LT1934ES6#PBF - LINEAR TECHNOLOGY

LT1934/LT1934-1

1934fe

TYPICAL APPLICATION

DESCRIPTION

Micropower Step-Down

Switching Regulators

in ThinSOT and DFN

The LT®1934 is a micropower step-down DC/DC converter

with internal 400mA power switch, packaged in a low

proﬁ le (1mm) ThinSOT. With its wide input range of 3.2V

to 34V, the LT1934 can regulate a wide variety of power

sources, from 4-cell alkaline batteries and 5V logic rails

to unregulated wall transformers and lead-acid batteries.

Quiescent current is just 12μA and a zero current shut-

down mode disconnects the load from the input source,

simplifying power management in battery-powered sys-

tems. Burst Mode® operation and the low drop internal

power switch result in high efﬁ ciency over a broad range

of load current.

The LT1934 provides up to 300mA of output current. The

LT1934-1 has a lower current limit, allowing optimum

choice of external components when the required output

current is less than 60mA. Fast current limiting protects

the LT1934 and external components against shorted

outputs, even at 34V input.

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

Burst Mode is a registered trademark of Linear Technology Corporation.

ThinSOT is a trademark of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

FEATURES

APPLICATIONS

n Wide Input Voltage Range: 3.2V to 34V

n Micropower Operation: IQ = 12μA

n 5V at 250mA from 6.5V to 34V Input (LT1934)

n 5V at 60mA from 6.5V to 34V Input (LT1934-1)

n 3.3V at 250mA from 4.5V to 34V Input (LT1934)

n 3.3V at 60mA from 4.5V to 34V Input (LT1934-1)

n Low Shutdown Current: <1μA

n Low VCESAT Switch: 200mV at 300mA

n Low Proﬁ le (1mm) SOT-23 (ThinSOT™) and

(2mm × 3mm × 0.8mm) 6-Pin DFN Package

n Wall Transformer Regulation

n Automotive Battery Regulation

n Standby Power for Portable Products

n Distributed Supply Regulation

n Industrial Control Supplies

3.3V Step-Down Converter Efﬁ ciency

BOOST

VIN

LT1934

SHDN

1934 TA01

2.2μF

0.22μF

10pF

C1: SANYO 4TPB100M

C2: TAIYO YUDEN GMK325BJ225MN

D1: ON SEMICONDUCTOR MBR0540

D2: CENTRAL CMDSH-3

L1: SUMIDA CDRH4D28-470

VOUT

3.3V

250mA

604k

47μH

VIN

4.5V TO 34V

ON OFF

GND

100μF

LOAD CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100

1934 TA02

LT1934

VIN = 12V

VOUT = 5V

VOUT = 3.3V

LT1934/LT1934-1

1934fe

ABSOLUTE MAXIMUM RATINGS

Input Voltage (VIN) ................................................... 34V

BOOST Pin Voltage ................................................. 40V

BOOST Pin Above SW Pin ........................................ 20V

SHDN Pin ................................................................. 34V

FB Voltage .................................................................. 6V

SW Voltage ............................................................... VIN

(Note 1)

BOOST 1

GND 2

FB 3

6 SW

5 VIN

4SHD

TOP VIEW

S6 PACKAGE

6-LEAD PLASTIC TSOT-23

TJMAX = 125°C, θJA = 250°C/ W, θJC = 102°C/W

TOP VIEW

GND

SHD

BOOST

VIN

DCB PACKAGE

6-LEAD (2mm s 3mm) PLASTIC DFN

θJA = 73.5°C/ W, θJC = 12°C/W

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

PIN CONFIGURATION

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL S6 PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT1934ES6#PBF LT1934ES6#TRPBF LTXP 6-Lead Plastic TSOT-23 –40°C to 85°C

LT1934ES6-1#PBF LT1934ES6-1#TRPBF LTF8 6-Lead Plastic TSOT-23 –40°C to 85°C

LT1934IS6#PBF LT1934IS6#TRPBF LTAJB 6-Lead Plastic TSOT-23 –40°C to 125°C

LT1934IS6-1#PBF LT1934IS6-1#TRPBF LTAJC 6-Lead Plastic TSOT-23 –40°C to 125°C

LT1934IDCB#PBF LT1934IDCB#TRPBF LCFZ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LT1934EDCB#PBF LT1934EDCB#TRPBF LCFZ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C

LT1934IDCB-1#PBF LT1934IDCB-1#TRPBF LDHC 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LT1934EDCB-1#PBF LT1934EDCB-1#TRPBF LDHC 6-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C

LEAD BASED FINISH TAPE AND REEL S6 PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT1934ES6 LT1934ES6#TR LTXP 6-Lead Plastic TSOT-23 –40°C to 85°C

LT1934ES6-1 LT1934ES6-1#TR LTF8 6-Lead Plastic TSOT-23 –40°C to 85°C

LT1934IS6 LT1934IS6#TR LTAJB 6-Lead Plastic TSOT-23 –40°C to 125°C

LT1934IS6-1 LT1934IS6-1#TR LTAJC 6-Lead Plastic TSOT-23 –40°C to 125°C

LT1934IDCB LT1934IDCB#TR LCFZ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LT1934EDCB LT1934EDCB#TR LCFZ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C

LT1934IDCB-1 LT1934IDCB-1#TR LDHC 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

LT1934EDCB-1 LT1934EDCB-1#TR LDHC 6-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C

