (CIN) Selection

The input capacitance, CIN, is needed to filter the square

wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring, a low

ESR input capacitor sized for the maximum RMS current

should be used. The maximum RMS current is given by:

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

applicaTions inForMaTion

the load transient response. The output ripple, ∆VOUT, is

determined by:

VOUT < IL

8•f•COUT

+ESR







∆ ∆

The output ripple is highest at maximum input voltage

since ∆IL increases with input voltage. Multiple capaci-

tors placed in parallel may be needed to meet the ESR

and RMS current handling requirements. Dry tantalum,

special polymer

, aluminum electrolytic, and ceramic

capacitors are all available in surface mount packages.

Special polymer capacitors are very low ESR but have

lower capacitance density than other types. Tantalum

capacitors have the highest capacitance density but it is

important to only use types that have been surge tested

for use in switching power supplies. Aluminum electrolytic

capacitors have significantly higher ESR, but can be used

in cost-sensitive applications provided that consideration

is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteristics

and small footprints.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input

and the power is supplied by a wall adapter through long

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

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

be mistaken as loop instability. At worst, a sudden inrush

of current through the long wires can potentially cause

a voltage spike at VIN large enough to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R and X7R dielectric formulations. These

dielectrics have the best temperature and voltage char-

acteristics of all the ceramics for a given value and size.

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 to support the load

until the feedback loop raises the switch current enough

to support the load. Typically, five cycles are required to

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

output voltage drop linearly. The output droop, VDROOP, is

usually about three times the linear drop of the first cycle.

Thus, a good place to start with the output capacitor value

is approximately:

COUT = 3 ∆IOUT

f•VDROOP

More capacitance may be required 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. A 10μF ceramic capacitor is usually enough

for these conditions. Place this input capacitor as close

to the VIN pin as possible.

Output Power Good

In the MS8E package, when the LTC3621’s output voltage

is within the ±7.5% window of the regulation point, the

output voltage is good and the PGOOD pin is pulled high

with an external resistor. Otherwise, an internal open-drain

pull-down device (275Ω) will pull the PGOOD pin low.

To prevent unwanted PGOOD glitches during transients

or dynamic VOUT changes, the LTC3621’s PGOOD fall-

ing edge includes a blanking delay of approximately 32

switching cycles.

Frequency Sync Capability

The LTC3621 has the capability to sync to a frequency within

a ±40% range of the internal programmed frequency. It

takes 2 to 3 cycles of external clock pulses to engage the

sync mode. If the external clock signal were to stop switch-

ing during operation, it will take roughly 7μs for the part’s

internal sync signal to go low and respond accordingly.

Once engaged in sync, the LTC3621 immediately runs at

the external clock frequency in forced continuous mode.

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

applicaTions inForMaTion

Inductor Selection

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple current:

I∆ L=VOUT

f•L1– VOUT

VIN(MAX)







Lower ripple current reduces power losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). To guarantee that ripple

current does not exceed a specified maximum, the induc-

tance should be chosen according to:

L = VOUT

f•∆IL(MAX)

1– VOUT

VIN(MAX)







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

be selected. Actual core loss is independent of core size

for a fixed inductor value, but is very dependent on the

inductance selected. As the inductance or frequency in-

creases, core losses decrease. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses will increase. Copper losses also increase

as frequency increases.

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 saturation.

Ferrite core material saturates “hard”, which means that

inductance collapses abruptly when the peak design current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

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. New designs

for surface mount inductors are available from Coilcraft,

Toko, Vishay, NEC/Tokin, TDK and Würth Electronik. Refer

to Table 1 for more details.

Checking Transient Response

The regular loop response can be checked by looking at the

load transient response. Switching regulators take several

cycles to respond to a step in load current. When a load step

occurs, VOUT immediately shifts by an amount equal to the

∆ILOAD • ESR, where ESR is the effective series resistance

of COUT. ∆ILOAD also begins to charge or discharge COUT

generating a feedback error signal used by the regulator to

return VOUT to its steady-state value. During this recovery

time, VOUT can be monitored for overshoot or ringing that

would indicate a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine

phase margin. In addition, a feedforward capacitor can

be added to improve the high frequency response, as

shown in Figure 1. Capacitor CFF provides phase lead by

creating a high frequency zero with R2, which improves

the phase margin.

