Datasheet

R
FB1
=
- 1
V
OUT
0.8
x R
FB2
LM20323
SNVS557C –MAY 2008–REVISED APRIL 2013 www.ti.com
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
52.3 10 5.0
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.
(6)
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
150 µF, 6.3V POSCAP (6TPB150MAZB) output capacitor.
Table 2. Recommended Compensation for
C
OUT
= 150 µF, I
OUT
= 3A
V
IN
V
OUT
L (µH) R
C
(kΩ) C
C1
(nF)
12 5 6.8 45.3 4.7
12 3.3 5.6 32.4 4.7
12 2.5 4.7 30.9 3.3
12 1.5 3.3 19.1 3.3
12 1.2 2.2 21.5 2.2
12 0.8 1.5 15 2.2
5 3.3 2.2 29.4 2.2
5 2.5 3.3 37.4 2.2
5 1.5 2.2 26.7 2.2
5 1.2 2 22.1 2.2
5 0.8 1.5 15 2.2
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 25 shows the transfer functions for power stage,
feedback/compensation network, and the resulting compensated loop for the LM20323.
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