AST1S31HF datasheet(14/33 Pages) STMICROELECTRONICS

14 / 33 page

Functional description

AST1S31HF

14/33

DocID026968 Rev 2

Figure 6. Small signal model for the error amplifier

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

Where GEA = Gm · Ro.

The poles of this transfer function are (if Cc >> C0 + CP):

Equation 11

Equation 12

whereas the zero is defined as:

Equation 13

The embedded compensation network is RC = 80 k, CC = 55 pF while CP and CO can be

considered as negligible. The error amplifier output resistance is 98 M

so the relevant

singularities are:

Equation 14

Gm*Vd

VFB

VREF

GEA s



GEA0 1s

Rc Cc







2 R

C0 Cp

 R

c Cc

sR0 Cc



R0 C0 Cp

 R

c Cc



 1







-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

fP LF

 R

0 Cc





----------------------------------

fP HF

 R

C0 Cp







----------------------------------------------------

 R

c Cc





---------------------------------

36 2 kHz



fP LF

13 6 Hz



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