Datasheet

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LT1931/LT1931A

1931fa

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Fixed Frequency 1.2MHz/2.2MHz Operation

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Very Low Noise: 1mV

P-P

Output Ripple

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–5V at 350mA from 5V Input

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–12V at 150mA from 5V Input

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Uses Small Surface Mount Components

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Wide Input Range: 2.6V to 16V

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Low Shutdown Current: <1µA

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Low V

CESAT

Switch: 400mV at 1A

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Pin-for-Pin Compatible with the LT1611

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Low Profile (1mm) ThinSOT

Package

The LT

1931/LT1931A are the industry’s highest power

inverting SOT-23 current mode DC/DC converters. Both

parts include a 1A integrated switch allowing high current

outputs to be generated in a small footprint. The LT1931

switches at 1.2MHz while the LT1931A switches at 2.2MHz.

These high speeds enable the use of tiny, low cost

capacitors and inductors 2mm or less in height. The

LT1931 is capable of generating –5V at 350mA or –12V

at 150mA from a 5V supply, while the LT1931A can

generate –5V at 300mA using significantly smaller induc-

tors. Both parts are easy pin-for-pin upgrades for higher

power LT1611 applications.

The LT1931/LT1931A operate in a dual inductor inverting

topology that filters both the input side and output side

current. Very low output voltage ripple approaching 1mV

P-P

can be achieved when ceramic output capacitors are used.

Fixed frequency switching ensures a clean output free

from low frequency noise typically present with charge

pump solutions. The low impedance output remains within

1% of nominal during large load steps. The 36V switch

allows V

to V

OUT

differential of up to 34V.

The LT1931/LT1931A are available in the 5-lead ThinSOT

package.

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Disk Drive MR Head Bias

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Digital Camera CCD Bias

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LCD Bias

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GaAs FET Bias

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Local Low Noise/Low Impedance Negative Supply

1.2MHz/2.2MHz Inverting

DC/DC Converters in ThinSOT

Figure 1. 5V to –5V, 350mA Inverting DC/DC Converter

OUT

–5V

350mA

1931 F01

L1A

10µH

L1B

10µH

GND

LT1931

C1: TAIYO YUDEN X5R JMK212BJ475MG

C2: TAIYO YUDEN X5R LMK212BJ105MG

C3: TAIYO YUDEN X5R JMK325BJ226MM

D1: ON SEMICONDUCTOR MBR0520

L1: SUMIDA CLS62-100

4.7µF

22µF

1µF

10k

29.4k

220pF

NFB

SHDN

LOAD CURRENT (mA)

EFFICIENCY (%)

100

200

250

1931 TA01

150

300

350

Efficiency

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a

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

respective owners.

Summary of content (12 pages)