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

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

This product is only offered in trays. For more information go to: http://www.linear.com/packaging/

Operating Temperature Range (Note 2)

LT1934E/LT1934E-1 .............................– 40°C to 85°C

LT1934I/LT1934I-1 .............................–40°C to 125°C

Maximum Junction Temperature .......................... 125°C

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

Lead Temperature (Soldering, 10 sec)

TSOT-23 ............................................................ 300°C

LT1934/LT1934-1

1934fe

ELECTRICAL CHARACTERISTICS

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 LT1934E and LT1934E-1 are guaranteed to meet performance

speciﬁ cations from 0°C to 85°C. Speciﬁ cations over the –40°C to 85°C

SYMBOL CONDITIONS MIN TYP MAX UNITS

Undervoltage Lockout

–40°C ≤ TA ≤ 85°C

–40°C ≤ TA ≤ 125°C

3.2

3.6

Quiescent Current VFB = 1.3V

–40°C ≤ TA ≤ 85°C

–40°C ≤ TA ≤ 125°C

μA

VSHDN = 0V 0.01 2 μA

FB Comparator Trip Voltage VFB Falling –40°C ≤ TA ≤ 85°C

–40°C ≤ TA ≤ 125°C

1.22

1.21

1.25

1.27

FB Comparator Hysteresis 10 mV

FB Pin Bias Current VFB = 1.25V –40°C ≤ TA ≤ 85°C

–40°C ≤ TA ≤ 125°C

±15

±60

FB Voltage Line Regulation 4V < VIN < 34V 0.007 %/V

Switch Off Time VFB > 1V

VFB = 0V

1.4 1.8

2.3 μs

μs

Maximum Duty Cycle VFB = 1V –40°C ≤ TA ≤ 85°C

–40°C ≤ TA ≤ 125°C

Switch VCESAT ISW = 300mA (LT1934, S6 Package)

ISW = 300mA (LT1934, DCB Package)

ISW = 75mA (LT1934-1, S6 Package)

ISW = 75mA (LT1934-1, DCB Package)

200

225

300

120

Switch Current Limit LT1934

LT1934-1

350

400

120

490

160

BOOST Pin Current ISW = 300mA (LT1934)

ISW = 75mA (LT1934-1)

8.5

6.0

Minimum Boost Voltage (Note 3) ISW = 300mA (LT1934)

ISW = 75mA (LT1934-1)

1.8

1.7

2.5

Switch Leakage Current 2μA

SHDN Pin Current VSHDN = 2.3V

VSHDN = 34V

0.5

1.5 5

μA

SHDN Input Voltage High 2.3 V

SHDN Input Voltage Low 0.25 V

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. VIN = 10V, VBOOST = 15V, unless otherwise noted.

operating temperature range are assured by design, characterization and

correlation with statistical process controls. The LT1934I and LT1934I-1

speciﬁ cations are guaranteed over the –40°C to 125°C temperature range.

Note 3: This is the minimum voltage across the boost capacitor needed to

guarantee full saturation of the internal power switch.

LT1934/LT1934-1

1934fe

TYPICAL PERFORMANCE CHARACTERISTICS

LT1934-1 Efﬁ ciency, VOUT = 3.3V Current Limit vs Temperature Off Time vs Temperature

Frequency Foldback VFB vs Temperature

SHDN Bias Current

vs SHDN Voltage

LT1934 Efﬁ ciency, VOUT = 5V LT1934 Efﬁ ciency, VOUT = 3.3V LT1934-1 Efﬁ ciency, VOUT = 5V

LOAD CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100

1934 G01

LT1934

VOUT = 5V

L = 47μH

TA = 25°C VIN = 12V

VIN = 24V

LOAD CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100

1934 G02

LT1934

VOUT = 3.3V

L = 47μH

TA = 25°C

VIN = 24V

VIN = 5V

VIN = 12V

LOAD CURRENT (mA)

0.1

EFFICIENCY (%)

100

1 10 100

1934 G03

LT1934-1

VOUT = 5V

L = 150μH

TA = 25°C

VIN = 24V

VIN = 12V

LOAD CURRENT (mA)

0.1

EFFICIENCY (%)

100

1 10 100

1934 G04

LT1934-1

VOUT = 3.3V

L = 100μH

TA = 25°C

VIN = 24V

VIN = 12V

TEMPERATURE (°C)

–50 –25

SWITCH CURRENT LIMIT (mA)

200

500

050 75

1934 G05

100

400

300

25 100 125

LT1934

LT1934-1

TEMPERATURE (°C)

–50

OFF TIME (μs)

2.0

2.5

3.0

25 75

1934 G06

1.5

1.0

–25 0 50 100 125

0.5

FEEDBACK PIN VOLTAGE (V)

SWITCH OFF TIME (μs)

0.6 1.0

1934 G07

00.2 0.4 0.8

1.2

TA = 25oC

TEMPERATURE (°C)

–50 –25

1.22

FEEDBACK VOLTAGE (V)

1.24

1.27

050 75

1934 G08

1.23

1.26

1.25

25 100 125

SHDN PIN VOLTAGE (V)

SHDN PIN CURRENT (μA)

