Datasheet

DocID022801 Rev 5 21/42
A7986A Application information
Figure 10. Type III compensation network
In Figure 11, the Bode diagram of the PWM and LC filter transfer function (G
PW0
· G
LC
(f))
and the open loop gain (G
LOOP
(f) = G
PW0
· G
LC
(f) · G
TYPEIII
(f)) are drawn.
Figure 11. Open loop gain: module Bode diagram
The guidelines for positioning the poles and the zeroes and for calculating the component
values can be summarized as follows:
1. Choose a value for R
1
, usually between 1 kΩ and 5 kΩ.
2. Choose a gain (R
4
/R
1
) in order to have the required bandwidth (BW), that means:
Equation 24
where K is the feed-forward constant and 1/K is equal to 18.
3. Calculate C
4
by placing the zero at 50% of the output filter double pole frequency (f
LC
):
R
4
BW
f
LC
---------- KR
1
⋅⋅=