LM20144 datasheet(16/30 Pages) TI

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RT =

154750

fSW

- 55

RFB1 =

- 1

VOUT

0.8

x RFB2

LM20144

SNVS529G – OCTOBER 2007 – REVISED MARCH 2013

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As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty

cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output

current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance

capacitors to provide the best input filtering for the device.

SETTING THE OUTPUT VOLTAGE (RFB1, RFB2)

The resistors RFB1 and RFB2 are selected to set the output voltage for the device. Table 1, shown below, provides

suggestions for RFB1 and RFB2 for common output voltages.

Table 1. Suggested Values for RFB1 and RFB2

RFB1(kΩ)

RFB2(kΩ)

VOUT

short

open

0.8

4.99

1.2

8.87

10.2

1.5

12.7

10.2

1.8

21.5

10.2

2.5

31.6

10.2

3.3

If different output voltages are required, RFB2 should be selected to be between 4.99 kΩ to 49.9 kΩ and RFB1 can

be calculated using the equation below.

(7)

ADJUSTING THE OPERATING FREQUENCY (RT)

The operating frequency of the LM20144 can be adjusted by connecting a resistor from the RT pin to ground.

The equation shown below can be used to calculate the value of RT for a given operating frequency.

where

•

fSW is the switching frequency in kHz

•

RT is the frequency adjust resistor in kΩ

(8)

Please refer to the curve Oscillator Frequency verses RT in the typical performance characteristics section If the

RT resistor is omitted, the device will not operate.

LOOP COMPENSATION (RC1, CC1)

The purpose of loop compensation is to meet static and dynamic performance requirements while maintaining

adequate stability. Optimal loop compensation depends on the output capacitor, inductor, load, and the device

itself. Table 2 below gives values for the compensation network that will result in a stable system when using a

100 µF, 6.3V ceramic X5R output capacitor and 1 µH inductor.

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