0.5

1.0

1.5

2.0

2468

1934 G09

10 12

TA = 25°C

LT1934/LT1934-1

1934fe

TYPICAL PERFORMANCE CHARACTERISTICS

Minimum Input Voltage

VOUT = 5V

Quiescent Current

vs Temperature

Undervoltage Lockout

vs Temperature

Minimum Input Voltage

VOUT = 3.3V

TEMPERATURE (°C)

–50

QUIESCENT CURRENT (μA)

–25 0 25 50

1934 G10

75 100 125

TEMPERATURE (°C)

–50

2.0

UVLO (V)

2.5

3.0

3.5

4.0

–25 0 25 50

1934 G11

75 100 125

LOAD CURRENT (mA)

3.5

INPUT VOLTAGE (V)

4.0

4.5

5.0

5.5

6.0

0.1 10 100

1934 G12

3.0

LT1934

VOUT = 3.3V

TA = 25°C

BOOST DIODE TIED TO OUTPUT

VIN TO START

VIN TO RUN

LOAD CURRENT (mA)

INPUT VOLTAGE (V)

0.1 10 100

1934 G13

LT1934

VOUT = 5V

TA = 25°C

BOOST DIODE TIED TO OUTPUT

VIN TO START

VIN TO RUN

PIN FUNCTIONS

BOOST (Pin 1/Pin 1): The BOOST pin is used to provide a

drive voltage, higher than the input voltage, to the internal

bipolar NPN power switch.

GND (Pin 2/Pin 5): Tie the GND pin to a local ground plane

below the LT1934 and the circuit components. Return the

feedback divider to this pin.

FB (Pin 3/Pin 6): The LT1934 regulates its feedback pin

to 1.25V. Connect the feedback resistor divider tap to this

pin. Set the output voltage according to VOUT = 1.25V

(1 + R1/R2) or R1 = R2 (VOUT/1.25 – 1).

SHDN (Pin 4/Pin 4): The SHDN pin is used to put the LT1934

in shutdown mode. Tie to ground to shut down the LT1934.

Apply 2.3V or more for normal operation. If the shutdown

feature is not used, tie this pin to the VIN pin.

VIN (Pin 5/Pin 3): The VIN pin supplies current to the

LT1934’s internal regulator and to the internal power

switch. This pin must be locally bypassed.

SW (Pin 6/Pin 2): The SW pin is the output of the internal

power switch. Connect this pin to the inductor, catch diode

and boost capacitor.

Exposed Pad (Pin 7, DFN Package): This pin must be

soldered to ground plane.

(TSOT-23/DFN)

LT1934/LT1934-1

1934fe

BLOCK DIAGRAM

–

12μs DELAY

ON TIME

OFF TIME

1.8μs DELAY

BOOST

R2 R1

1934 BD

VOUT

VIN

C2 +

VIN

ON OFF

GND

ENABLE

FEEDBACK

COMPARATOR

SHDN VREF 1.25V

LT1934/LT1934-1

1934fe

OPERATION

The LT1934 uses Burst Mode control, combining both low

quiescent current operation and high switching frequency,

which result in high efﬁ ciency across a wide range of load

currents and a small total circuit size.

A comparator monitors the voltage at the FB pin of the

LT1934. If this voltage is higher than the internal 1.25V

reference, the comparator disables the oscillator and power

switch. In this state, only the comparator, reference and

undervoltage lockout circuits are active, and the current into

the VIN pin is just 12μA. As the load current discharges the

output capacitor, the voltage at the FB pin falls below 1.25V

and the comparator enables the oscillator. The LT1934

begins to switch, delivering current to the output capaci-

tor. The output voltage rises, and when it overcomes the

feedback comparator’s hysteresis, the oscillator is disabled

and the LT1934 returns to its micropower state.

The oscillator consists of two one-shots and a ﬂ ip-ﬂ op.

A rising edge from the off-time one-shot sets the ﬂ ip-ﬂ op,

which turns on the internal NPN power switch. The switch

remains on until either the on-time one-shot trips or the

current limit is reached. A sense resistor and ampliﬁ er

monitor the current through the switch and resets the

(Refer to Block Diagram)

ﬂ ip-ﬂ op when this current reaches 400mA (120mA for

the LT1934-1). After the 1.8μs delay of the off-time one-

shot, the cycle repeats. Generally, the LT1934 will reach

current limit on every cycle—the off time is ﬁ xed and

the on time is regulated so that the LT1934 operates at

the correct duty cycle. The 1.8μs off time is lengthened

when the FB pin voltage falls below 0.8V; this foldback

behavior helps control the output current during start-up

and overload. Figure 1 shows several waveforms of an

LT1934 producing 3.3V from a 10V input. When the switch

is on, the SW pin voltage is at 10V. When the switch is

off, the inductor current pulls the SW pin down until it is

clamped near ground by the external catch diode.

The switch driver operates from either the input or from

the BOOST pin. An external capacitor and diode are used

to generate a voltage at the BOOST pin that is higher than

the input supply. This allows the driver to fully saturate

the bipolar switch for efﬁ cient operation.

If the SHDN pin is grounded, all internal circuits are turned

off and VIN current reduces to the device leakage current,

typically a few nA.

VOUT

50mV/DIV

VSW

10V/DIV

Figure 1. Operating Waveforms of the LT1934 Converting

10V to 3.3V at 180mA (Front Page Schematic)

1934 F01a

ISW

0.5A/DIV

ILI

0.5A/DIV

5μs/DIV

LT1934/LT1934-1

1934fe

Which One to Use: LT1934 or LT1934-1?

