Soft-start capacitor for LM5119

Yaseen Khan

Other Parts Discussed in Thread: LM5119

Hello everyone,

I am using LM5119 as a buck converter in Interleaved mode. I was wondering if there is any limit on soft-start

capacitance. From the block diagram, I can see 10uA current source that follows - Current x time = Capacitance x Voltage or Css = (tss x 10uA) /0.8V (Page 17 of datasheet)

For my application, I need to start the power supply very slowly so I tried 1uF then 10uF capacitor. It worked so far. But when I tried 20uF capacitor, something

went wrong and the chip got blown along with MosFETs (not visible damage though). I had to replace LM5119 and transistors...

Can anyone tell me if there is any limit on the soft-start capacitor?

Thank you, really appreciate your help.

Regards,

Yaseen Khan

over 13 years ago

0 EricLee over 13 years ago

TI__Guru 58171 points

Hi Yaseen

There is no reason IC goes wrong with 22uF Css.

I recommend you to check dead-times and any excessive voltage spike at every node of LM5119.

Regards,

Eric

0 Yaseen Khan over 13 years ago in reply to EricLee

Prodigy 60 points

Hi Eric,

Yes, that's what I thought too. It should still work with higher capacitance.

How do I verify the dead times? Do you mean the dead-time between gate signal turning on and off? I can try seeing it on a scope.

I am using 24V input and converting to about 6V. Excessive voltage spikes as in more than 40V?

Thanks.

Regards,

Yaseen

0 EricLee over 13 years ago in reply to Yaseen Khan

TI__Guru 58171 points

Hi Yaseen

Yes, I meant the dead-times between high-side gate-source and low-side gate-source. Please refer LM5119 datasheet for the absolute maximum voltage ratings of each pin.

Regards,

Eric

0 Yaseen Khan over 13 years ago in reply to EricLee

Prodigy 60 points

Thanks Eric.

I will check them out. I will also try 15uF Css today.

I will keep you posted.

Thanks.

Regards,

Yaseen

0 Yaseen Khan over 13 years ago in reply to Yaseen Khan

Prodigy 60 points

I tried Css 15uF. It worked nicely, the votlage went up slowly to my regulated output.

Thanks Eric, appreciate it.

Regards,

Yaseen

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Soft-start capacitor for LM5119