Datasheet

DocID022450 Rev 7 21/45
L7986 Application information
45
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 k Ω and 5 k Ω .
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
):
Equation 25
4. Calculate C
5
by placing the second pole at four times the system bandwidth (BW):
Equation 26
5. Set also the first pole at four times the system bandwidth and also the second zero at
the output filter double pole:
R
4
BW
f
LC
---------- KR
1
⋅⋅=
C
4
1
π R
4
f
LC
⋅⋅
---------------------------=
C
5
C
4
2π R
4
C
4
4BW⋅ 1–⋅⋅⋅
--------------------------------------------------------------=