The only difference between the LT1934 and LT1934-1

is the peak current through the internal switch and the

inductor. If your maximum load current is less than 60mA,

use the LT1934-1. If your maximum load is higher, use

the LT1934; it can supply up to ~300mA.

While the LT1934-1 can’t deliver as much output current,

it has other advantages. The lower peak switch current

allows the use of smaller components (input capacitor,

inductor and output capacitor). The ripple current at the

input of the LT1934-1 circuit will be smaller and may be

an important consideration if the input supply is current

limited or has high impedance. The LT1934-1’s current

draw during faults (output overload or short) and start-

up is lower.

The maximum load current that the LT1934 or LT1934-1

can deliver depends on the value of the inductor used.

Table 1 lists inductor value, minimum output capacitor

and maximum load for 3.3V and 5V circuits. Increasing

the value of the capacitor will lower the output voltage

ripple. Component selection is covered in more detail in

the following sections.

Minimum Input Voltage

The minimum input voltage required to generate a par-

ticular output voltage is determined by either the LT1934’s

undervoltage lockout of ~3V or by its maximum duty cycle.

APPLICATIONS INFORMATION

The duty cycle is the fraction of time that the internal

switch is on and is determined by the input and output

voltages:

DC = (VOUT + VD)/(VIN – VSW + VD)

where VD is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

(~0.3V at maximum load for the LT1934, ~0.1V for the

LT1934-1). This leads to a minimum input voltage of:

IN(MIN) = (VOUT + VD)/DCMAX – VD + VSW

with DCMAX = 0.85.

Inductor Selection

A good ﬁ rst choice for the inductor value is:

L = 2.5 • (VOUT + VD) • 1.8μs/ILIM

where ILIM is the switch current limit (400mA for the

LT1934 and 120mA for the LT1934-1). This choice provides

a worst-case maximum load current of 250mA (60mA for

the LT1934-1). The inductor’s RMS current rating must

be greater than the load current and its saturation current

should be greater than ILIM. To keep efﬁ ciency high, the

series resistance (DCR) should be less than 0.3Ω (1Ω

for the LT1934-1). Table 2 lists several vendors and types

that are suitable.

This simple rule may not provide the optimum value for

your application. If the load current is less, then you can

relax the value of the inductor and operate with higher

ripple current. This allows you to use a physically smaller

inductor, or one with a lower DCR resulting in higher

efﬁ ciency. The following provides more details to guide

inductor selection. First, the value must be chosen so that

the LT1934 can supply the maximum load current drawn

from the output. Second, the inductor must be rated ap-

propriately so that the LT1934 will function reliably and

the inductor itself will not be overly stressed.

Detailed Inductor Selection and

Maximum Load Current

The square wave that the LT1934 produces at its switch

pin results in a triangle wave of current in the inductor. The

LT1934 limits the peak inductor current to ILIM. Because

Table 1

PART VOUT L

MINIMUM

COUT

MAXIMUM

LOAD

LT1934 3.3V 100μH

47μH

33μH

100μH

47μH

33μH

300mA

250mA

200mA

5V 150μH

68μH

47μH

33μH

22μH

300mA

250mA

200mA

LT1934-1 3.3V 150μH

100μH

68μH

15μH

10μH

60mA

45mA

20mA

5V 220μH

150μH

100μH

10μH

4.7μH

60mA

45mA

20mA

LT1934/LT1934-1

1934fe

the average inductor current equals the load current, the

maximum load current is:

OUT(MAX) = IPK – ΔIL/2

where IPK is the peak inductor current and ΔIL is the

peak-to-peak ripple current in the inductor. The ripple

current is determined by the off time, tOFF = 1.8μs, and

the inductor value:

ΔIL = (VOUT + VD) • tOFF/L

IPK is nominally equal to ILIM. However, there is a slight

delay in the control circuitry that results in a higher peak

current and a more accurate value is:

PK = ILIM + 150ns • (VIN – VOUT)/L

These expressions are combined to give the maximum

load current that the LT1934 will deliver:

IOUT(MAX) = 350mA + 150ns • (VIN – VOUT)/L – 1.8μs

• (VOUT + VD)/2L (LT1934)

IOUT(MAX) = 90mA + 150ns • (VIN – VOUT)/L – 1.8μs

• (VOUT + VD)/2L (LT1934-1)

The minimum current limit is used here to be conservative.

The third term is generally larger than the second term,

so that increasing the inductor value results in a higher

output current. This equation can be used to evaluate

a chosen inductor or it can be used to choose L for a

given maximum load current. The simple, single equation

rule given above for choosing L was found by setting

ΔIL = ILIM/2.5. This results in IOUT(MAX) ~0.8ILIM (ignoring

the delay term). Note that this analysis assumes that the

inductor current is continuous, which is true if the ripple

current is less than the peak current or ΔIL < IPK.

The inductor must carry the peak current without satu-

rating excessively. When an inductor carries too much

current, its core material can no longer generate ad-

ditional magnetic ﬂ ux (it saturates) and the inductance

drops, sometimes very rapidly with increasing current.

This condition allows the inductor current to increase

at a very high rate, leading to high ripple current and

decreased overload protection.

Inductor vendors provide current ratings for power induc-

tors. These are based on either the saturation current or

on the RMS current that the inductor can carry without

dissipating too much power. In some cases it is not clear

which of these two determine the current rating. Some data

sheets are more thorough and show two current ratings,

one for saturation and one for dissipation. For LT1934 ap-

plications, the RMS current rating should be higher than

the load current, while the saturation current should be

higher than the peak inductor current calculated above.

