LM5574 strange ripple when works in parallel with another SMPS converter

Denis Kotlar

Other Parts Discussed in Thread: LM5574

I'm using LM5574 to generate +/-9V from 50-55Vin. The same power input is shared with another SMPS circuit. The LM5574 output looks fine while another SMPS is disabled:

But when another SMPS is enabled a strange ripple appears:

... and this is schematic:

Please let me know what could be done to find a reason why this is happens. Many thanks in advance.

over 7 years ago

0 Denis Kotlar over 7 years ago

Intellectual 790 points

Maybe that is also valuable information: the switching frequencies are synchronized where LM5574 acts as master (SW2). Their outputs looks fine:

Unfortunately, the mentioned ripple is present in both case: with and without synchronized switching frequencies.

0 Anston Lobo over 7 years ago in reply to Denis Kotlar

TI__Genius 16025 points

Hi Denis,

The two regulators are constructively interacting with each other, Please synchronize them 180 degrees out of phase to remedy the ripple response.

Thanks,
Anston

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LM5574 strange ripple when works in parallel with another SMPS converter