 English |
|
|
|
|
|
|
|
| AST1S31HF Datasheet, PDF (14/33 Pages) STMicroelectronics – Internal soft-start and enable | |||
| |||
| ◁ |
Functional description
Figure 6. Small signal model for the error amplifier
V FB
AST1S31HF
Vd
VREF
Gm*Vd
Ro
Co CRcc
Cp
Cc
RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect
system stability and can be neglected.
So GEA(s) results:
Equation 10
GEAï¨sï© = s----2----ï---R----0----ï---ï¨---C----0-----+----C-----p---ï©----ï---R----c----ï---GC-----cE---A+----0-s---ï--ï-ï¨--ï¨1--R---+--0---s-ï---Cï---R-c----c+----ï-R--C---0-c---ïï©---ï¨--C-----0----+-----C----p---ï©----+-----R----c----ï---C-----c--ï©----+-----1--
Where GEA = Gm · Ro.
The poles of this transfer function are (if Cc >> C0 + CP):
Equation 11
fP LF = 2-----ï---ï°-----ï---1R-----0----ï---C----c-
Equation 12
fP HF = -2----ï---ï°-----ï---R-----c----ï1---ï¨--C-----0----+-----C----p---ï©
whereas the zero is defined as:
Equation 13
fZ = 2-----ï---ï°-----ï---1R-----c----ï---C----c-
The embedded compensation network is RC = 80 kï, CC = 55 pF while CP and CO can be
considered as negligible. The error amplifier output resistance is 98 Mïï so the relevant
singularities are:
Equation 14
fZ = 36ï¬ 2 kHz
fP LF = 13ï¬ 6 Hz
14/33
DocID026968 Rev 2
|
▷ |
German
Russian
Spanish
Italian
Polish
Chinese
Japanese
Korean
French
Portuguese