Input Capacitor

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT1934 and to force this switching current into

a tight local loop, minimizing EMI. The input capacitor

must have low impedance at the switching frequency to

do this effectively. A 2.2μF ceramic capacitor (1μF for the

LT1934-1) satisﬁ es these requirements.

If the input source impedance is high, a larger value ca-

pacitor may be required to keep input ripple low. In this

case, an electrolytic of 10μF or more in parallel with a 1μF

ceramic is a good combination. Be aware that the input

Table 2. Inductor Vendors

VENDOR PHONE URL PART SERIES COMMENTS

Murata (404) 426-1300 www.murata.com LQH3C Small, Low Cost, 2mm Height

Sumida (847) 956-0666 www.sumida.com CR43

CDRH4D28

CDRH5D28

Coilcraft (847) 639-6400 www.coilcraft.com DO1607C

DO1608C

DT1608C

Würth

Electronics

(866) 362-6673 www.we-online.com WE-PD1, 2, 3, 4

APPLICATIONS INFORMATION

LT1934/LT1934-1

1934fe

capacitor is subject to large surge currents if the LT1934

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be speciﬁ ed for such use.

Output Capacitor and Output Ripple

The output capacitor ﬁ lters the inductor’s ripple current and

stores energy to satisfy the load current when the LT1934

is quiescent. In order to keep output voltage ripple low, the

impedance of the capacitor must be low at the LT1934’s

switching frequency. The capacitor’s equivalent series

resistance (ESR) determines this impedance. Choose one

with low ESR intended for use in switching regulators. The

contribution to ripple voltage due to the ESR is approxi-

mately ILIM • ESR. ESR should be less than ~150mΩ for

the LT1934 and less than ~500mΩ for the LT1934-1.

The value of the output capacitor must be large enough

to accept the energy stored in the inductor without a large

change in output voltage. Setting this voltage step equal to

1% of the output voltage, the output capacitor must be:

OUT > 50 • L • (ILIM/VOUT)2

For example, an LT1934 producing 3.3V with L = 47μH

requires 33μF. This value can be relaxed if small circuit

size is more important than low output ripple.

Sanyo’s POSCAP series in B-case and C-case sizes

provides very good performance in a small package for

the LT1934. Similar performance in traditional tantalum

capacitors requires a larger package (C- or D-case).

APPLICATIONS INFORMATION

The LT1934-1, with its lower switch current, can use a

B-case tantalum capacitor.

With a high quality capacitor ﬁ ltering the ripple current

from the inductor, the output voltage ripple is determined

by the hysteresis and delay in the LT1934’s feedback

comparator. This ripple can be reduced further by adding

a small (typically 10pF) phase lead capacitor between the

output and the feedback pin.

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT1934.

Not all ceramic capacitors are suitable. X5R and X7R

types are stable over temperature and applied voltage

and give dependable service. Other types (Y5V and Z5U)

have very large temperature and voltage coefﬁ cients of

capacitance. In the application circuit they may have only

a small fraction of their nominal capacitance and voltage

ripple may be much larger than expected.

Ceramic capacitors are piezoelectric. The LT1934’s switch-

ing frequency depends on the load current, and at light

loads the LT1934 can excite the ceramic capacitor at audio

frequencies, generating audible noise. If this is unaccept-

able, use a high performance electrolytic capacitor at the

output. The input capacitor can be a parallel combination

of a 2.2μF ceramic capacitor and a low cost electrolytic

capacitor. The level of noise produced by the LT1934-1

Table 3. Capacitor Vendors

VENDOR PHONE URL PART SERIES COMMENTS

Panasonic (714) 373-7366 www.panasonic.com Ceramic,

Polymer,

Tantalum

EEF Series

Kemet (864) 963-6300 www.kemet.com Ceramic,

Tantalum T494, T495

Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,

Polymer,

Tantalum

POSCAP

Murata (404) 436-1300 www.murata.com Ceramic,

AVX www.avxcorp.com Ceramic,

Tantalum TPS Series

Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic

LT1934/LT1934-1

1934fe

APPLICATIONS INFORMATION

when used with ceramic capacitors will be lower and may

be acceptable.

A ﬁ nal precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT1934. A ceramic

input capacitor combined with trace or cable inductance

forms a high quality (under damped) tank circuit. If the

LT1934 circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT1934’s rating. This situation is easily avoided; see

the Hot Plugging Safely section.

Catch Diode

A 0.5A Schottky diode is recommended for the catch

diode, D1. The diode must have a reverse voltage rating

equal to or greater than the maximum input voltage. The

ON Semiconductor MBR0540 is a good choice; it is rated

for 0.5A forward current and a maximum reverse voltage

of 40V.

Schottky diodes with lower reverse voltage ratings usu-

ally have a lower forward drop and may result in higher

efﬁ ciency with moderate to high load currents. However,

these diodes also have higher leakage currents. This leakage

current mimics a load current at the output and can raise

the quiescent current of the LT1934 circuit, especially at

elevated temperatures.

BOOST Pin Considerations

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.1μF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 2 shows two

ways to arrange the boost circuit. The BOOST pin must

be more than 2.5V above the SW pin for best efﬁ ciency.

