R5972D datasheet(16/35 Pages) STMICROELECTRONICS

16 / 35 page

Closing the loop

R5972D

16/35

DocID026873 Rev 1

7.3

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.

Equation 13

Where K is equal to 0.076. Therefore the PWM gain is also equal to:

Equation 14

This means that even if the input voltage changes, the error amplifier does not change its

value to keep the loop in regulation, thus ensuring a better line regulation and line transient

response.

In summary, the open loop gain can be expressed as:

Equation 15

Example 1

Considering RC = 4.7 k, CC = 22 nF and CP = 220 pF, the poles and zeroes of A0 are:

FP1 = 9 Hz

FP2 = 150 kHz

FZ1 = 1.5 kHz

If L = 33 µH, COUT = 100 µF and ESR = 80 m, the poles and zeroes of ALC become:

FPLC = 3.3 kHz

FZESR = 19.89 kHz

Finally R1 = 5.6 k and R2 = 3.3 k.

The gain and phase bode diagrams are plotted respectively in Figure 8 and Figure 9.

GPWM s



Vcc

VOSCMAX VOSCMIN

–



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

VOSCMAX VOSCMIN

–

KVCC



GPWM s



----

const



GPWM s



R1 R2

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



AO s





ALC





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