LT1938EDD#PBF - LINEAR TECHNOLOGY

LT1938

1938fa

25V, 2.2A, 2.8MHz

Step-Down

Switching Regulator

The LT®1938 is an adjustable frequency (300kHz to

2.8MHz) monolithic buck switching regulator that accepts

input voltages up to 25V. A high efﬁciency 0.18Ω switch

is included on the die along with a boost Schottky diode

and the necessary oscillator, control and logic circuitry.

Current mode topology is used for fast transient response

and good loop stability. The LT1938’s high operating fre-

quency allows the use of small, low cost inductors and

ceramic capacitors resulting in low output ripple while

keeping total solution size to a minimum. The low current

shutdown mode reduces input supply current to less than

1µA while a resistor and capacitor on the RUN/SS pin

provide a controlled output voltage ramp (soft-start). A

power good ﬂag signals when VOUT reaches 90% of the

programmed output voltage. The LT1938 is available in

a 3mm × 3mm DFN package with Exposed Pad for low

thermal resistance.

■ Automotive Battery Regulation

■ Power for Portable Products

■ Distributed Supply Regulation

■ Industrial Supplies

■ Wall Transformer Regulation

■ Wide Input Voltage Range: 3.6V to 25V

■ 2.2A Maximum Output Current

■ Adjustable Switching Frequency: 300kHz to 2.8MHz

■ Low Shutdown Current: IQ < 1µA

■ Integrated Boost Diode

■ Power Good Flag

■ Saturating Switch Design: 0.18Ω On-Resistance

■ 1.265V Feedback Reference Voltage

■ Output Voltage: 1.265V to 20V

■ Soft-Start Capability

■ Small 10-Pin Thermally Enhanced (3mm × 3mm)

DFN Package

3.3V Step-Down Converter Efﬁciency (VOUT = 3.3V)

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

BIAS

VIN BD

VIN

4.5V TO

25V

VOUT

3.3V

2.2A

4.7µF

0.47µF

680pF

22µF

200k

16.2k

60.4k

4.7µH

324k

GND

OFF ON

LT1938

1938 TA01

RUN/SS BOOST

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

All other trademarks are the property of their respective owners.

LOAD CURRENT (A)

EFFICIENCY (%)

1.6

1938 G02

50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 2.22.0

VIN = 12V

VIN = 7V

VIN = 24V

L: NEC PLC-0745-4R7

f = 800kHz

LT1938

1938fa

ELECTRICAL ChARACTERISTICS

VIN, RUN/SS Voltage .................................................25V

BOOST Pin Voltage ...................................................50V

BOOST Pin Above SW Pin .........................................25V

FB, RT, VC Voltage .......................................................5V

BIAS, PG, BD Voltage ................................................ 25V

Operating Junction Temperature Range (Note 2)

LT1938E ............................................. –40°C to 125°C

LT1938I .............................................. –40°C to 125°C

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

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage ●3 3.6 V

Quiescent Current from VIN VRUN/SS = 0.2V 0.01 0.5 µA

VBIAS = 3V, Not Switching ●0.4 0.8 mA

VBIAS = 0, Not Switching 1.2 2.0 mA

Quiescent Current from BIAS VRUN/SS = 0.2V 0.01 0.5 µA

VBIAS = 3V, Not Switching ●0.85 1.5 mA

VBIAS = 0, Not Switching 0 0.1 mA

Minimum Bias Voltage 2.7 3 V

Feedback Voltage

●

1.25

1.24

1.265

1.28

1.29

FB Pin Bias Current (Note 3) ●30 100 nA

FB Voltage Line Regulation 4V < VIN < 25V 0.002 0.02 %/V

The ● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBIAS = 3.3V unless otherwise

noted. (Note 2)

AbSOLUTE MAxIMUM RATINgS

(Note 1)

PIN CONFIgURATION

TOP VIEW

DD PACKAGE

10-LEAD (3mm × 3mm) PLASTIC DFN

311

1RT

BIAS

BOOST

VIN

RUN/SS

TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

ORDER INFORMATION

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

LT1938EDD#PBF LT1938EDD#TRPBF LDFT 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C

LT1938IDD#PBF LT1938IDD#TRPBF LDFT 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°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/

LT1938

1938fa

PARAMETER CONDITIONS MIN TYP MAX UNITS

Error Amp gm330 µMho

Error Amp Gain 1000

VC Source Current 75 µA

VC Sink Current 100 µA

VC Pin to Switch Current Gain 3.5 A/V

VC Clamp Voltage 2 V

Switching Frequency RT = 8.66k

RT = 29.4k

RT = 187k

2.7

1.25

250

3.0

1.4

300

3.3

1.55

350

MHz

kHz

Minimum Switch Off-Time ●100 150 nS

Switch Current Limit Duty Cycle = 5% 3.1 3.6 4.0 A

Switch VCESAT ISW = 2A 360 mV

Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.02 2 µA

Minimum Boost Voltage (Note 4) ●1.6 2.1 V

BOOST Pin Current ISW = 1A 18 30 mA

RUN/SS Pin Current VRUN/SS = 2.5V 5 10 µA

RUN/SS Input Voltage High 2.5 V

RUN/SS Input Voltage Low 0.2 V

PG Threshold Offset from Feedback Voltage VFB Rising 100 mV

PG Hysteresis 10 mV

PG Leakage VPG = 5V 0.1 1 µA

PG Sink Current VPG = 0.4V ●100 300 µA

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 LT1938E is guaranteed to meet performance speciﬁcations

from 0°C to 125°C. Speciﬁcations over the –40°C to 125°C operating

temperature range are assured by design, characterization and correlation

with statistical process controls. The LT1938I speciﬁcations are

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

Note 3: Bias current measured in regulation. Bias current ﬂows into the FB

pin.