For outputs of 3.3V and above, the standard circuit (Fig-

ure 2a) is best. For outputs between 2.8V and 3V, use a

0.22μF capacitor and a small Schottky diode (such as the

BAT-54). For lower output voltages the boost diode can be

tied to the input (Figure 2b). The circuit in Figure 2a is more

efﬁ cient because the BOOST pin current comes from a lower

voltage source. You must also be sure that the maximum

voltage rating of the BOOST pin is not exceeded.

The minimum operating voltage of an LT1934 applica-

tion is limited by the undervoltage lockout (~3V) and by

the maximum duty cycle as outlined above. For proper

start-up, the minimum input voltage is also limited by the

boost circuit. If the input voltage is ramped slowly, or the

LT1934 is turned on with its SHDN pin when the output

is already in regulation, then the boost capacitor may not

be fully charged. Because the boost capacitor is charged

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

running properly. This minimum load will depend on input

and output voltages, and on the arrangement of the boost

circuit. The minimum load generally goes to zero once the

circuit has started. Figure 3 shows a plot of minimum load

to start and to run as a function of input voltage. In many

cases the discharged output capacitor will present a load

to the switcher which will allow it to start. The plots show

the worst-case situation where VIN is ramping very slowly.

Use a Schottky diode (such as the BAT-54) for the lowest

start-up voltage.

At light loads, the inductor current becomes discontinu-

ous and the effective duty cycle can be very high. This

reduces the minimum input voltage to approximately

300mV above VOUT. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

VIN

BOOST

GND

VIN

LT1934

(2a)

VOUT

VBOOST – VSW VOUT

MAX VBOOST VIN + VOUT

VIN

BOOST

GND

VIN

LT1934

(2b)

1934 F02

VOUT

VBOOST – VSW VIN

MAX VBOOST 2VIN

Figure 2. Two Circuits for Generating the Boost Voltage

LT1934/LT1934-1

1934fe

APPLICATIONS INFORMATION

maximum duty cycle of the LT1934, requiring a higher

input voltage to maintain regulation.

Shorted Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, an LT1934 buck regulator will tolerate a shorted

output. There is another situation to consider in systems

where the output will be held high when the input to the

LT1934 is absent. This may occur in battery charging ap-

plications or in battery backup systems where a battery

or some other supply is diode OR-ed with the LT1934’s

output. If the VIN pin is allowed to ﬂ oat and the SHDN pin

is held high (either by a logic signal or because it is tied

Figure 3. The Minimum Input Voltage Depends

on Output Voltage, Load Current and Boost Circuit

Minimum Input Voltage VOUT = 3.3V

Minimum Input Voltage VOUT = 5V

LOAD CURRENT (mA)

3.5

INPUT VOLTAGE (V)

4.0

4.5

5.0

5.5

6.0

0.1 10 100

1934 G12

3.0

LT1934

VOUT = 3.3V

TA = 25°C

BOOST DIODE TIED TO OUTPUT

VIN TO START

VIN TO RUN

LOAD CURRENT (mA)

INPUT VOLTAGE (V)

0.1 10 100

1934 G13

LT1934

VOUT = 5V

TA = 25°C

BOOST DIODE TIED TO OUTPUT

VIN TO START

VIN TO RUN

to VIN), then the LT1934’s internal circuitry will pull its

quiescent current through its SW pin. This is ﬁ ne if your

system can tolerate a few mA in this state. If you ground

the SHDN pin, the SW pin current will drop to essentially

zero. However, if the VIN pin is grounded while the output

is held high, then parasitic diodes inside the LT1934 can

pull large currents from the output through the SW pin

and the VIN pin. Figure 4 shows a circuit that will run only

when the input voltage is present and that protects against

a shorted or reversed input.

Figure 4. Diode D4 Prevents a Shorted Input from Discharging

a Backup Battery Tied to the Output; It Also Protects the Circuit

from a Reversed Input. The LT1934 Runs Only When the Input

is Present

BOOST

GND FB

SHDN SW

100k

LT1934

1934 F07

OUT

BACKUP

D4: MBR0530

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 5 shows

the high current paths in the buck regulator circuit. Note

that large, switched currents ﬂ ow in the power switch,

the catch diode (D1) and the input capacitor (C2). The

loop formed by these components should be as small as

possible. Furthermore, the system ground should be tied

to the regulator ground in only one place; this prevents

the switched current from injecting noise into the system

ground. These components, along with the inductor and

output capacitor, should be placed on the same side of

the circuit board, and their connections should be made

on that layer. Place a local, unbroken ground plane below

these components, and tie this ground plane to system

ground at one location, ideally at the ground terminal of

the output capacitor C1. Additionally, the SW and BOOST

nodes should be kept as small as possible. Finally, keep

the FB node as small as possible so that the ground pin

LT1934/LT1934-1

1934fe

APPLICATIONS INFORMATION

and ground traces will shield it from the SW and BOOST

nodes. Figure 6 shows component placement with trace,

ground plane and via locations. Include two vias near

the GND pin of the LT1934 to help remove heat from the

LT1934 to the ground plane.

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT1934 and LT1934-1 circuits. How-

ever, these capacitors can cause problems if the LT1934

is plugged into a live supply (see Linear Technology

Application Note 88 for a complete discussion). The low

loss ceramic capacitor combined with stray inductance in

series with the power source forms an under damped tank

circuit, and the voltage at the VIN pin of the LT1934 can

ring to twice the nominal input voltage, possibly exceeding

the LT1934’s rating and damaging the part. If the input

supply is poorly controlled or the user will be plugging

the LT1934 into an energized supply, the input network

should be designed to prevent this overshoot.

