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TPS5430 buck convertor - oscillates & wouldn't start

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zserg:
Hi all!

I have a buck regulator based on TPS5430 from TI, which wouldn't start properly.
The design has been done using TI WEBENCH tool & all the components have been chosen from their recommended BoM.
However, when I supply 12V in, the output voltage gets stuck at 1.255 V and the PH pin scope trace just shows weird oscillations.
What is also weird, the Vsense pin also shows 1.255V, while according to the datasheet it should be 1.221V (internal REF).
I tried playing with input/output capacitance - no luck. Again, the input and output caps were chosen according to WEBENCH recommendations, I've checked them against the datasheet and they fall within specs.
What I've found is that if I briefly short the bootstrap capacitor, the circuit would start, though the PH trace still looks weird on the scope.
If I switch it down and let Cout fully discharge, it wouldn't start again without kicking Cboot.
Now, the datasheet explicitly says to use Cboot of 10n, which is exactly what I've done. I've tried 4.7n and 2x10n in parallel instead - that didn't help.

Any ideas what the reason could be?

Colt45:
Try putting an electrolytic across the input cap, see if it starts.

Sometimes with a long lead from the wall wart, and only ceramic input caps, and no TVS, the input will ring. Seems to sometimes confuse the buck chips.

zserg:
Tried already, unfortunately no alterations to input/output capacitors seem to help. :/

SiliconWizard:

--- Quote from: zserg on November 28, 2019, 05:25:59 pm ---However, when I supply 12V in, the output voltage gets stuck at 1.255 V and the PH pin scope trace just shows weird oscillations.
What is also weird, the Vsense pin also shows 1.255V, while according to the datasheet it should be 1.221V (internal REF).

--- End quote ---

Vsense is the feedback input of the converter, which in normal regulating conditions will "servo" it to Vref. Not exactly the same thing. It's clear here that the converter is not in normal regulating conditions.

IMO, the second part should be a big hint: how can Vsense, which is the output of a resistive divider, be equal to the input voltage of said divider? Can you check both resistors and your wiring first?

If all is well with this, we could then suspect the Vsense pad is fried. But check all your wiring first.

zserg:
Thanks a lot for the reply!

--- Quote from: SiliconWizard on November 28, 2019, 08:00:34 pm ---Vsense is the feedback input of the converter, which in normal regulating conditions will "servo" it to Vref. Not exactly the same thing. It's clear here that the converter is not in normal regulating conditions.

--- End quote ---
Yup, that's what I meant, was just trying to be brief.

--- Quote from: SiliconWizard on November 28, 2019, 08:00:34 pm ---IMO, the second part should be a big hint: how can Vsense, which is the output of a resistive divider, be equal to the input voltage of said divider? Can you check both resistors and your wiring first?

--- End quote ---
Fair point! I've checked the wiring & resistor values first thing - it's all OK. Will check it one more time tomorrow though.
For now, it can be seen from the PCB layout that the voltage divider is where it should be - those two 0805 resistors right between the IC and the inductor.



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