AN937 Datasheet, PDF (5/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

AN937
Power supply, UVLO and voltage reference
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.
ΔV_osc
=
V_{-----C---C-----–-----}1_--
6
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
fsw
(KHz)
500
200
100
50
20
10
5
0
D97IN630
Tamb=25˚C
0.82nF
1.2nF
2.2nF
3.3nF
4.7nF
5.6nF
20
40
60
80 R2(KΩ)
Figure 6. Voltage feed forward function
V1
V2-3
Vi=30V
Vi=15V
Vc
t
Vi=30V
Vi=15V
t
D97IN684
Doc ID 5655 Rev 15
5/29