Figure 6. A Good PCB Layout Ensures Proper, Low EMI Operation

SHUTDOWN

VIAS TO LOCAL GROUND PLANE

OUTLINE OF LOCAL GROUND PLANE

OUT

1934 F06

SYSTEM

GROUND

Figure 5. Subtracting the Current When the Switch is On (a) from the Current When the Switch is Off (b) Reveals the Path of the High

Frequency Switching Current (c). Keep This Loop Small. The Voltage on the SW and BOOST Nodes Will Also be Switched; Keep These

Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane

GND

(5a)

C2 D1 C1

1934 F05

GND

(5c)

GND

(5b)

LT1934/LT1934-1

1934fe

APPLICATIONS INFORMATION

Figure 7 shows the waveforms that result when an LT1934

circuit is connected to a 24V supply through six feet of

24-gauge twisted pair. The ﬁ rst plot is the response with

a 2.2μF ceramic capacitor at the input. The input voltage

rings as high as 35V and the input current peaks at 20A.

One method of damping the tank circuit is to add another

capacitor with a series resistor to the circuit. In Figure 7b

an aluminum electrolytic capacitor has been added. This

capacitor’s high equivalent series resistance damps the

circuit and eliminates the voltage overshoot. The extra

capacitor improves low frequency ripple ﬁ ltering and can

slightly improve the efﬁ ciency of the circuit, though it is

likely to be the largest component in the circuit. An alterna-

tive solution is shown in Figure 7c. A 1Ω resistor is added

LT1934

2.2μF

10V/DIV

10A/DIV

10μs/DIV

CLOSING SWITCH

SIMULATES HOT PLUG

(7a)

(7b)

(7c)

(7d)

(7e)

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

LT1934

2.2μF

10μF

35V

AI.EI.

LT1934

2.2μF0.1μF

1Ω

LT1934-1

1μF

LT1934-1

1μF

1934 F07

0.1μF

4.7Ω

Figure 7. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT1934 is Connected to a Live Supply

LT1934/LT1934-1

1934fe

APPLICATIONS INFORMATION

in series with the input to eliminate the voltage overshoot

(it also reduces the peak input current). A 0.1μF capacitor

improves high frequency ﬁ ltering. This solution is smaller

and less expensive than the electrolytic capacitor. For high

input voltages its impact on efﬁ ciency is minor, reducing

efﬁ ciency less than one half percent for a 5V output at full

load operating from 24V.

Voltage overshoot gets worse with reduced input capaci-

tance. Figure 7d shows the hot plug response with a 1μF

ceramic input capacitor, with the input ringing above 40V.

The LT1934-1 can tolerate a larger input resistance, such

as shown in Figure 7e where a 4.7Ω resistor damps the

voltage transient and greatly reduces the input current

glitch on the 24V supply.

High Temperature Considerations

The die temperature of the LT1934 must be lower than the

maximum rating of 125°C. This is generally not a concern

unless the ambient temperature is above 85°C. For higher

temperatures, care should be taken in the layout of the

circuit to ensure good heat sinking of the LT1934. The

maximum load current should be derated as the ambient

temperature approaches 125°C.

The die temperature is calculated by multiplying the LT1934

power dissipation by the thermal resistance from junction

to ambient. Power dissipation within the LT1934 can be

estimated by calculating the total power loss from an

efﬁ ciency measurement and subtracting the catch diode

loss. The resulting temperature rise at full load is nearly

independent of input voltage. Thermal resistance depends

on the layout of the circuit board, but a value of 150°C/W

is typical for the TSOT-23 and 75°C/W for the DFN.

The temperature rise for an LT1934 (TSOT-23) producing

5V at 250mA is approximately 25°C, allowing it to deliver

full load to 100°C ambient. Above this temperature the

load current should be reduced. For 3.3V at 250mA the

temperature rise is 15°C. The DFN temperature rise will

be roughly one-half of these values.

Finally, be aware that at high ambient temperatures the

external Schottky diode, D1, is likely to have signiﬁ cant

leakage current, increasing the quiescent current of the

LT1934 converter.

Outputs Greater Than 6V

For outputs greater than 6V, tie a diode (such as a 1N4148)

from the SW pin to VIN to prevent the SW pin from ringing

above VIN during discontinuous mode operation. The 12V

output circuit in Typical Applications shows the location of

this diode. Also note that for outputs above 6V, the input

voltage range will be limited by the maximum rating of

the BOOST pin. The 12V circuit shows how to overcome

this limitation using an additional Zener diode.

