Datasheet

Application informations L7981
24/44 Doc ID 15182 Rev 3
6.4.2 Type II compensation network
If the equivalent series resistance (ESR) of the output capacitor introduces a zero with a
frequency lower than the desired bandwidth (that is: 2π∗ESR∗COUT>1/BW), this zero helps
stabilize the loop. Electrolytic capacitors show not negligible ESR (>30 mΩ), so with this
kind of output capacitor the type II network combined with the zero of the ESR allows
stabilizing the loop.
In Figure 13 the type II network is shown.
Figure 13. Type II compensation network
The singularities of the network are:
In Figure 14 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
TYPEII
(f)) are drawn.
f
Z1
1
2π R
4
C
4
⋅⋅
------------------------------= f
P0
0= f
P1
1
2π R
4
C
4
C
5
⋅
C
4
C
5
+
--------------------
⋅⋅
------------------------------------------- -=,,