ST1S50 datasheet(12/29 Pages) STMICROELECTRONICS

12 / 29 page

Functional description

ST1S50

12/29

DocID025723 Rev 4

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

(poles and zeroes) to stabilize the loop. In Figure 5 is shown the small signal model of the

error amplifier with the internal compensation network.

Figure 5. Small signal model for the error amplifier

RC and CC are the compensation network, RO represents output resistance of the error

amplifier, Co represents the internal low frequency pole of 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 8

Where GEA = Gm · Ro

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

Equation 9

Equation 10

whereas the zero is defined as:

Equation 11

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





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

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