Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Boost, SEPIC MSOP 8 EP

LT1946A

sn1946a 1946afs

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

The LT

1946A is a fixed frequency step-up DC/DC con-

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

generating 12V at 430mA from a 5V input, the LT1946A is

ideal for powering large TFT-LCD panels. The LT1946A

switches at 2.7MHz, 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

1.1mm high.

■

TFT-LCD Bias Supplies

■

GPS Receivers

■

DSL Modems

■

Local Power Supply

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

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1.5A, 36V Internal Switch

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2.7MHz Switching Frequency

■

Integrated Soft-Start Function

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Adjustable Output from V

to 35V

■

Low V

CESAT

Switch: 300mV at 1.5A (Typical)

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12V at 430mA from a 5V Input

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Small Thermally Enhanced 8-Lead MSOP Package

2.7MHz Boost DC/DC

Converter with 1.5A Switch

and Soft-Start

COMP

LT1946A

2.2µH

100nF

270pF

27.4k

OUT

12V

430mA

1946A TA01

2.2µF

GND*SS

SHDN

C1: 2.2µF, X5R or X7R, 6.3V

C2: 2.2µF, X5R or X7R, 16V

D1: MICROSEMI UPS120 OR EQUIVALENT

L1: SUMIDA CR43-2R2

* EXPOSED PAD MUST ALSO BE GROUNDED

182k

21k

OFF ON

LOAD CURRENT (mA)

100 200 300 400 500

EFFICIENCY (%)

1946A TA01

Efficiency

Figure 1. 5V to 12V, 430mA Step-Up DC/DC Converter

Summary of content (12 pages)

PAGE 1
LT1946A 2.7MHz Boost DC/DC Converter with 1.5A Switch and Soft-Start U FEATURES ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 1.5A, 36V Internal Switch 2.7MHz Switching Frequency Integrated Soft-Start Function Adjustable Output from VIN to 35V Low VCESAT Switch: 300mV at 1.
PAGE 2
LT1946A 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 FB Voltage .............................................................. 2.5V Current into FB Pin ............................................... ±1mA SHDN Voltage .......................................................... 16V Maximum Junction Temperature ...............
PAGE 3
LT1946A U W TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency 3000 1.27 2700 1.26 1.25 1.24 1.23 1.22 1.21 2400 TA = –30°C 2100 TA = 100°C 1800 1500 TA = 25°C 1200 900 600 300 1.20 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 0 125 0 0.2 0.4 0.6 0.8 FEEDBACK VOLTAGE (V) 1946A G01 1.2 –25 0 25 50 75 TEMPERATURE (°C) 100 125 1946A G03 Switching Waveforms for Figure 1 Circuit Quiescent Current 0.35 4.0 VOUT 100mV/DIV AC COUPLED 3.8 QUIESCENT CURRENT (mA) 0.30 0.
PAGE 4
LT1946A U U U PI FU CTIO S 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. External compensation consists of placing a resistor and capacitor in series from VC to GND. Typical capacitor range is from 90pF to 270pF. Typical resistor range is from 25k to 120k. FB (Pin 2): Feedback Pin. Reference voltage is 1.25V. Connect resistive divider tap here. Minimize trace area at FB.
PAGE 5
LT1946A W BLOCK DIAGRA SS VC COMP 8 1 7 4µA 120k 90pF – DRIVER A2 VIN 6 1.25V REFERENCE + + A1 FB 0.5V Q1 + 0.01Ω – + R2 (EXTERNAL) 4 GND A3 ÷3 2.7MHz OSCILLATOR – SHDN S Q RAMP GENERATOR R1 (EXTERNAL) SHUTDOWN R Σ – VOUT 5 SW COMPARATOR 3 2 EXPOSED PAD 1946A F02 FB Figure 2.
PAGE 6
LT1946A U OPERATIO The LT1946A 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 7
LT1946A U W U U APPLICATIO S I FOR ATIO capacitor is sufficient for most applications, but systems with very low output currents may need only a 1µF or smaller output capacitor. Solid tantalum or OSCON capacitors can be used, but they will occupy more board area than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating. Ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the LT1946A. A 2.
PAGE 8
LT1946A U W U U APPLICATIO S I FOR ATIO 1 2 • π • ESR • C OUT As with any feedback loop, identifying the gain and phase contribution of the various elements in the loop is critical. Figure 4 shows the key equivalent elements of a boost converter. Because of the fast current control loop, the power stage of the IC, inductor, and diode have been replaced by the equivalent transconductance amplifier GMP. GMP acts as a current source where the output current is proportional to the VC voltage.
PAGE 9
LT1946A U W U U APPLICATIO S I FOR ATIO Setting Output Voltage 100 To set the output voltage, select the values of R1 and R2 (see Figure 1) according to the following equation: GAIN (f) 50 V  R1 = R2  OUT – 1  1.25V  0 A good range for R2 is from 5k to 30k. –50 100 1k 10k 100k FREQUENCY (Hz) 1M 1946A FO5a PHASE (f) 0 –100 Layout Hints The high speed operation of the LT1946A demands careful attention to board layout. You will not get advertised performance with careless layouts.
PAGE 10
LT1946A U TYPICAL APPLICATIO S Low Profile (< 1.1mm Tall) Triple Output TFT Supply (10V, –10V, 20V) D2 D3 VON 20V 5mA C5 0.1µF L1 1.5µH VIN 5V OFF ON 3 8 + C1 4.7µF 7 CSS 100nF D1 6 VIN 5 SW R1 75k SHDN SS LT1946A COMP VC FB AVDD 10V 475mA 2 C2 20µF GND* 1 RC 59k CC 150pF 4 C1–C6: X5R or X7R C1: 4.7µF, 6.3V C2: 2× 10µF, 10V C3: 1µF, 25V C4: 2.2µF, 10V C5–C6: 0.
PAGE 11
LT1946A U TYPICAL APPLICATIO S Triple Output TFT Supply Uses SEPIC Topology for Output Disconnect D2 VON 23V 10mA C4 0.22µF D3 VOFF –12V 10mA C5 0.22µF L1 10µH VIN 12V ± 10% 3 OFF ON 8 + 1 C1 2.2µF D1 6 VIN 5 SW C1–C5: X5R or X7R C1: 2.2µF, 6.3V C2: 2× 10µF, 16V C3: 1µF, 25V C4: 0.22µF, 25V C5: 0.22µF, 16V L2 10µH SHDN SS LT1946A FB R1 84.5k 2 C2 20µF VC COMP GND* 4 7 CSS 100nF C3 1µF AVDD 12V 250mA R2 9.
PAGE 12
LT1946A U TYPICAL APPLICATIO S Low Profile (< 1.1mm Tall) Triple Output TFT Supply (8V, – 8V, 24V) D2 D4 D3 C6 0.1µF C5 0.1µF D5 VON 23V 5mA C7 0.1µF Efficiency OFF ON 3 8 + C1 4.7µF 7 6 5 VIN SW LT1946A COMP VC 1 FB AVDD 8V 375mA 2 C2 20µF GND* 4 C4 1µF R3 5.23k CSS 100nF 85 80 R2 28.7k SHDN SS 90 D1 EFFICIENCY (%) L1 1.2µH VIN 3.3V 75 70 65 60 VON LOAD = 5mA VOFF LOAD = 10mA 55 C1–C8: X5R or X7R C1: 4.7µF, 6.3V C2: 2× 10µF, 10V C3: 2.2µF, 10V C4: 1µF, 25V C5, C6, C8: 0.