Note 4: This is the minimum voltage across the boost capacitor needed to

guarantee full saturation of the switch.

The ● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 10V, VRUN/SS = 10V VBOOST = 15V, VBIAS = 3.3V unless otherwise

noted. (Note 2)

ELECTRICAL ChARACTERISTICS

LT1938

1938fa

Efﬁciency (VOUT = 5.0V) Efﬁciency (VOUT = 3.3V)

Maximum Load Current

Switch Voltage Drop Boost Pin Current

Switch Current Limit

Maximum Load Current

Switch Current Limit

Efﬁciency

LOAD CURRENT (A)

EFFICIENCY (%)

100

1.6

1938 G01

50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 2.22.0

VIN = 12V

VIN = 24V

L: NEC PLC-0745-4R7

f = 800kHz

LOAD CURRENT (A)

EFFICIENCY (%)

1.6

1938 G02

50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 2.22.0

VIN = 12V

VIN = 7V

VIN = 24V

L: NEC PLC-0745-4R7

f = 800kHz

SWITCHING FREQUENCY (MHz)

EFFICIENCY (%)

1.5

1938 G03

0.5 1 2

2.5 3

VIN = 12V

VIN = 24V

VOUT = 3.3V

L = 10µH

LOAD = 1A

INPUT VOLTAGE (V)

LOAD CURRENT (A)

1938 G04

2.5

10 20

1.5

1.0

4.0

3.5

3.0

2.0

TYPICAL

MINIMUM

VOUT = 3.3V

TA = 25°C

L = 4.7µH

f = 800kHz

INPUT VOLTAGE (V)

LOAD CURRENT (A)

1938 G05

2.5

10 20

1.5

1.0

4.0

3.5

3.0

2.0

TYPICAL

MINIMUM

VOUT = 5V

TA = 25°C

L = 4.7µH

f = 800kHz

DUTY CYCLE (%)

SWITCH CURRENT LIMIT (A)

1938 G06

2.5

20 60

1.5

1.0

4.0

3.5

3.0

2.0

80 100

TEMPERATURE (°C)

–50

SWITCH CURRENT LIMIT (A)

2.0

2.5

3.0

125

1938 G07

1.5

1.0

00–25 5025 10075

0.5

4.5

4.0 DUTY CYCLE = 10 %

DUTY CYCLE = 90 %

SWITCH CURRENT (mA)

400

500

700

1500 2500

1938 G08

300

200

500 1000 2000 3000 3500

100

600

VOLTAGE DROP (mV)

SWITCH CURRENT (mA)

BOOST PIN CURRENT (mA)

2000

1938 G09

1000

500 2500 3000

1500 3500

(TA = 25°C unless otherwise noted)

TYPICAL PERFORMANCE ChARACTERISTICS

LT1938

1938fa

Feedback Voltage Switching Frequency Frequency Foldback

Minimum Switch On-Time Soft-Start RUN/SS Pin Current

Boost Diode Error Amp Output Current Minimum Input Voltage

TEMPERATURE (°C)

–50

FEEDBACK VOLTAGE (V)

1.270

1.280

125

1938 G10

1.260

1.250 0 50 100–25 25 75

1.290

1.265

1.275

1.255

1.285

TEMPERATURE (°C)

–50

FREQUENCY (MHz)

1.00

1.10

125

1938 G11

0.90

0.80 0 50 100–25 25 75

1.20 RT = 45.3k

0.95

1.05

0.85

1.15

FB PIN VOLTAGE (mV)

SWITCHING FREQUENCY (kHz)

800

1000

1200

600 1000

1938 G12

600

400

200 400 800 1200 1400

200

RT = 45.3k

TEMPERATURE (°C)

–50

MINIMUM SWITCH ON-TIME (ns)

100

120

1938 G13

–25 0 50 75 100 125

140

RUN/SS PIN VOLTAGE (V)

SWITCH CURRENT LIMIT (A)

3.5

1.5

1938 G14

2.0

1.0

0.5 1 2

0.5

4.0

3.0

2.5

1.5

2.5 3 3.5

RUN/SS PIN VOLTAGE (V)

RUN/SS PIN CURRENT (µA)

1938 G15

5 10 20 25

BOOST DIODE CURRENT (A)

BOOST DIODE Vf (V)

0.8

1.0

1.2

2.0

1938 G16

0.6

0.4

00.5 1.0 1.5

0.2

1.6

1.4

FB PIN VOLTAGE (V)

1.065

–80

VC PIN CURRENT (µA)

–60

–20

1.265 1.465

100

1938 G17

–40

1.165 1.365

LOAD CURRENT (A)

0.001

INPUT VOLTAGE (V)

3.0

3.5

1938 G18

2.5

2.0

0.01 0.1 1

4.5

4.0

VOUT = 3.3V

TA = 25°C

L = 4.7µH

f = 800kHz

TYPICAL PERFORMANCE ChARACTERISTICS

(TA = 25°C unless otherwise noted)

LT1938

1938fa

VC Voltages

Minimum Input Voltage Power Good Threshold

Switching Waveforms

(Discontinuous Operation)

Switching Waveforms

(Continuous Operation)

LOAD CURRENT (A)