The output voltage settling behavior is related to the sta-

bility of the closed-loop system and will demonstrate the

actual overall supply performance. L

TpowerCAD™ and

LTSpice

can be used to check control loop and transient

performance.

In some applications, a more severe transient can be caused

by switching in loads with large (>1µF) load capacitors.

The discharged load capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT. No regulator

can deliver enough current to prevent this problem if the

switch connecting the load has low resistance and is driven

quickly. The solution is to limit the turn-on speed of the

load switch driver. A Hot Swap™ controller is designed

specifically for this purpose and usually incorporates

current limiting, short-circuit protection and soft-starting.

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

applicaTions inForMaTion

Efficiency Considerations

The percent 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. Percent efficiency can

be expressed as:

% Efficiency = 100% – (Loss1 + Loss2 + …)

where Loss1, Loss2, etc. are the individual losses as a

percentage of input power. Although all dissipative elements

in the circuit produce losses, three main sources usually

account for most of the losses in LTC3621 circuits: 1) I2R

losses, 2) switching and biasing losses, 3) other losses.

1. I2R losses are calculated from the DC resistances of

the internal switches, RSW, and external inductor, RL.

In continuous mode, the average output current flows

through inductor L but is “chopped” between the

internal top and bottom power MOSFETs. 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 Characteristics

curves. Thus to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

Table 1. Inductor Selection Table

INDUCTOR

INDUCTANCE

(µH)

DCR

(mΩ)

MAX CURRENT

(A)

DIMENSIONS

(mm)

HEIGHT

(mm) MANUFACTURER

IHLP-1616BZ-11 Series 1.0

2.2

4.7

4.5

3.25

1.7

4.3 × 4.7

Vishay

www.vishay.com

IHLP-2020BZ-01 Series 1

2.2

3.3

4.7

5.6

6.8

18.9

45.6

79.2

108

113

139

4.2

3.3

2.8

2.5

2.4

5.4 × 5.7

FDV0620 Series 1

2.2

3.3

4.7

5.7

3.2

2.8

6.7 × 7.4

Toko

www.toko.com

MPLC0525L Series 1

1.5

2.2

6.4

5.2

4.1

6.2 × 5.4

2.5

NEC/Tokin

www.nec-tokin.com

XFL4020 Series 1.0

1.5

2.2

3.3

4.7

10.8

14.4

21.3

34.8

52.2

5.1

4.4

3.5

2.5

4 × 4

2.1

Coilcraft

www.coilcraft.com

RLF7030 Series 1

1.5

2.2

3.3

4.7

6.8

8.8

9.6

6.4

6.1

5.4

4.1

3.4

2.8

6.9 × 7.3

3.2

TDK

www.tdk.com

WE-TPC 4828 Series 1.2

1.8

2.2

2.7

3.3

3.9

4.7

3.1

2.7

2.5

2.35

2.15

1.72

1.55

4.8 × 4.8

2.8

Würth Elektronik

www.we-online.com

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

applicaTions inForMaTion

2. The switching current is the sum of the MOSFET driver

and control currents. The power MOSFET driver current

results from switching the gate capacitance of the power

MOSFETs. Each time a power MOSFET 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 a

current out of VIN that is typically much larger than the

DC control bias current. In continuous mode, IGATECHG

= f(QT + QB), where QT and QB are the gate charges of

the internal top and bottom power MOSFETs and f is

the switching frequency. The power loss is thus:

Switching Loss = IGATECHG • VIN

The gate charge loss is proportional to VIN and f and

thus their effects will be more pronounced at higher

supply voltages and higher frequencies.

