- 1

0.8

LM20133, LM20133Q

SNVS526F – 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.

short

open

0.8

4.99

10

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

be calculated using the equation below.

(7)

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.

5.00

3.30

4.7

16.2

5.00

2.50

4.7

11.3

5.00

1.80

4.7

8.45

5.00

1.50

4.7

5.23

5.00

1.20

4.7

3.32

5.00

0.80

4.7

1.62

3.30

1.80

4.7

9.53

3.30

1.50

4.7

4.87

3.30

1.20

4.7

3.24

3.30

0.80

4.7

1.62

If the desired solution differs from the table above the loop transfer function should be analyzed to optimize the

loop compensation. The overall loop transfer function is the product of the power stage and the feedback network

transfer functions. For stability purposes, the objective is to have a loop gain slope that is -20db/decade from a

very low frequency to beyond the crossover frequency. Figure 32, shown below, shows the transfer functions for

power stage, feedback/compensation network, and the resulting closed loop system for the LM20133.

16

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