0.001

INPUT VOLTAGE (V)

5.0

5.5

1938 G19

4.5

4.0

0.01 0.1 1

6.5

6.0

VOUT = 5V

TA = 25°C

L = 4.7µH

f = 800kHz

TEMPERATURE (°C)

–50

THRESHOLD VOLTAGE (V)

1.50

2.00

2.50

25 75 125

1938 G20

1.00

0.50

–25 0 50 100

CURRENT LIMIT CLAMP

SWITCHING THRESHOLD

TEMPERATURE (°C)

–50

THRESHOLD VOLTAGE (V)

1.160

1.180

1.200

25 75 125

1938 G21

1.140

1.120

1.100

–25 0 50 100

PG RISING

0.5A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

1938 G22

1µs/DIV

VIN = 12V, FRONT PAGE APPLICATION

ILOAD = 140mA

0.5A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

1938 G23

1µs/DIV

VIN = 12V, FRONT PAGE APPLICATION

ILOAD = 1A

TYPICAL PERFORMANCE ChARACTERISTICS

(TA = 25°C unless otherwise noted)

LT1938

1938fa

BD (Pin 1): This pin connects to the anode of the boost

Schottky diode.

BOOST (Pin 2): This pin is used to provide a drive

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

NPN power switch.

SW (Pin 3): The SW pin is the output of the internal power

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

boost capacitor.

VIN (Pin 4): The VIN pin supplies current to the LT1938’s

internal regulator and to the internal power switch. This

pin must be locally bypassed.

RUN/SS (Pin 5): The RUN/SS pin is used to put the

LT1938 in shutdown mode. Tie to ground to shut down

the LT1938. Tie to 2.3V or more for normal operation. If

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

pin. RUN/SS also provides a soft-start function; see the

Applications Information section.

PG (Pin 6): The PG pin is the open collector output of an

internal comparator. PG remains low until the FB pin is

within 10% of the ﬁnal regulation voltage. PG output is

valid when VIN is above 3.5V and RUN/SS is high.

BIAS (Pin 7): The BIAS pin supplies the current to the

LT1938’s internal regulator. Tie this pin to the lowest

available voltage source above 3V (typically VOUT). This

architecture increases efﬁciency especially when the input

voltage is much higher than the output.

FB (Pin 8): The LT1938 regulates the FB pin to 1.265V.

Connect the feedback resistor divider tap to this pin.

VC (Pin 9): The VC pin is the output of the internal error

ampliﬁer. The voltage on this pin controls the peak switch

current. Tie an RC network from this pin to ground to

compensate the control loop.

(Pin 10): Oscillator Resistor Input. Connecting a resistor

to ground from this pin sets the switching frequency.

Exposed Pad (Pin 11): Ground. The Exposed Pad must

be soldered to the PCB.

PIN FUNCTIONS

LT1938

1938fa

bLOCK DIAgRAM

–

OSCILLATOR

300kHz–2.8MHz

VC CLAMP

SOFT-START

SLOPE COMP

INTERNAL 1.265V REF

VIN

BIAS

RUN/SS

BOOST

SWITCH

LATCH

VOUT

GND

ERROR AMP

1.12V

∑

1938 BD

11 8

LT1938

1938fa

The LT1938 is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

enables an RS ﬂip-ﬂop, turning on the internal power

switch. An ampliﬁer and comparator monitor the current

ﬂowing between the VIN and SW pins, turning the switch

off when this current reaches a level determined by the

voltage at VC. An error ampliﬁer measures the output

voltage through an external resistor divider tied to the FB

pin and servos the VC pin. If the error ampliﬁer’s output

increases, more current is delivered to the output; if it

decreases, less current is delivered. An active clamp on the

VC pin provides current limit. The VC pin is also clamped to

the voltage on the RUN/SS pin; soft-start is implemented

by generating a voltage ramp at the RUN/SS pin using an

external resistor and capacitor.

An internal regulator provides power to the control cir-

cuitry. The bias regulator normally draws power from the

VIN pin, but if the BIAS pin is connected to an external

voltage higher than 3V bias power will be drawn from the

external source (typically the regulated output voltage).

This improves efﬁciency. The RUN/SS pin is used to place

the LT1938 in shutdown, disconnecting the output and

reducing the input current to less than 1µA.

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 internal bipolar NPN power switch for efﬁcient

operation.

The oscillator reduces the LT1938’s operating frequency

when the voltage at the FB pin is low. This frequency

foldback helps to control the output current during startup

and overload.

The LT1938 contains a power good comparator which trips

when the FB pin is at 90% of its regulated value. The PG

output is an open-collector transistor that is off when the

output is in regulation, allowing an external resistor to pull

the PG pin high. Power good is valid when the LT1938 is

enabled and VIN is above 3.6V.

OPERATION

LT1938

1938fa

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=R2 VOUT

1.265 – 1

⎛

⎝

⎜⎞

⎠

⎟

Reference designators refer to the Block Diagram.

Setting the Switching Frequency

The LT1938 uses a constant frequency PWM architecture

that can be programmed to switch from 300kHz to 2.8MHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Figure 1.

SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)

0.2

0.3

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

267

187

133

84.5

60.4

45.3

36.5

29.4

23.7

20.5

16.9

14.3

12.1

10.2

8.66

Operating Frequency Tradeoffs

Selection of the operating frequency is a tradeoff between

efﬁciency, component size, minimum dropout voltage, and

maximum input voltage. The advantage of high frequency

operation is that smaller inductor and capacitor values may

be used. The disadvantages are lower efﬁciency, lower

maximum input voltage, and higher dropout voltage. The

highest acceptable switching frequency (fSW(MAX)) for a

given application can be calculated as follows:

fVV

tVVV

SW MAXDOUT

ON MINDI

() ()

()

–

where VIN is the typical input voltage, VOUT is the output

voltage, is the catch diode drop (~0.5V), VSW is the internal

switch drop (~0.5V at max load). This equation shows

that slower switching frequency is necessary to safely

accommodate high VIN/VOUT ratio. Also, as shown in

the next section, lower frequency allows a lower dropout

voltage. The reason input voltage range depends on the

switching frequency is because the LT1938 switch has

ﬁnite minimum on and off times. The switch can turn on

for a minimum of ~150ns and turn off for a minimum of

~150ns. This means that the minimum and maximum

duty cycles are:

DC ft

MINSWON MIN

MAXSWOFFMIN

()

1–

where fSW is the switching frequency, the tON(MIN) is the

minimum switch on time (~150ns), and the tOFF(MIN) is

the minimum switch off time (~150ns). These equations

show that duty cycle range increases when switching

frequency is decreased.

A good choice of switching frequency should allow ad-

equate input voltage range (see next section) and keep

the inductor and capacitor values small.

Input Voltage Range

The maximum input voltage for LT1938 applications de-

pends on switching frequency, the Absolute Maximum Rat-

ings on VIN and BOOST pins, and on operating mode.

If the output is in start-up or short-circuit operating modes,

then VIN must be below 25V and below the result of the

following equation:

VVV

IN MAXOUT D

SW ON MIN

() ()

=++–

where VIN(MAX) is the maximum operating input voltage,

VOUT is the output voltage, VD is the catch diode drop

(~0.5V), VSW is the internal switch drop (~0.5V at max

load), fSW is the switching frequency (set by RT), and

tON(MIN) is the minimum switch on time (~150ns). Note that

a higher switching frequency will depress the maximum

operating input voltage. Conversely, a lower switching

Figure 1. Switching Frequency vs RT Value

APPLICATIONS INFORMATION

LT1938

1938fa

frequency will be necessary to achieve safe operation at

high input voltages.

If the output is in regulation and no short-circuit or start-up

events are expected, then input voltage transients of up to

25V are acceptable regardless of the switching frequency.

In this mode, the LT1938 may enter pulse skipping opera-

tion where some switching pulses are skipped to maintain

output regulation. In this mode the output voltage ripple

and inductor current ripple will be higher than in normal

operation.

The minimum input voltage is determined by either the

LT1938’s minimum operating voltage of ~3.6V or by its

maximum duty cycle (see equation in previous section).

The minimum input voltage due to duty cycle is:

VVV

IN MINOUT D

SW OFFMIN

() ()

1– –

where VIN(MIN) is the minimum input voltage, and tOFF(MIN)

is the minimum switch off time (150ns). Note that higher

switching frequency will increase the minimum input

voltage. If a lower dropout voltage is desired, a lower

switching frequency should be used.

Inductor Selection

For a given input and output voltage, the inductor value

and switching frequency will determine the ripple current.

The ripple current ∆IL increases with higher VIN or VOUT

and decreases with higher inductance and faster switch-

ing frequency. A reasonable starting point for selecting

the ripple current is:

∆IL = 0.4(IOUT(MAX))

where IOUT(MAX) is the maximum output load current. To

guarantee sufﬁcient output current, peak inductor current

must be lower than the LT1938’s switch current limit (ILIM).

The peak inductor current is:

IL(PEAK) = IOUT(MAX) + ∆IL/2

where IL(PEAK) is the peak inductor current, IOUT(MAX) is

the maximum output load current, and ∆IL is the inductor

ripple current. The LT1938’s switch current limit (ILIM) is

at least 3.5A at low duty cycles and decreases linearly to

2.5A at DC = 0.8. The maximum output current is a func-

tion of the inductor ripple current:

IOUT(MAX) = ILIM – ∆IL/2

Be sure to pick an inductor ripple current that provides

sufﬁcient maximum output current (IOUT(MAX)).

The largest inductor ripple current occurs at the highest

VIN. To guarantee that the ripple current stays below the

speciﬁed maximum, the inductor value should be chosen

according to the following equation:

L=VOUT +V

fΔIL

⎛

⎝

⎜⎞

⎠

⎟1– VOUT +V

IN MAX

( )

⎛

⎝

⎜

⎞

⎠

⎟

where VD is the voltage drop of the catch diode (~0.4V),

VIN(MAX) is the maximum input voltage, VOUT is the output

voltage, fSW is the switching frequency (set by RT), and L

is in the inductor value.

The inductor’s RMS current rating must be greater than the

maximum load current and its saturation current should be

about 30% higher. For robust operation in fault conditions

(start-up or short circuit) and high input voltage (>30V),

the saturation current should be above 3A. To keep the

efﬁciency high, the series resistance (DCR) should be less

than 0.1Ω, and the core material should be intended for

high frequency applications. Table 1 lists several vendors

and suitable types.