3. Other “hidden” losses such as transition loss and cop-

per trace and internal load resistances can account for

additional efficiency degradations in the overall power

system. It is very important to include these “system”

level losses in the design of a system. Transition loss

arises from the brief amount of time the top power

MOSFET spends in the saturated region during switch

node transitions. The LTC3621 internal power devices

switch quickly enough that these losses are not sig-

nificant compared to other sources. These losses plus

other losses, including diode conduction losses during

dead-time and inductor core losses, generally account

for less than 2% total additional loss.

Thermal Conditions

In a majority of applications, the LTC3621 does not dis-

sipate much heat due to its high efficiency and low thermal

resistance of its exposed pad package. However, in ap-

plications where the LTC3621 is running at high ambient

temperature, high VIN, high switching frequency, and

maximum output current load, the heat dissipated may

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 160°C,

both power switches will be turned off until the temperature

drops about 15°C cooler.

To avoid the LTC3621 from exceeding the maximum junc-

tion temperature, the user will need to do some thermal

analysis. The goal of the thermal analysis is to determine

whether the power dissipated exceeds the maximum

junction temperature of the part. The temperature rise is

given by:

TRISE = PD • θJA

As an example, consider the case when the LTC3621

is used in applications where VIN = 12V, IOUT = 1A,

f = 2.25MHz, VOUT = 1.8V. The equivalent power MOSFET

resistance RSW is:

RSW =RDS(ON)TOP •VOUT

VIN

+RDS(ON)BOT •1– VOUT

VIN







= 370mΩ •1.8V

12V +150mΩ •1– 1.8V

12V







=183mΩ

The VIN current during 2.25MHz force continuous opera-

tion with no load is about 5mA, which includes switching

and internal biasing current loss, transition loss, inductor

core loss and other losses in the application. Therefore,

the total power dissipated by the part is:

PD = IOUT2 • RSW + VIN • IIN(Q)

= 1A2 • 183mΩ + 12V • 5mA

= 243mW

The DFN 2mm × 3mm package junction-to-ambient thermal

resistance, θJA, is around 64°C/W. Therefore, the junction

temperature of the regulator operating in a 25°C ambient

temperature is approximately:

TJ = 0.243W • 64°C/W + 25°C = 40.6°C

Remembering that the above junction temperature is

obtained from an RDS(ON) at 25°C, we might recalculate

the junction temperature based on a higher RDS(ON) since

it increases with temperature. Redoing the calculation

assuming that RSW increased 5% at 40.6°C yields a new

junction temperature of 41.1°C. If the application calls

for a higher ambient temperature and/or higher switching

frequency, care should be taken to reduce the temperature

rise of the part by using a heat sink or forced air flow.

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

VOUT

COUT

3621 F03

VIN

CIN

GND

Board Layout Considerations

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the LTC3621 (refer to Figure 3). Check the following in

your layout:

5. Keep sensitive components away from the SW pin. The

feedback resistors and INTVCC bypass capacitors should

be routed away from the SW trace and the inductor.

6. A ground plane is preferred.

7. Flood all unused areas on all layers with copper, which

reduces the temperature rise of power components.

These copper areas should be connected to GND.

Design Example

As a design example, consider using the LTC3621 in an

application with the following specifications:

VIN = 10.8V to 13.2V

VOUT = 3.3V

IOUT(MAX) = 1A

IOUT(MIN) = 0A

fSW = 2.25MHz

Because efficiency and quiescent current is important at

both 500mA and 0A current states, Burst Mode operation

will be utilized.

Given the internal oscillator of 2.25MHz, we can calcu-

late the inductor value for about 40% ripple current at

maximum VIN:

L = 3.3V

2.25MHz •0.4A





1– 3.3V

13.2V





= 2.75µH

Given this, a 2.7µH or 3.3µH, >1.2A inductor would suffice.

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

22µF ceramic capacitor will be used.

CIN should be sized for a maximum current rating of:

IRMS =1A 3.3V

13.2V







13.2V

3.3V –1







1/2

= 0.43A

Decoupling the VIN pin with 10µF ceramic capacitors is

adequate for most applications.

applicaTions inForMaTion

1. Do the capacitors CIN connect to the VIN pin and GND

pin as close as possible? These capacitors provide the

AC current to the internal power MOSFETs and their

drivers.

