AN1517 datasheet(13/30 Pages) STMICROELECTRONICS

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DocID8827 Rev 3

13/30

AN1517

Closing the loop

If RLOAD >> ESR, the previous expression of ALC can be simplified and becomes:

Equation 7

The zero of this transfer function is given by:

Equation 8

F0 is the zero introduced by the ESR of the output capacitor and it is very important to

increase the phase margin of the loop.

The poles of the transfer function can be calculated through the following expression:

Equation 9

In the denominator of ALC, the typical second order system equation can be recognized:

Equation 10

If the damping coefficient

 is very close to zero, the roots of the equation become a double

root whose value is

n.

Similarly, for ALC the poles can usually be defined as a double pole whose value is:

Equation 11

4.3

PWM comparator

The PWM gain is given by the following formula:

Equation 12

where VOSCMAX is the maximum value of a sawtooth waveform and VOSCMIN is the

minimum value. A voltage feed-forward is implemented to ensure a constant GPWM. This is

obtained by generating a sawtooth waveform directly proportional to the input voltage VCC.

ALC s



1 ESR COUT



LCOUT



ESR COUT



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





ESR



COUT



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

FPLC1 2



ESR COUT







COUT



–





–



COUT



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











FPLC





LCOUT



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

GPWM s



Vcc

VOSCMAX VOSCMIN

–



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

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