Table 1. Inductor Vendors

VENDOR URL PART SERIES TYPE

Murata www.murata.com LQH55D Open

TDK www.componenttdk.com SLF7045

SLF10145

Shielded

Toko www.toko.com D62CB

D63CB

D75C

D75F

Shielded

Open

Sumida www.sumida.com CR54

CDRH74

CDRH6D38

CR75

Open

Shielded

Open

APPLICATIONS INFORMATION

LT1938

1938fa

Of course, such a simple design guide will not always re-

sult in the optimum inductor for your application. A larger

value inductor provides a slightly higher maximum load

current and will reduce the output voltage ripple. If your

load is lower than 2A, then you can decrease 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 efﬁciency. There are

several graphs in the Typical Performance Characteristics

section of this data sheet that show the maximum load

current as a function of input voltage and inductor value

for several popular output voltages. Low inductance may

result in discontinuous mode operation, which is okay

but further reduces maximum load current. For details of

maximum output current and discontinuous mode opera-

tion, see Linear Technology Application Note 44. Finally,

for duty cycles greater than 50% (VOUT/VIN > 0.5), there

is a minimum inductance required to avoid subharmonic

oscillations. See AN19.

Input Capacitor

Bypass the input of the LT1938 circuit with a ceramic capaci-

tor of X7R or X5R type. Y5V types have poor performance

over temperature and applied voltage, and should not be

used. A 4.7µF to 10µF ceramic capacitor is adequate to

bypass the LT1938 and will easily handle the ripple current.

Note that larger input capacitance is required when a lower

switching frequency is used. If the input power source has

high impedance, or there is signiﬁcant inductance due to

long wires or cables, additional bulk capacitance may be

necessary. This can be provided with a low performance

electrolytic 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 LT1938 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 4.7µF capacitor is capable of this task, but only if it is

placed close to the LT1938 and the catch diode (see the

PCB Layout section). A second precaution regarding the

ceramic input capacitor concerns the maximum input

voltage rating of the LT1938. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (under damped) tank circuit. If the LT1938 circuit

is plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT1938’s

voltage rating. This situation is easily avoided (see the Hot

Plugging Safety section).

For space sensitive applications, a 2.2µF ceramic capaci-

tor can be used for local bypassing of the LT1938 input.

However, the lower input capacitance will result in in-

creased input current ripple and input voltage ripple, and

may couple noise into other circuitry. Also, the increased

voltage ripple will raise the minimum operating voltage

of the LT1938 to ~3.7V.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it ﬁlters the square wave generated by the

LT1938 to produce the DC output. In this role it determines

the output ripple, and low impedance at the switching

frequency is important. The second function is to store

energy in order to satisfy transient loads and stabilize the

LT1938’s control loop. Ceramic capacitors have very low

equivalent series resistance (ESR) and provide the best

ripple performance. A good starting value is:

CVf

OUT OUT SW

=100

where fSW is in MHz, and COUT is the recommended

output capacitance in µF. Use X5R or X7R types. This

choice will provide low output ripple and good transient

response. Transient performance can be improved with a

higher value capacitor if the compensation network is also

adjusted to maintain the loop bandwidth. A lower value

of output capacitor can be used to save space and cost

but transient performance will suffer. See the Frequency

Compensation section to choose an appropriate compen-

sation network.

APPLICATIONS INFORMATION

LT1938

1938fa

When choosing a capacitor, look carefully through the

data sheet to ﬁnd out what the actual capacitance is under

operating conditions (applied voltage and temperature).

A physically larger capacitor, or one with a higher voltage

rating, may be required. High performance tantalum or

electrolytic capacitors can be used for the output capacitor.

Low ESR is important, so choose one that is intended for

use in switching regulators. The ESR should be speciﬁed

by the supplier, and should be 0.05Ω or less. Such a

capacitor will be larger than a ceramic capacitor and will

have a larger capacitance, because the capacitor must be

large to achieve low ESR. Table 2 lists several capacitor

vendors.

Catch Diode

The catch diode conducts current only during switch off

time. Average forward current in normal operation can

be calculated from:

ID(AVG) = IOUT (VIN – VOUT)/VIN

where IOUT is the output load current. The only reason to

consider a diode with a larger current rating than necessary

for nominal operation is for the worst-case condition of

shorted output. The diode current will then increase to the

typical peak switch current. Peak reverse voltage is equal

to the regulator input voltage. Use a diode with a reverse

voltage rating greater than the input voltage. Table 3 lists

several Schottky diodes and their manufacturers.

Table 3. Diode Vendors

PART NUMBER

(V)

IAVE

(A)

VF AT 1A

(mV)

VF AT 2A

(mV)

On Semiconductor

MBRM120E

530

595

Diodes Inc.

B120

B130

B220

B230

DFLS230L

500

International Rectiﬁer

10BQ030

20BQ030

420

470

Frequency Compensation

The LT1938 uses current mode control to regulate the

output. This simpliﬁes loop compensation. In particular, the

LT1938 does not require the ESR of the output capacitor

for stability, so you are free to use ceramic capacitors to

achieve low output ripple and small circuit size. Frequency

compensation is provided by the components tied to the

VC pin, as shown in Figure 2. Generally a capacitor (CC)

and a resistor (RC) in series to ground are used. In addi-

tion, there may be lower value capacitor in parallel. This

capacitor (CF) is not part of the loop compensation but

is used to ﬁlter noise at the switching frequency, and is

required only if a phase-lead capacitor is used or if the

output capacitor has high ESR.

VENDOR PHONE URL PART SERIES COMMANDS

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 (408) 436-1300 www.murata.com Ceramic

AVX www.avxcorp.com Ceramic,

Tantalum TPS Series

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

Table 2. Capacitor Vendors

APPLICATIONS INFORMATION

LT1938

1938fa

Loop compensation determines the stability and transient

performance. Designing the compensation network is a

bit complicated and the best values depend on the ap-

plication and in particular the type of output capacitor. A

practical approach is to start with one of the circuits in

this data sheet that is similar to your application and tune

the compensation network to optimize the performance.

