AN938 Datasheet, PDF (8/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Power supply, UVLO and voltage reference
Figure 7. Voltage feed-forward function.
AN938
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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.
Equation 2
ΔV_osc
=
V_{-----C---C-----–-----}1_--
6
Also the ΔV/Δt of the sawtooth is directly proportional to the supply voltage. As V_cc
increases, the T_on time of the power transistor decreases in such a way to provide to the
choke, and finally also the load, the product volt. x sec. as a constant.
Figure 8 shows how the duty cycle varies as a result of the change on the ΔV/Δt of the
sawtooth with V_cc. The output of the error amplifier doesn’t change to maintain the output
voltage constant and in regulation. With this function on the 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 8.
Maximum duty cycle vs. R_osc and C_osc as parameter
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Doc ID 5410 Rev 10