TPS40428: Current readout (READ_IOUT) does not seem to be correct.

Michael Williamson

Expert 1995 points

Part Number: TPS40428

Tool/software:

Hello,

We are using a TPS40428 in multiphase output with a pair of CSD95378B devices to drive an FPGA core voltage at 0.75 Volts, pretty much per the schematic in the datasheet.

I am testing the PMBUS READ_IOUT command, and I am getting a value of 1 A (read value was 0xE010). I am expecting it to be closer to 10 Amps. 1 AMP is too low.

I believe the PMBUS transactions are OK, as when I read the output voltage, I get 0.748047 volts and I am able to properly read the VOUT_COMMAND register, etc.

I have checked that the IOUT_CAL_GAIN register is set to 0.5 mOhm (0x8021).

Is there something I should be checking on the schematic?

Thanks,

Mike

over 1 year ago

0 Mahmoud Harmouch over 1 year ago

TI__Mastermind 28340 points

hello,

the IOUT_CAL_GAIN register is set to 0.5 mOhm is the right value with CSD95378B. What is the input supply current?

Regards

0 Michael Williamson over 1 year ago in reply to Mahmoud Harmouch

Expert 1995 points

We are running the whole board (a couple of other supplies) from a +12V source, which is sinking 0.7A (about 8.5 Watts). I can't measure the current to just the TPS40428. The core voltage rail should be consuming at least 60-70% of the load of the board, if not more. We have another board design using the same part with a similar bitstream and the current draw is closer to 10 Amps at 0.8V, which is what I would expect. I don't think the FPGA would operate correctly if the core was only consuming 750 mW of power.

0 Mahmoud Harmouch over 1 year ago in reply to Michael Williamson

TI__Mastermind 28340 points

Hello,

can you please measure the signal on IOUT relative to REFIN? using differential probe.

Regards

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TPS40428: Current readout (READ_IOUT) does not seem to be correct.