TPS53515: Frequency instability issue at Fsw=600KHz

Tess Chen

Intellectual 2675 points

Part Number: TPS53515

Vin=12V, Vout=0.8V/8A application, Cin=44uF and Cout=350PF. The detail schematic as follows:

When Fsw=600KHZ, the frequency isn't satble. The wave for SW point is in the below:

But the circuit works well with Fsw=400KHZ, please find the wave for SW point and BODE in the two following picture:

I wonder what cause this phenomena, and how can I fix this issue by adjusting the component's parameter? Thanks.

over 6 years ago

0 JohnTucker over 6 years ago

TI__Guru**** 191445 points

tess,

It appears you have MODE pulled up with 0 ohm to PGOOD. That is not a valid condition. Try using 20 k ohm.

0 Tess Chen over 6 years ago in reply to JohnTucker

TI__Intellectual 2675 points

Dear John,

Thanks for your reply. In my schematic, MODE pulled up with 0 ohm to PGOOD but also pulled up with VREG. And I find MODE could be pulled up with VREG with 0ohm for FCCM as shown in the below table of its datasheet. So whether it is valid in this case or I should disconnect the PGOOD and VREG pin? Thanks.

0 JohnTucker over 6 years ago in reply to Tess Chen

TI__Guru**** 191445 points

Tess,

My mistake, the schematic was hard to read on a small display. 0 ohm to VREG will select the long time constant. Your design may work better with short time constant as you do not have a lot of output capacititance. Please try 20 kohm pull up to PGOOD. Let me know if that works for you.

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TPS53515: Frequency instability issue at Fsw=600KHz