Stability should then be checked across all operating

conditions, including load current, input voltage and

temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

how to test the stability using a transient load. Figure 2

shows an equivalent circuit for the LT1938 control loop.

The error ampliﬁer is a transconductance ampliﬁer with

ﬁnite output impedance. The power section, consisting of

the modulator, power switch and inductor, is modeled as

a transconductance ampliﬁer generating an output cur-

rent proportional to the voltage at the VC pin. Note that

the output capacitor integrates this current, and that the

capacitor on the VC pin (CC) integrates the error ampli-

ﬁer output current, resulting in two poles in the loop. In

most cases a zero is required and comes from either the

output capacitor ESR or from a resistor RC in series with

CC. This simple model works well as long as the value

of the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency.

A phase lead capacitor (CPL) across the feedback divider

Figure 3. Transient Load Response of the LT1938 Front Page

Application as the Load Current is Stepped from 500mA to

1500mA. VOUT = 3.3V

Figure 2. Model for Loop Response

APPLICATIONS INFORMATION

–

1.265V

VCGND

LT1938

1938 F02

OUTPUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm = 3.5mho

gm =

330µmho

POLYMER

TANTALUM

CERAMIC

CPL

may improve the transient response. Figure 3 shows the

transient response when the load current is stepped from

500mA to 1500mA and back to 500mA.

BOOST and BIAS Pin Considerations

Capacitor C3 and the internal boost Schottky diode (see

the Block Diagram) are used to generate a boost volt-

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

a 0.22µF capacitor will work well. Figure 2 shows three

ways to arrange the boost circuit. The BOOST pin must be

more than 2.3V above the SW pin for best efﬁciency. For

outputs of 3V and above, the standard circuit (Figure 4a)

is best. For outputs between 2.8V and 3V, use a 1µF boost

capacitor. A 2.5V output presents a special case because it

is marginally adequate to support the boosted drive stage

while using the internal boost diode. For reliable BOOST pin

operation with 2.5V outputs use a good external Schottky

diode (such as the ON Semi MBR0540), and a 1µF boost

capacitor (see Figure 4b). For lower output voltages the

boost diode can be tied to the input (Figure 4c), or to

another supply greater than 2.8V. The circuit in Figure 4a

is more efﬁcient because the BOOST pin current and BIAS

pin quiescent current comes from a lower voltage source.

You must also be sure that the maximum voltage ratings

of the BOOST and BIAS pins are not exceeded.

The minimum operating voltage of an LT1938 application

is limited by the minimum input voltage (3.6V) and by the

maximum duty cycle as outlined in a previous section. For

1938 F03

1A/DIV

VOUT

100mV/DIV

10µs/DIV

VOUT = 12V, FRONT PAGE APPLICATION

LT1938

1938fa

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

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

or the LT1938 is turned on with its RUN/SS 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 5 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 and the minimum input

to start will be the same as the minimum input to run.

This occurs, for example, if RUN/SS is asserted after VIN

is applied. The plots show the worst-case situation where

VIN is ramping very slowly. For lower start-up voltage, the

boost diode can be tied to VIN; however, this restricts the

input range to one-half of the absolute maximum rating

of the BOOST pin.

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

maximum duty cycle of the LT1938, requiring a higher

input voltage to maintain regulation.

Figure 4. Three Circuits For Generating The Boost Voltage

Figure 5. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

APPLICATIONS INFORMATION

VIN

BOOST

VIN

VOUT

4.7µF

GND

LT1938

VIN

BOOST

VIN

VOUT

4.7µF

GND

LT1938

VIN

BOOST

VIN

VOUT

4.7µF

GND

LT1938

1938 F04

(4a) For VOUT > 2.8V

(4b) For 2.5V < VOUT < 2.8V

(4c) For V

OUT

< 2.5V

1938 F05

LOAD CURRENT (A)

0.001

INPUT VOLTAGE (V)

4.0

4.5

5.0

3.5

3.0

2.0

0.01 0.1 1

2.5

6.0

5.5

TO START

TO RUN

VOUT = 3.3V

TA = 25°C

L = 4.7µH

f = 800kHz

LOAD CURRENT (A)

0.001

INPUT VOLTAGE (V)

5.0

6.0

7.0

4.0

2.0

0.01 0.1 1

3.0

8.0

TO START

TO RUN

VOUT = 5V

TA = 25 °C

L = 4.7µH

f = 800kHz

LT1938

1938fa

Soft-Start

The RUN/SS pin can be used to soft-start the LT1938,

reducing the maximum input current during start-up.

The RUN/SS pin is driven through an external RC ﬁlter to

create a voltage ramp at this pin. Figure 7 shows the start-

up and shut-down waveforms with the soft-start circuit.

By choosing a large RC time constant, the peak start-up

current can be reduced to the current that is required to

regulate the output, with no overshoot. Choose the value

of the resistor so that it can supply 20µA when the RUN/SS

pin reaches 2.3V.

LT1938 can pull large currents from the output through

the SW pin and the VIN pin. Figure 7 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 8 shows

the recommended component placement with trace,

ground plane and via locations. Note that large, switched

currents ﬂow in the LT1938’s VIN and SW pins, the catch

diode (D1) and the input capacitor (C1). The loop formed

by these components should be as small as possible. 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.