LT1934/LT1934-1

1934fe

TYPICAL APPLICATIONS

3.3V Step-Down Converter

BOOST

LT1934-1

SHDN

1934 TA04

1μF

0.1μF

10pF

C1: TAIYO YUDEN JMK316BJ226ML

C2: TAIYO YUDEN GMK316BJ105ML

D1: ZETEX ZHCS400 OR ON SEMI MBR0540

D2: CENTRAL CMDSH-3

L1: COILCRAFT DO1608C-104 OR

WURTH ELECTRONICS WE-PD4 TYPE S

OUT

3.3V

45mA

604k

100μH

4.5V TO 34V

ON OFF

GND

22μF

5V Step-Down Converter

BOOST

LT1934-1

SHDN

1934 TA05

1μF

0.1μF

10pF

C1: TAIYO YUDEN JMK316BJ226ML

C2: TAIYO YUDEN GMK316BJ105ML

D1: ZETEX ZHCS400 OR ON SEMI MBR0540

D2: CENTRAL CMPD914

L1: COILCRAFT DO1608C-154 OR

WURTH ELECTRONICS WE-PD4 TYPE S

OUT

45mA

332k

150μH

6.5V TO 34V

ON OFF

GND

22μF

LT1934/LT1934-1

1934fe

TYPICAL APPLICATIONS

1.8V Step-Down Converter

BOOST

LT1934

SHDN

1934 TA06

2.2μF

0.1μF

C1: SANYO 2R5TPB100M

C2: TAIYO YUDEN EMK316BJ225ML

D1: ZETEX ZHCS400 OR ON SEMI MBR0540

D2: CENTRAL CMPD914

L1: SUMIDA CR43-330

OUT

1.8V

250mA

332k

147k

33μH

3.6V TO 16V

ON OFF

GND

100μF

BOOST

LT1934-1

SHDN

1934 TA08

1μF

10V

10pF

D1: ON SEMICONDUCTOR MBR0540

D2, D3: BAT54

D4: CENTRAL CMPZ5240B

L1: COILTRONICS CTX50-1

ZENER DIODE D4 PROVIDES AN UNDERVOLTAGE LOCKOUT,

REDUCING THE INPUT CURRENT REQUIRED AT START-UP

OUT

9mA

ISOLATED

OUT

3mA

715k

390k

L1A

50μH

L1B

50μH

14V TO 32V

<3.6mA

GND

33μF

•

10μF

Loop Powered 3.3V Supply with Additional Isolated Output

LT1934/LT1934-1

1934fe

BOOST

LT1934

SHDN

1934 TA07a

1μF

0.1μF

C1: SANYO 6TPB47M (619) 661-6835

C2: TAIYO YUDEN GMK316BJ105ML (408) 573-4150

D1, D3: ON SEMICONDUCTOR MBR0540 (602) 244-6600

D2: CENTRAL CMDSH-3 (516) 435-1110

L1: SUMIDA CR43-470 (847) 956-0667

332k

1k 10k

47μH

7V TO 28V SW

GND

CHRG

LTC4052

ACPR

GATE

SENSE

BAT

350mA

1-CELL 4.2V

Li-Ion

BATTERY

GNDTIMER

47μF

CHARGE STATUS

AC PRESENT

10μF

0.047μF

TIMER

0.1μF

0.022μF

BATTERY VOLTAGE (V)

2.5

CHARGE CURRENT (mA)

200

300

4.5

1934 TA07b

100

033.5 4

500

400

VIN = 12V

VIN = 8V

VIN = 24V

TYPICAL APPLICATIONS

Standalone 350mA Li-Ion Battery Charger

12V Step-Down Converter

BOOST

LT1934

SHDN

1934 TA09

2.2μF

0.1μF

C1: KEMET T495D226K020AS

C2: TAIYO YUDEN GMK325BJ225MN

D1: ON SEMI MBR0540

D2, D4: CENTRAL CMPD914

D3: CENTRAL CMPZ5234B 6.2V ZENER

L1: TDK SLF6028T-101MR42

OUT

12V

170mA

100k

866k

100μH

15V TO 32V

ON OFF

GND

22μF

LT1934/LT1934-1

1934fe

PACKAGE DESCRIPTION

1.50 – 1.75

(NOTE 4)

2.80 BSC

0.30 – 0.45

6 PLCS (NOTE 3)

DATUM ‘A’

0.09 – 0.20

(NOTE 3) S6 TSOT-23 0302 REV B

2.90 BSC

(NOTE 4)

0.95 BSC

1.90 BSC

0.80 – 0.90

1.00 MAX 0.01 – 0.10

0.20 BSC

0.30 – 0.50 REF

PIN ONE ID

NOTE:

1. DIMENSIONS ARE IN MILLIMETERS

2. DRAWING NOT TO SCALE

3. DIMENSIONS ARE INCLUSIVE OF PLATING

4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR

5. MOLD FLASH SHALL NOT EXCEED 0.254mm

6. JEDEC PACKAGE REFERENCE IS MO-193

3.85 MAX

0.62

MAX

0.95

REF

RECOMMENDED SOLDER PAD LAYOUT

PER IPC CALCULATOR

1.4 MIN

2.62 REF

1.22 REF

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

s 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

S6 Package

6-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1636)

DCB Package

6-Lead Plastic DFN (2mm × 3mm)

(Reference LTC DWG # 05-08-1715)

LT1934/LT1934-1

1934fe

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

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

LT 0209 REV E • PRINTED IN USA

BOOST

LT1934

SHDN

1934 TA03

2.2μF

0.1μF

10pF

C1: SANYO 6TPB68M

C2: TAIYO YUDEN GMK325BJ225MN

D1: ZETEX ZHCS400 OR ON SEMI MBR0540

D2: CENTRAL CMPD914

L1: SUMIDA CDRH5D28-680

OUT

250mA

332k

68μH

6.5V TO 34V

ON OFF

GND

68μF

TYPICAL APPLICATION

5V Step-Down Converter

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