2. Are COUT and L closely connected? The (–) plate of

COUT returns current to GND.

3. The resistive divider, R1 and R2, must be connected

between the (+) plate of COUT and a ground line ter-

minated near GND. The feedback signal VFB should be

routed away from noisy components and traces, such

as the SW line, and its trace should be minimized. Keep

R1 and R2 close to the IC.

4. Solder the exposed pad (Pin 7 for DFN, Pin 9 for MSOP)

on the bottom of the package to the GND plane. Connect

this GND plane to other layers with thermal vias to help

dissipate heat from the LTC3621.

Figure 3. Sample PCB Layout

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

3.00 ±0.10

(2 SIDES)

2.00 ±0.10

(2 SIDES)

NOTE:

1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (TBD)

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

0.40 ±0.10

BOTTOM VIEW—EXPOSED PAD

1.65 ±0.10

(2 SIDES)

0.75 ±0.05

R = 0.115

TYP

R = 0.05

TYP

1.35 ±0.10

(2 SIDES)

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

0.200 REF

0.00 – 0.05

(DCB6) DFN 0405

0.25 ±0.05

0.50 BSC

PIN 1 NOTCH

R0.20 OR 0.25

45° CHAMFER

0.25 ±0.05

1.35 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

1.65 ±0.05

(2 SIDES)

2.15 ±0.05

0.70 ±0.05

3.55 ±0.05

PACKAGE

OUTLINE

0.50 BSC

DCB Package

6-Lead Plastic DFN (2mm × 3mm)

(Reference LTC DWG # 05-08-1715 Rev A)

package DescripTion

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

package DescripTion

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

MSOP (MS8E) 0213 REV K

0.53 ±0.152

(.021 ±.006)

SEATING

PLANE

NOTE:

1. DIMENSIONS IN MILLIMETER/(INCH)

2. DRAWING NOT TO SCALE

3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.

MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.

INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE

5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX

6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD

SHALL NOT EXCEED 0.254mm (.010") PER SIDE.

0.18

(.007)

0.254

(.010)

1.10

(.043)

MAX

0.22 – 0.38

(.009 – .015)

TYP

0.86

(.034)

REF

0.65

(.0256)

BSC

0° – 6° TYP

DETAIL “A”

GAUGE PLANE

1 2 34

4.90 ±0.152

(.193 ±.006)

BOTTOM VIEW OF

EXPOSED PAD OPTION

765

3.00 ±0.102

(.118 ±.004)

(NOTE 3)

3.00 ±0.102

(.118 ±.004)

(NOTE 4)

0.52

(.0205)

REF

1.68

(.066)

1.88

(.074)

5.10

(.201)

MIN

3.20 – 3.45

(.126 – .136)

1.68 ±0.102

(.066 ±.004)

1.88 ±0.102

(.074 ±.004) 0.889 ±0.127

(.035 ±.005)

RECOMMENDED SOLDER PAD LAYOUT

0.65

(.0256)

BSC

0.42 ±0.038

(.0165

±.0015)

TYP

0.1016 ±0.0508

(.004 ±.002)

DETAIL “B”

CORNER TAIL IS PART OF

THE LEADFRAME FEATURE.

FOR REFERENCE ONLY

NO MEASUREMENT PURPOSE

0.05 REF

0.29

REF

MS8E Package

8-Lead Plastic MSOP, Exposed Die Pad

(Reference LTC DWG # 05-08-1662 Rev K)

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

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 08/13 Updated Efficiency curve

