ST1S31 datasheet(12/35 Pages) STMICROELECTRONICS

12 / 35 page

Functional description

ST1S31

12/35

DocID022998 Rev 4

The resistor to adjust the output voltage that gives the term from output voltage to the FB

pin. GDIV(s) is:

The transfer function from FB to Vc (output of E/A) introduces the singularities (poles and

zeroes) to stabilize the loop. The small signal model of the error amplifier with the internal

compensation network can be seen in Figure 6.

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 9

where GEA = Gm · Ro.

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

Equation 10

Equation 11

whereas the zero is defined as:

Equation 12

GDIV s



R1 R2

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

Gm*Vd

VFB

VREF

AM11419v1

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





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

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