Datasheet

DocID023654 Rev 2 11/27
ST1S41 Functional description
The transfer function from FB to Vc (output of E/A) introduces the singularities (poles and
zeroes) to stabilize the loop. Figure 5 shows the small signal model of the error amplifier
with the internal compensation network.
Figure 5. Small signal model for the error amplifier
R
C
and C
C
introduce a pole and a zero in the open loop gain. C
P
does not significantly affect
system stability and can be neglected.
So G
EA
(s) results:
Equation 9
where G
EA
= G
m
· R
o
.
The poles of this transfer function are (if C
c
>> C
0
+C
P
):
Equation 10
Equation 11
whereas the zero is defined as:
Equation 12
The embedded compensation network is R
C
=70 kΩ, C
C
=195 pF while C
P
and C
O
can be
considered as negligible. The error amplifier output resistance is 240 MΩ
so the relevant
singularities are:
Equation 13
CoRo
Cc
Rc
Cp
Gm * Vd
V
FB
V
REF
Vd
AM15062v1
G
EA
s()
G
EA0
1s+ R
c
C
c
⋅⋅()⋅
s
2
R
0
C
0
C
p
+()R
c
C
c
sR
0
C
c
⋅ R
0
C
0
C
p
+()R
c
C
c
⋅+⋅+()1+⋅+⋅⋅ ⋅⋅
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=
f
P LF
1
2 π R
0
C
c
⋅⋅ ⋅
------------------------------------------=
f
P HF
1
2 π R
c
C
0
C
p
+()⋅⋅ ⋅
------------------------------------------------------------=
f
Z
1
2 π R
c
C
c
⋅⋅ ⋅
----------------------------------------- -=
f
Z
11.6 kHz f
PLF
3.4 Hz==