LM73605: LM73605 switching current not stable

Duan Wu

Part Number: LM73605

Hi;

We are developing a DC-DC circuit which turn a 12V input to 5V output with out current below 5A. We choose TI LM73605 ,the circuit is posted. As pointed out in the datasheet, if we let RT pin floating, the switching frequency will be 500 KHz. But actually, we got a strange switching waveform as the figure below. We are confuzed. Please help us.

Donald.

over 6 years ago

0 Katelyn Wiggenhorn over 6 years ago

TI__Genius 11040 points

Hello,

One possibility is that there is an instability in your loop response. Have you taken loop response measurements of this design?

I would recommend first increasing the output capacitor, C5, to about 88uF of ceramic capacitance to see if this fixes the problem. At first look the output capacitance is on the low side.

Best Regards,
Katelyn

0 Duan Wu over 6 years ago in reply to Katelyn Wiggenhorn

Prodigy 250 points

Hi, Katelyn;
THX for your rapid reply. But my schematic I posted has a error, actually, C5 is a 100uF Tan capacitpor. So the problem is not the output capacitor. Would you please give us some more advice?

Donald

0 Robert Blattner over 6 years ago in reply to Duan Wu

TI__Expert 6245 points

Hello Donald,

Tantalums can have a high ESR. The ESR being a problem can be checked by adding a 22 uF ceramic. If the ESR is a problem, consider a polymer electrolytic cap. They are inexpensive and have low ESR.

Regards, Robert

0 Ambreesh Tripathi over 6 years ago in reply to Robert Blattner

TI__Expert 6860 points

Donald ,

Did the addition of cermaic caps as suggested by RObert fixed the issue ?

Ambreesh

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LM73605: LM73605 switching current not stable