The SW and BOOST nodes should be as small as possible.

Finally, keep the FB and VC nodes small so that the ground

traces will shield them from the SW and BOOST nodes.

The Exposed Pad on the bottom of the package must be

soldered to ground so that the pad acts as a heat sink. To

keep thermal resistance low, extend the ground plane as

much as possible, and add thermal vias under and near

the LT1938 to additional ground planes within the circuit

board and on the bottom side.

Figure 6. To Soft-Start the LT1938, Add a Resisitor

and Capacitor to the RUN/SS Pin

APPLICATIONS INFORMATION

1938 F06

1A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

RUN/SS

GND

0.22µF

RUN

15k

2ms/DIV

Shorted and Reversed Input Protection

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

sively, an LT1938 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

LT1938 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 LT1938’s

output. If the VIN pin is allowed to ﬂoat and the RUN/SS

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

tied to VIN), then the LT1938’s internal circuitry will pull

its quiescent current through its SW pin. This is ﬁne if

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

ground the RUN/SS 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

Figure 7. 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 LT1938

Runs Only When the Input is Present

VIN BOOST

GND FB

RUN/SS

MBRS140

VIN

LT1938

1938 F07

VOUT

BACKUP

LT1938

1938fa

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT1938 circuits. However, these capaci-

tors can cause problems if the LT1938 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 LT1938 can ring to twice the

nominal input voltage, possibly exceeding the LT1938’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LT1938 into an

energized supply, the input network should be designed

to prevent this overshoot. Figure 9 shows the waveforms

that result when an LT1938 circuit is connected to a 24V

supply through six feet of 24-gauge twisted pair. The

ﬁrst plot is the response with a 4.7µF ceramic capacitor

at the input. The input voltage rings as high as 50V and

the input current peaks at 26A. A good solution is shown

in Figure 9b. A 0.7Ω resistor is added in series with the

APPLICATIONS INFORMATION

Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation

VIAS TO LOCAL GROUND PLANE

VIAS TO VOUT

VIAS TO RUN/SS

VIAS TO PG

VIAS TO VIN

OUTLINE OF LOCAL

GROUND PLANE

1938 F08

L1 C2

RRT

RPG

VOUT

D1 C1

GND

input to eliminate the voltage overshoot (it also reduces

the peak input current). A 0.1µF capacitor improves high

frequency ﬁltering. For high input voltages its impact on

efﬁciency is minor, reducing efﬁciency by 1.5 percent for

a 5V output at full load operating from 24V.

High Temperature Considerations

The PCB must provide heat sinking to keep the LT1938

cool. The Exposed Pad on the bottom of the package must

be soldered to a ground plane. This ground should be tied

to large copper layers below with thermal vias; these lay-

ers will spread the heat dissipated by the LT1938. Place

additional vias can reduce thermal resistance further. With

these steps, the thermal resistance from die (or junction)

to ambient can be reduced to θJA = 35°C/W or less. With

100 LFPM airﬂow, this resistance can fall by another 25%.

Further increases in airﬂow will lead to lower thermal re-

sistance. Because of the large output current capability of

the LT1938, it is possible to dissipate enough heat to raise

the junction temperature beyond the absolute maximum of

125°C. When operating at high ambient temperatures, the

LT1938

1938fa

maximum load current should be derated as the ambient

temperature approaches 125°C.

Power dissipation within the LT1938 can be estimated by

calculating the total power loss from an efﬁciency measure-

ment and subtracting the catch diode loss and inductor

loss. The die temperature is calculated by multiplying the

LT1938 power dissipation by the thermal resistance from

junction to ambient.

Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation when the LT1938 is Connected to a Live Supply

APPLICATIONS INFORMATION

LT1938

4.7µF

VIN

20V/DIV

IIN

10A/DIV

20µs/DIV

VIN

CLOSING SWITCH

SIMULATES HOT PLUG

IIN

(9a)

(9b)

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

LT1938

4.7µF0.1µF

0.7Ω VIN

20V/DIV

IIN

10A/DIV

20µs/DIV

DANGER

RINGING VIN MAY EXCEED

ABSOLUTE MAXIMUM RATING

(9c)

LT1938

4.7µF

22µF

35V

AI.EI.

1938 F09

VIN

20V/DIV

IIN

10A/DIV

20µs/DIV

Other Linear Technology Publications

Application Notes 19, 35 and 44 contain more detailed

descriptions and design information for buck regulators

and other switching regulators. The LT1376 data sheet

has a more extensive discussion of output ripple, loop

compensation and stability testing. Design Note 100

shows how to generate a bipolar output supply using a

buck regulator.