PAGE 1
LT1931/LT1931A 1.2MHz/2.2MHz Inverting DC/DC Converters in ThinSOT U FEATURES DESCRIPTIO ■ The LT®1931/LT1931A are the industry’s highest power inverting SOT-23 current mode DC/DC converters. Both parts include a 1A integrated switch allowing high current outputs to be generated in a small footprint. The LT1931 switches at 1.2MHz while the LT1931A switches at 2.2MHz. These high speeds enable the use of tiny, low cost capacitors and inductors 2mm or less in height.
PAGE 2
LT1931/LT1931A W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) VIN Voltage .............................................................. 16V SW Voltage ................................................– 0.4V to 36V NFB Voltage ............................................................. – 2V Current Into NFB Pin ............................................ ±1mA SHDN Voltage .......................................................... 16V Maximum Junction Temperature ..........
PAGE 3
LT1931/LT1931A U W TYPICAL PERFOR A CE CHARACTERISTICS Quiescent Current Feedback Pin Voltage 7.0 90 NOT SWITCHING 80 6.5 LT1931A 5.5 5.0 4.5 LT1931 4.0 SHDN PIN CURRENT (µA) –1.27 6.0 FEEDBACK VOLTAGE (V) QUIESCENT CURRENT (mA) Shutdown Pin Current –1.28 –1.26 –1.25 –1.24 60 50 40 30 LT1931 20 0 3.0 –50 –25 0 50 25 TEMPERATURE (°C) 75 –1.22 –50 100 –25 0 25 50 TEMPERATURE (°C) 75 –10 100 0 Current Limit 1.2 VCESAT (V) 0.6 TA = 25°C 0.40 2.3 0.35 2.1 0.25 0.20 0.
PAGE 4
LT1931/LT1931A W BLOCK DIAGRA VIN 5 VIN R5 80k R6 80k 1 SW + – Q1 VOUT CPL (OPTIONAL) Q2 x10 – A1 gm RC Σ RAMP GENERATOR + COMPARATOR A2 R LATCH S DRIVER Q3 Q + CC R3 30k R1 (EXTERNAL) NFB R2 (EXTERNAL) 0.01Ω – 1.2MHz OSCILLATOR R4 150k SHDN 3 NFB 4 SHUTDOWN 2 GND 1931 BD Figure 2 U OPERATIO The LT1931 uses a constant frequency, current mode control scheme to provide excellent line and load regulation.
PAGE 5
LT1931/LT1931A U W U U APPLICATIO S I FOR ATIO LT1931A AND LT1931 DIFFERENCES: Switching Frequency The key difference between the LT1931A and LT1931 is the faster switching frequency of the LT1931A. At 2.2MHz, the LT1931A switches at nearly twice the rate of the LT1931. Care must be taken in deciding which part to use.
PAGE 6
LT1931/LT1931A U W U U APPLICATIO S I FOR ATIO The inductors shown in Table 2 for use with the LT1931A were chosen for their small size. For better efficiency, use similar valued inductors with a larger volume. For instance, the Sumida CR43 series, in values ranging from 3.3µH to 10µH, will give a LT1931A application a few percentage points increase in efficiency. CAPACITOR SELECTION Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage.
PAGE 7
LT1931/LT1931A U W U U APPLICATIO S I FOR ATIO VOUT 20mV/DIV AC COUPLED VOUT 2V/DIV IL1A + IL1B 0.5A/DIV AC COUPLED IIN 0.5A/DIV AC COUPLED LOAD 200mA CURRENT 100mA VSHDN 5V 0V 100µs/DIV 1931 F04 500µs/DIV Figure 4. Transient Response of Inverting Converter with 220pF Phase Lead Capacitor Figure 6. Start-Up Waveforms for 5V to – 5V Application (Figure 1). No Soft-Start Circuit. VOUT Reaches – 5V in 500µs; Input Current Peaks at 800mA VOUT 0.
PAGE 8
LT1931/LT1931A U W U U APPLICATIO S I FOR ATIO DIODE SELECTION VOUT 2V/DIV IIN 0.5A/DIV AC COUPLED VSS 5V 0V 200µs/DIV 1931 F08 A Schottky diode is recommended for use with the LT1931/ LT1931A. The Motorola MBR0520 is a very good choice. Where the input to output voltage differential exceeds 20V, use the MBR0530 (a 30V diode). These diodes are rated to handle an average forward current of 0.5 A. In applications where the average forward current of the diode exceeds 0.
PAGE 9
LT1931/LT1931A U TYPICAL APPLICATIO S 5V to –12V Inverting Converter C2 1µF L1A 10µH VIN 5V Efficiency 100 L1B 10µH 95 90 D1 SHDN R1 84.5k LT1931 C1 4.
PAGE 10
LT1931/LT1931A U TYPICAL APPLICATIO S 2.2MHz, 5V to –5V Converter Uses Tiny Chip Inductors C2 1µF L1 3.3µH VIN 5V Efficiency 80 L2 3.3µH D1 VIN SW SHDN C1 2.2µF R1 28.7k LT1931A C4 68pF C3 4.
PAGE 11
LT1931/LT1931A U TYPICAL APPLICATIO S SLIC Power Supply with – 21.6V and – 65V Outputs, Uses Soft-Start L1 10µH VIN 5V C1 4.7µF 16V R1 1Ω VIN C2 1µF 35V SW D1 RSS 15k 3 LT1931 SHDN VSS 2 NFB GND 1 VOUT1 –21.6V 48mA* R3 16.2k R2 1k C8 1000pF CSS 68nF COM C5 4.7µF 25V C3 1µF 35V R4 2.7k D2 3 *TOTAL OUTPUT POWER NOT TO EXCEED 1.3W C1 TO C7: X5R OR X7R D1, D2: BAV99 OR EQUIVALENT L1: SUMIDA CR43-100 2 1 C4 1µF 35V C6 4.7µF 25V D3 3 2 1 C7 4.
PAGE 12
LT1931/LT1931A U TYPICAL APPLICATIO 2.2MHz, 12V to – 5V Converter Uses Low Profile Coupled Inductor C2 0.1µF L1A 4.7µH VIN 12V L1B 4.7µH D1 VIN SHDN R1 28.7k LT1931A C1 2.2µF VOUT –5V 450mA SW C3 4.7µF NFB GND R2 10k C1: TAIYO YUDEN Y5V EMK212F225ZG C2: 0.