A5975D datasheet(22/50 Pages) STMICROELECTRONICS

22 / 50 page

Closing the loop

A5975D

22/50

Doc ID 018760 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, therefore ensuring a better line regulation and line

transient response.

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

Equation 15

Example:

Considering RC = 10 kΩ, CC = 10 nF and CP = 120 pF, the poles and zeroes of A0 are:

FP1 = 20 Hz

FP2 = 130 kHz

FZ1 = 1.6 kHz

If L = 12 µH, COUT = 220 µF and ESR = 25 mΩ, the poles and zeroes of ALC become:

FPLC = 2.5 kHz

F0 = 38 kHz

Finally, R1 = 5.6 kΩ and R2 = 3.3 kΩ.

The gain and phase bode diagrams are plotted respectively in Figure 11 and 12.

PWM s

()

OSCMAX

OSCMIN

–

()

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

OSCMAX

OSCMIN

–

⋅

PWM s

()

----

const

()

PWM s

()

-------------------- A

O s

() A

⋅⋅

⋅

()

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