Datasheet

R
FB1
=
- 1
V
OUT
0.8
x R
FB2
LM20125
www.ti.com SNVS525E –OCTOBER 2007–REVISED MARCH 2013
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 (R
FB1
, R
FB2
)
The resistors R
FB1
and R
FB2
are selected to set the output voltage for the device. Table 1, shown below, provides
suggestions for R
FB1
and R
FB2
for common output voltages.
Table 1. Suggested Values for R
FB1
and R
FB2
R
FB1
(kΩ) R
FB2
(kΩ) V
OUT
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
If different output voltages are required, R
FB2
should be selected to be between 4.99 kΩ to 49.9 kΩ and R
FB1
can
be calculated using the equation below.
(7)
LOOP COMPENSATION (R
C1
, C
C1
)
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.
Table 2. Recommended Compensation for
C
OUT
= 100 µF and L = 1.5 µH
V
IN
V
OUT
C
C1
(nF) R
C1
(kΩ)
5.00 3.30 3.3 17.8
5.00 2.50 3.3 12.1
5.00 1.80 3.3 9.31
5.00 1.50 3.3 6.81
5.00 1.20 3.3 4.87
5.00 0.80 3.3 3.16
3.30 2.50 3.3 14.3
3.30 1.80 3.3 7.87
3.30 1.50 3.3 5.49
3.30 1.20 3.3 3.65
3.30 0.80 4.7 2.74
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 27, shown below, shows the transfer functions for
power stage, feedback/compensation network, and the resulting closed loop system for the LM20125.
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