We design a buck converter on L4971D and expected output 3.3V but only get 0.65V with 48VDC Input.we tune OSC pin resistor up to 5K then output is 3.3V but when input decreases below 35VDC then output again 0.65 to 1.2V.

RMish.2 · ‎2022-05-19

R3 = zero ohm C5, R4 = DNM

Peter BENSCH · ‎2022-05-20

Welcome, @RMish.2, to the community!

Unfortunately, you got confused when choosing the feedback voltage divider, for which there is a table in the data sheet in figure 4.

For Vout= 3.3V you have to:

remove R4 and
set R3 to zero ohms.

However, it should also work if you only remove R4.

Does it answer your question?

Good Luck and regards

/Peter

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