TPS54873: TPS54873 output voltage is raise about 1V at a momentary and pulled to GND when the device is started up.

Hi Kazuya-san,

Thank you for providing your waveform.  Was this tested on the EVM or on their own circuit?  Can you send the schematic?

If they can, it would be good to see the short pulse with a smaller timescale.  Can they take another waveform at 50 usec/div and add the PH node?

Thanks,

Chris

Hi Chris,

Thank you very much for your reply.

I got the customer schematic.
But they don't want to disclose the schematic on E2E.
So could you please give me your e-mail address? I'd like to discuss this issue with you off line.

Thank you again and best regards,
Kazuya.

Hi Kazuya-san,

Thank you for sending the schematic.

This is a difference between the TPS54873 (which does not sink current during startup) and the TPS54810 (which does sink current).  The TPS54873 has this extra description in its D/S: 

DISABLED SINKING DURING START-UP (DSDS) The DSDS feature enables minimal voltage drooping of output precharge capacitors at start-up. The TPS54873 is designed to disable the low-side MOSFET to prevent sinking current from a precharge output capacitor during start-up. Once the high-side MOSFET has been turned on to the maximum duty cycle limit, the low-side MOSFET is allowed to switch. Once the maximum duty cycle condition is met, the converter functions as a sourcing converter until the SS/ENA is pulled low.

The TPS54873 typical application circuit also shows (optional) diodes pre-charging Vout from Vin.  It seems that this 20-year old IC was designed to intentionally startup smoothly into pre-biased outputs (without dragging them down to 0V).  

The maximum duty cycle operation is probably what is creating the spike of Vout.  You could measure the same waveform with a shorter timescale to see how quickly the duty cycle reduces once it has started switching.

I would expect this Vout overshoot to be reduced by increasing the amount of output capacitance.  This should absorb the excess energy with less voltage increase.

Finally, it would be a good idea to migrate to a newer family, such as the 6A TPSM82866A module (or TPS62867 discrete) or the 6A/9A/12A/15A TPS62873/0.

Chris