Switching frequency

[PZha.1]

HI Sir,

the switching frequency below input 24V 0.04A output 12V

is that normal.

 the switching frequency below, input 24V 0.01A output 12V

the switching frequency below input 24V 0.33A output 12V

 the circuit see below:

 

[Peter BENSCH]

You didn't mention which switching regulator you currently have, but I'm guessing the L7987?

Why wouldn't the switching waveforms be ok - the L7987 can deliver 3A and is slightly underpowered at 0.01A?

Regards
/Peter

[PZha.1]

HI Peter,

Yes, the regulator is L7987 you can see it from the schematic

L7987 is used in each of our products, it's about 12K per year.

I don't know why the switching frequency looks abnormal sometimes at light load.

any suggestion.

thanks,

With best regards,

PZha

[PZha.1]

HI Peter,

The switching frequency is 641KHz  after Change the FSW resistor from 47K to 33K

i

seems better than 520KHz 

[Peter BENSCH · Accepted solution]

Well, as mentioned, the L7987 can certainly supply 3A. The L7987 has a so-called light-load management for low outputs (see data sheet, section 4.3), but with 0.3% or 1.2% you are well below the lower limit. In this respect, the observed signals can certainly be regarded as realistic.

Regards
/Peter

[PZha.1]

HI Peter， thanks for your great support.

With best regards, PZha