Datasheet

L7985 Application information
Doc ID 022446 Rev 3 25/43
Figure 14. 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 1k and 5k, in order to have values of C4 and
C5 not comparable with parasitic capacitance of the board.
2. Choose a gain (R
4
/R
1
) in order to have the required bandwidth (BW), that means:
Equation 28
where f
ESR
is the ESR zero:
Equation 29
and Vs is the sawtooth amplitude. The voltage feed-forward keeps the ratio Vs/Vin constant.
3. Calculate C
4
by placing the zero one decade below the output filter double pole:
Equation 30
4. Then calculate C
3
in order to place the second pole at four times the system bandwidth
(BW):
R
4
f
ESR
f
LC
-------------
⎝⎠
⎜⎟
⎛⎞
2
BW
f
ESR
-------------
V
S
V
IN
---------
R
1
⋅⋅⋅=
f
ESR
1
2π ESR C
OUT
⋅⋅
---------------------------------------------------=
C
4
10
2π R
4
f
LC
⋅⋅
-------------------------------------=