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LT1946

1946fb

3.3V

LT1946

4.7µH

100nF

470pF

49.9k

OUT

430mA

1946 F01

20µF

2.2µF

GNDCOMPSS

SHDNOFF ON

C1: 2.2µF, X5R OR X7R, 6.3V

C2: 2 × 10µF, X5R OR X7R, 10V

D1: MICROSEMI UPS120 OR EQUIVALENT

L1: TDK RLF5018T-4R7M1R4

28.7k

5.23k

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TFT-LCD Bias Supplies

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GPS Receivers

■

DSL Modems

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Local Power Supplies

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1.5A, 36V Internal Switch

■

1.2MHz Switching Frequency

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Integrated Soft-Start Function

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Output Voltage Up to 34V

■

Low V

CESAT

Switch: 300mV at 1.5A (Typ)

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8V at 430mA from a 3.3V Input

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Small 8-Lead MSOP Package

1.2MHz Boost

DC/DC Converter with

1.5A Switch and Soft-Start

Figure 1. 3.3V to 8V, 430mA Step-Up DC/DC Converter

The LT

1946 is a fixed frequency step-up DC/DC converter

containing an internal 1.5A, 36V switch. Capable of gener-

ating 8V at 430mA from a 3.3V input, the LT1946 is ideal

for large TFT-LCD panel power supplies. The LT1946

switches at 1.2MHz, allowing the use of tiny, low profile

inductors and low value ceramic capacitors. Loop com-

pensation can be either internal or external, giving the user

flexibility in setting loop compensation and allowing opti-

mized transient response with low ESR ceramic output

capacitors. Soft-start is controlled with an external capaci-

tor, which determines the input current ramp rate during

start-up.

The 8-lead MSOP package and high switching frequency

ensure a low profile overall solution less than 1.2mm high.

LOAD CURRENT (mA)

EFFICIENCY (%)

300

500

1946 F01b

100 200 400

Efficiency

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

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

All other trademarks are the property of their respective owners.

Summary of content (12 pages)