Input quiescent current limits changed

Oscillator frequency (fOSC) conditions changed

B 03/14 Clarified Features and Description

Clarified options

Clarified ordering info and Absolute Maximum Ratings

Added Note 7

Clarified electrical specifications

Clarified pin descriptions, Block Diagram

Clarified Operation description

Added box to figure

Clarified Applications Information

Clarified Typical Application

Swapped locations of CFB and R1

2 - 3

9 - 13

C 04/15 Added H-Grade Options and Specifications

Added H-Grade Options and Specifications

Clarified Graphs to Accommodate 150°C Performance

2, 3

5, 6

LTC3621/LTC3621-2

3621fc

For more information www.linear.com/LTC3621

 LINEAR TECHNOLOGY CORPORATION 2013

LT 0415 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/LTC3621

relaTeD parTs

Typical applicaTion

PART NUMBER DESCRIPTION COMMENTS

LTC3646/

LTC3646-1

40V, 1A (IOUT), 3MHz Synchronous Step-Down

DC/DC Converter

95% Efficiency, VIN: 4V to 40V, VOUT(MIN) = 0.6V, IQ = 140µA, ISD < 8µA,

3mm × 4mm DFN-14, MSOP-16E Packages

LTC3600 1.5A, 15V

, 4MHz Synchronous Rail-to-Rail Single

Resistor Step-Down Regulator

95% Efficiency, VIN: 4V to 15V, VOUT(MIN) = 0V, IQ = 700µA, ISD < 1µA,

3mm × 3mm DFN-12, MSOP-12E Packages

LTC3601 15V

, 1.5A (IOUT) 4MHz Synchronous Step-Down

DC/DC Converter

95% Efficiency, VIN: 4.5V to 15V, VOUT(MIN) = 0.6V, IQ = 300µA, ISD < 1µA,

4mm × 4mm QFN-20, MSOP-16E Packages

LTC3603 15V

, 2.5A (IOUT) 3MHz Synchronous Step-Down

DC/DC Converter

95% Efficiency, VIN: 4.5V to 15V, VOUT(MIN) = 0.6V, IQ = 75µA, ISD < 1µA,

4mm × 4mm QFN-20, MSOP-16E Packages

LTC3633/

TC3633A

15V/20V, Dual 3A (IOUT) 4MHz Synchronous Step-Down

DC/DC Converter

95% Efficiency, VIN: 3.6V to 15V/20V, VOUT(MIN) = 0.6V, IQ = 500µA, ISD < 15µA,

4mm × 5mm QFN-28, TSSOP-28E Packages. A Version Up to 20VIN

LTC3605/

LTC3605A

15V/20V, 5A (IOUT) 4MHz Synchronous Step-Down

DC/DC Converter

95% Efficiency, VIN: 4V to 15V/20V, VOUT(MIN) = 0.6V, IQ = 2mA, ISD < 15µA,

4mm × 4mm QFN-24 Package. A Version Up to 20VIN

LTC3604 15V, 2.5A (IOUT) 4MHz Synchronous Step-Down

DC/DC Converter

95% Efficiency, VIN: 3.6V to 15V, VOUT(MIN) = 0.6V, IQ = 300µA, ISD < 14µA,

3mm × 3mm QFN-16, MSOP-16E Packages

LTC1877 600mA (IOUT) 550kHz Synchronous Step-Down

DC/DC Converter

VIN: 2.7V to 10V, VOUT(MIN) = 0.8V, IO = 10µA, ISD < 1µA, MSOP-8 Package

LT8610/LT8611 42V, 2.5A (IOUT) Synchronous Step-Down

DC/DC Converter

96% Efficiency, VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA,

MSOP-16E Package

187k

CFB

22pF

1µF

COUT

22µF

3621 TA02

3.3µH VOUT

25.5k

VIN

RUN

VIN

12V

LTC3621-2

GND

MODE/SYNC

INTVCC

CIN

10µF

3621 TA02

5VOUT with 400mA Burst Mode Operation, 2.25MHz

1.2VOUT, Forced Continuous Mode, 1MHz

1.2VOUT, Synchronized to 600kHz, Forced Continuous Mode

CFB

22pF

1µF

COUT

22µF

1.2V2.7V TO 17V

3621 TA03

3.3µH

604k

VIN

RUN

LTC3621

GND

MODE/SYNC

INTVCC

CIN

10µF

604k

VOUT

VIN

CFB

22pF

1µF

COUT

22µF

1.2V2.7V TO 17V

3621 TA04

4.7µH

604k

600kHz CLK

VIN

RUN

LTC3621

GND

MODE/SYNC

INTVCC

CIN

10µF

604k

OUT