Datasheet

DocID022446 Rev 8 21/44
L7985 Application information
44
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
):
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:
Equation 27
The suggested maximum system bandwidth is equal to the switching frequency divided by
3.5 (F
SW
/ 3.5), anyhow, lower than 100 kHz if the F
SW
is set higher than 500 kHz.
R
4
BW
f
LC
--------- - KR
1
=
C
4
1
 R
4
f
LC

---------------------------=
C
5
C
4
2 R
4
C
4
4BW 1–
------------------------------------------------------------- -=
R
3
R
1
4BW
f
LC
----------------- 1–
---------------------------= C
3
1
2 R
3
4BW
---------------------------------------- -=