TPS56C231: Delay between EN going high and switching start

More waveforms. The VREG5 and MODE pin are up and settled by 180us. But still there is a further delay of 2ms until voltage rise

This is the selected MODE pin setting

SS pin is floating, since we do not want additional delays

Hello,

Referring to the Start-up waveform in the datasheet. It seems there is a 1ms delay from EN to Vout. So a 1ms difference between your waveform.

Also, as you are floating the SS pin, the SS time should be 1.2ms by default.

Can you share a schematic so I can take a look?

Thanks,

Calan

Hi,

Here is the relevant part of the schematic. I will check with the team, how much of the schematic can be put out, but until then, I hope this serves as some useful information.

 Yes, we have the SS pin floating and with that we expect a 1.2ms soft start time.. But isnt it the Vout ramp time that should be 1.2ms? I am concerned about the time interval between EN going and Vout starting to ramp.

Another observation today. I probed the SW node waveform and I observe that, it very weakly tries to start at around the time that the SS pin starts ramping up. Ideally this should be the time SW node should start I believe. But due to some reason, it dies out and stays flat at 0, until again restarting at 2.1ms

This is the same glitch zoomed in

Hello,

What is your intention behind not populating the Power Good pin? The datasheet recommends 10kohm resistor on this pin.

Also, your schematic has far less input capacitance compared to the reference schematic in the datasheet. Your conditions and the datasheet schematic conditions are similar if I am not mistaken. Are you able to populate the PG pin and add more input capacitors and see the result?

Datasheet: https://www.ti.com/lit/ds/symlink/tps56c231.pdf?ts=1717610113642&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS56C231

Have you tried a fresh EVM?

Thanks,

Calan

Hello,

The Power Good pin is not used in the system. I left it floating since it was an Open Drain and we didnot have any use case of this signal.

The evaluation board works as expected. Unfortunately I have not brought out any trace from the PGOOD pin to try and pull it up. I also tried with more input capacitance but the result is the same

Another observation:

1. The SS pin charges up as per the expected delay, but the switching just wouldnt start until 2ms minimum. Tried with multiple loads, the minimum value is around 2ms, and increases with decreasing load current

Also, removed the pullup resistor in the EVM, to mimic my schematic, and it works fine in the EVM

Hello Venugopal, 

No, the Power Good Resistor has nothing to do with the start up. 

Believe the device is not stable, the fact that it tries to start and is not able to than tries again after 1.2ms, that tell me something else is going. 

the SCH you attached above, i am not able to see the full SCH. 

• The recommended inductor is 1.5uH with at least 20% above 5A of saturation level, please check that .
• The output capacitors has to be 10V or 16V rates and should be 47uF x2 
• Is your VIN measured right at the input pin of the device? I am wondering if you causing VIN UVLO thru inrush current during start up 

Thanks! 

Tahar