R5975D datasheet(19/47 Pages) STMICROELECTRONICS

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DocID026878 Rev 1

19/47

R5975D

Closing the loop

Figure 8. Block diagram of the loop

7.1

Error amplifier and compensation network

The output LC filter of a step-down converter contributes with 180-degree phase shift in the

control loop. For this reason a compensation network between the COMP pin and

GROUND is added. The simplest compensation network, together with the equivalent circuit

of the error amplifier, are shown in Figure 9. RC and CC introduce a pole and a zero in the

open loop gain. CP does not significantly affect system stability but it is useful to reduce the

noise of the COMP pin.

The transfer function of the error amplifier and its compensation network is:

Equation 2

where Avo = Gm · Ro.

A0 s



AV0 1s

Rc Cc







2 R

C0 Cp

 R

c Cc

sR0 Cc



R0 C0 Cp

 R

c Cc



 1







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