PM6641 datasheet(25/47 Pages) STMICROELECTRONICS

25 / 47 page

PM6641

Device description

25/47

In order to obtain the typical integrative loop transfer function the signal stage must

compensate for the power stage pole (due to the output capacitor and the load) and zero

(above the loop bandwidth if ceramic output capacitors are selected). The signal stage

transfer function is:

Equation 2

Where gM is the power stage transconductance, KL is a design parameter and α is the gain

due to the output resistor divider (0.8V / Vout). The external compensation network (Rc, Cc

and CRO) introduces:

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One zero, to compensate the power stage pole:

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One pole in order to delete the static output voltage error;

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One pole, if necessary, in order to compensate the high frequency zero due to the

output capacitor ESR:

The control loop gain is obtained by multiplying G(s) by H(s):

Equation 3

This model provides good results if the control loop cut-off frequency fCO is lower than about

fsw/10.

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