AN937

Power supply, UVLO and voltage reference

Doc ID 5655 Rev 15

5/29

The oscillator peak-to-valley voltage is proportional to the supply voltage, and the voltage

feed forward is operative from 8 V to 55 V of input supply.

Also the

ΔV/Δt of the sawtooth is directly proportional to the supply voltage. As Vcc

increases, the Ton time of the power transistor decreases in such a way to provide to the

choke, and finally also the load, the product Volt. sec constant.

Figure 6 show how the duty cycle varies as a result of the change on the

ΔV/Δt of the

sawtooth with the Vcc. The output of the error amplifier doesn’t change to maintain the

output voltage constant and in regulation. With this function on board, the output response

time is greatly reduced in presence of an abrupt change on the supply voltage, and the

output ripple voltage at the mains frequency is greatly reduced too.

Figure 5.

Device switching frequency vs Rosc and Cosc

Figure 6.

Voltage feed forward function

ΔV

osc

V

CC

1

–

6

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

=

0

20

40

60

80

R2(K

Ω)

5

10

20

50

100

200

500

fsw

(KHz)

D97IN630

0.82nF

1.2nF

2.2nF

3.3nF

4.7nF

5.6nF

Tamb=25˚C

V1

V2-3

D97IN684

Vi=30V

Vi=15V

Vc

t

Vi=30V

Vi=15V

t