LT1938

1938fa

TYPICAL APPLICATIONS

5V Step-Down Converter

3.3V Step-Down Converter

BIAS

VIN BD

VIN

6.3V TO 25V

VOUT

2.2A

4.7µF

0.47µF

22µF

200k

f = 800kHz

D: DIODES INC. DFLS230L

L: TAIYO YUDEN NP06DZB6R8M

20k

60.4k

6.8µH

590k

GND

680pF

ON OFF

LT1938

1938 TA02

RUN/SS BOOST

BIAS

VIN BD

VIN

4.4V TO 25V

VOUT

3.3V

2.2A

4.7µF

0.47µF

22µF

200k

f = 800kHz

D: DIODES INC. DFLS230L

L: TAIYO YUDEN NP06DZB4R7M

16.2k

60.4k

4.7µH

324k

GND

680pF

ON OFF

LT1938

1938 TA03

RUN/SS BOOST

LT1938

1938fa

TYPICAL APPLICATIONS

2.5V Step-Down Converter

5V, 2MHz Step-Down Converter

BIAS

VIN BD

VIN

4V TO 25V

VOUT

2.5V

2.2A

4.7µF

1µF

47µF

200k

f = 600kHz

D1: DIODES INC. DFLS230L

D2: MBR0540

L: TAIYO YUDEN NP06DZB4R7M

22.1k

84.5k

4.7µH

196k

GND

680pF

ON OFF

LT1938

3684 TA04

RUN/SS BOOST

BIAS

VIN BD

VIN

8.6V TO 22V

VOUT

2.2µF

0.47µF

10µF

200k

f = 2MHz

D: DIODES INC. DFLS230L

L: SUMIDA CDRH4D22/HP-2R2

20k

16.9k

2.2µH

590k

GND

680pF

ON OFF

LT1938

1938 TA05

RUN/SS BOOST

LT1938

1938fa

TYPICAL APPLICATIONS

12V Step-Down Converter

BIAS

VIN BD

VIN

15V TO 25V

VOUT

12V

2.2A

10µF

0.47µF

22µF

100k

f = 800kHz

D: DIODES INC. DFLS230L

L: NEC/TOKIN PLC-0755-100

30k

60.4k

10µH

845k

GND

680pF

ON OFF

LT1938

3684 TA06

RUN/SS BOOST

LT1938

1938fa

TYPICAL APPLICATIONS

1.8V Step-Down Converter

BIAS

VIN BD

VIN

3.5V TO 25V

VOUT

1.8V

2.2A

4.7µF

0.47µF

47µF

200k

f = 500kHz

D: DIODES INC. DFLS230L

L: TAIYO YUDEN NP06DZB3R3M

15.4k

105k

3.3µH

84.5k

GND

680pF

ON OFF

LT1938

1938 TA07

RUN/SS BOOST

LT1938

1938fa

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.

DD Package

10-Lead Plastic DFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1699)

PACKAgE DESCRIPTION

3.00 ±0.10

(4 SIDES)

NOTE:

1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).

CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT

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

0.38 ± 0.10

BOTTOM VIEW—EXPOSED PAD

1.65 ± 0.10

(2 SIDES)

0.75 ±0.05

R = 0.115

TYP

2.38 ±0.10

(2 SIDES)

106

PIN 1

TOP MARK

(SEE NOTE 6)

0.200 REF

0.00 – 0.05

(DD) DFN 1103

0.25 ± 0.05

2.38 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

1.65 ±0.05

(2 SIDES)2.15 ±0.05

0.50

BSC

0.675 ±0.05

3.50 ±0.05

PACKAGE

OUTLINE

0.25 ± 0.05

0.50 BSC

LT1938

1938fa

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

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

 LINEAR TECHNOLOGY CORPORATION 2007

LT 1107 REV A • PRINTED IN USA

PART NUMBER DESCRIPTION COMMENTS

LT1933 500mA (IOUT), 500kHz Step-Down Switching Regulator in

SOT-23

VIN: 3.6V to 36V, VOUT(MIN) = 12V, IQ = 1.6mA, ISD < 1µA, ThinSOTTM

Package

LT3437 60V, 400mA (IOUT), MicroPower Step-Down DC/DC

Converter with Burst Mode Operation

VIN: 3.3V to 80V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, DFN Package

LT1936 36V, 1.4A (IOUT), 500kHz High Efﬁciency Step-Down

DC/DC Converter

VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.9mA, ISD < 1µA, MS8E Package

LT3493 36V, 1.2A (IOUT), 750kHz High Efﬁciency Step-Down

DC/DC Converter

VIN: 3.6V to 40V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1µA, DFN Package

LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Efﬁciency Step-

Down DC/DC Converter with Burst Mode Operation

VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA, TSSOP16E

Package

LT1767 25V, 1.2A (IOUT), 1.1MHz, High Efﬁciency Step-Down

DC/DC Converter

VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 6µA, MS8E Package

LT1940 Dual 25V, 1.4A (IOUT), 1.1MHz, High Efﬁciency Step-Down

DC/DC Converter

VIN: 3.6V to 25V, VOUT(MIN) = 1.20V, IQ = 3.8mA, ISD < 30µA, TSSOP16E

Package

LT1766 60V, 1.2A (IOUT), 200kHz, High Efﬁciency Step-Down

DC/DC Converter

VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.5mA, ISD < 25µA, TSSOP16E

Package

LT3434/LT3435 60V, 2.4A (IOUT), 200/500kHz, High Efﬁciency Step-Down

DC/DC Converter with Burst Mode Operation

VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA, TSSOP16E

Package

LT3481 36V, 2A (IOUT), Micropower 2.8MHz, High Efﬁciency

Step-Down DC/DC Converter

VIN: 3.6V to 36V, VOUT(MIN) = 1.265V, IQ = 5µA, ISD < 1µA, 3mm × 3mm

DFN and MS10E Packages

1.265V Step-Down Converter

BIAS

VIN BD

VIN

3.6V TO 25V

VOUT

1.265V

2.2A

4.7µF

0.47µF

47µF

f = 500kHz

D: DIODES INC. DFLS240L

L: TAIYO YUDEN NP06DZB3R3M

13k

105k

3.3µH

GND

680pF

ON OFF

LT1938

1938 TA08

RUN/SS BOOST

TYPICAL APPLICATION

RELATED PARTS