PAGE 1
LT1946 1.2MHz Boost DC/DC Converter with 1.5A Switch and Soft-Start U FEATURES ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT®1946 is a fixed frequency step-up DC/DC converter containing an internal 1.5A, 36V switch. Capable of generating 8V at 430mA from a 3.3V input, the LT1946 is ideal for large TFT-LCD panel power supplies. The LT1946 switches at 1.2MHz, allowing the use of tiny, low profile inductors and low value ceramic capacitors.
PAGE 2
LT1946 W W W AXI U U ABSOLUTE RATI GS (Note 1) VIN Voltage ............................................................. 16V SW Voltage ............................................... – 0.4V to 36V FB Voltage ............................................................. 2.5V SHDN Voltage ......................................................... 16V Current Into FB Pin .............................................. ±1mA Maximum Junction Temperature .........................
PAGE 3
LT1946 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VSHDN = VIN unless otherwise specified. (Note 2) SYMBOL CONDITIONS MIN SHDN Input Voltage High TYP MAX UNITS 2.4 V SHDN Input Voltage Low SHDN Pin Bias Current VSHDN = 3V VSHDN = 0V 16 0 Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device.
PAGE 4
LT1946 U U U PI FU CTIO S VIN (Pin 6): Input Supply Pin. Must be locally bypassed. VC (Pin 1): Error Amplifier Output Pin. Tie external compensation network to this pin, or use the internal compensation network by shorting the VC pin to the COMP pin. COMP (Pin 7): Internal Compensation Pin. Provides an internal compensation network. Tie directly to the VC pin for internal compensation. Tie to GND if not used. FB (Pin 2): Feedback Pin. Reference voltage is 1.250V. Connect resistive divider tap here.
PAGE 5
LT1946 U OPERATIO The LT1946 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Please refer to Figure 2 for the following description of the part’s operation. At the start of the oscillator cycle, the SR latch is set, turning on the power switch Q1. The switch current flows through the internal current sense resistor generating a voltage.
PAGE 6
LT1946 U W U U APPLICATIO S I FOR ATIO Capacitor Selection Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage. Multilayer ceramic capacitors are an excellent choice, as they have an extremely low ESR and are available in very small packages. X5R or X7R dielectrics are preferred, as these materials retain the capacitance over wide voltage and temperature ranges. A 4.
PAGE 7
LT1946 U W U U APPLICATIO S I FOR ATIO Compensation—Theory – Like all other current mode switching regulators, the LT1946 needs to be compensated for stable and efficient operation. Two feedback loops are used in the LT1946: a fast current loop which does not require compensation, and a slower voltage loop which does. Standard Bode plot analysis can be used to understand and adjust the voltage feedback loop. gmp VOUT + COUT + VC 1.
PAGE 8
LT1946 U W U U APPLICATIO S I FOR ATIO Diode Selection 100 A Schottky diode is recommended for use with the LT1946. The Microsemi UPS120 is a very good choice. Where the input to output voltage differential exceeds 20V, use the UPS140 (a 40V diode). These diodes are rated to handle an average forward current of 1A. For applications where the average forward current of the diode is less than 0.
PAGE 9
LT1946 U TYPICAL APPLICATIO S Low Profile, Triple Output TFT Supply (10V, –10V, 20V) D2 D3 VON 20V 5mA C5 0.1µF L1 5.4µH VIN 3.3V TO 5V 3 OFF ON 8 + 7 C1 4.7µF 6 VIN 5 SW R1 75k SHDN SS LT1946 COMP VC 1 RC 33.3k CC 470pF CSS 100nF AVDD 10V 450mA, VIN = 5V 275mA, VIN = 3.3V D1 FB 2 C2 20µF GND 4 C1 TO C6: X5R OR X7R C1: 4.7µF, 6.3V C2: 2 × 10µF, 10V C3: 1µF, 25V C4: 2.2µF, 10V C5, C6: 0.
PAGE 10
LT1946 U TYPICAL APPLICATIO S 12V Output Boost Converter L1 4.7µH VIN 3.3V TO 5V OFF ON C1 4.7µF 3 1 RC 33.3k 6 VIN CSS 100nF 5 SW LT1946 FB VC 8 2 C2 4.7µF COMP GND 7 VOUT 12V 410mA, VIN = 5V 275mA, VIN = 3.3V R1 84.5k SHDN SS CC 470pF D1 R2 9.76k 4 C1: 4.7µF, X5R OR X7R, 6.3V C2: 4.7µF, X5R OR X7R, 16V D1: MICROSEMI UPS120 OR EQUIVALENT L1: TDK RLF5018T-4R7M1R4 1946 TA02 Efficiency Transient Response 90 VIN = 5V VOUT 100mV/DIV AC COUPLED 85 VIN = 3.
PAGE 11
LT1946 U PACKAGE DESCRIPTIO MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 3.20 – 3.45 (.126 – .136) 0.254 (.010) 0.42 ± 0.038 (.0165 ± .0015) TYP 0.65 (.0256) BSC 8 0.52 (.0205) REF 7 6 5 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) DETAIL “A” 0° – 6° TYP GAUGE PLANE 1 0.53 ± 0.152 (.021 ± .006) RECOMMENDED SOLDER PAD LAYOUT 2 3 4 1.10 (.043) MAX DETAIL “A” 0.86 (.
PAGE 12
LT1946 U TYPICAL APPLICATIO Low Profile, Triple Output TFT Supply (8V, – 8V, 23V) D2 D3 D4 C5 0.1µF L1 5.4µH VIN 3.3V OFF ON 3 8 + 7 C1 4.7µF C6 0.1µF C7 0.1µF AVDD 8V 375mA 5 SW R2 28.7k SHDN LT1946 FB COMP VC 2 C2 20µF GND 1 RC 49.9k CC 470pF CSS 100nF VON 23V 5mA D1 6 VIN SS D5 4 C1 TO C8: X5R OR X7R C1: 4.7µF, 6.3V C2: 2 × 10µF, 10V C3: 2.2µF, 10V C4: 1µF, 25V C5, C6, C8: 0.1µF, 10V C7: 0.