Hi.
I used MP8009 / PSR mode / Flyback Topology
The data sheet states that the frequency is fixed at 250 kHz.
The frequency is in the 100 kHz band.
Why does the frequency change with the load?
Where should I refer to the datasheet?
And MP8009 output is too noisy;
Why is that?
Please help me.
I’ll be waiting for your reply.
Best regards,
won.
Hi @seungyeon.won,
What does your input and output look like? Can you also measure the SW and check it’s frequency?
How are you measuring the output voltage? try to measure the Vout at the output caps away from the inductor. are you testing this on the EVB?
Thank you
Saurabh
Page 24 of the datasheet describes light load operation in PSR and yes despite the “fixed frequency” bullet point in bold at the beginning of the datasheet, the frequency is not fixed and can go as low as 30khz. Backed up as well by some of the scope captures.
The basic problem with PSR is the only way to learn what the output voltage is, is to push some energy into the output. You may learn that the voltage is too high! To help reduce this problem at light loads the “interogation” pulses are made less frequent. In the end you may find a minimum load requirement to maintain regulation of a few % of full load. This may be naturally provided by your application circuit or you might have to add some dummyt load.
