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TPS544C25: VOUT_COMMAND not adjusting Vout as expected

Christopher Arntzen
Intellectual 630 points
Part Number: TPS544C25
Other Parts Discussed in Thread: TPS546D24A

I have a TPS544C25 that I'm writing VOUT_COMMAND to and the output voltage is not changing as expected.

VOUT_SCALE_LOOP is set to 0.25.

The VOUT_COMMAND value stored in NVM is 06AE and it uses this upon power up.

When I write 06AF to VOUT_COMMAND, the write is successful and I verified that 06AF is in the VOUT_COMMAND register, but the output does not go up by ~2mV as it should have. The output voltage does not change at all. I also read the STATUS_CML register to verify there was no fault.

When I write 06B0 to VOUT_COMMAND, the output goes up by ~8mV, and not by ~4mV as expected.

I am not violating any of the relationships between the VOUT_COMMAND and VOUT_OV_FAULT_LIMIT, VOUT_OV_WARN_LIMIT, VOUT_MAX, MFR_VOUT_MIN, VOUT_UV_WARN_LIMIT, VOUT_UV_FAULT_LIMIT limits.

I am only writing VOUT_COMMAND and not VOUT_OV_FAULT_LIMIT, VOUT_OV_WARN_LIMIT, VOUT_MAX, MFR_VOUT_MIN, VOUT_UV_WARN_LIMIT, VOUT_UV_FAULT_LIMIT.

Any idea why the output voltage is not changing as expected?

  • +1
    TI__Guru 56795 points

     

    The Reference DAC used in the TPS544C25 has 2^-9V (1.953mV) LSB steps.  With VOUT_SCALE_LOOP = 0.25, the resolution for the output voltage adjustment is 2^-7V (7.8125mV)

    The internal hardware is calculating the required reference voltage by VOUT_COMMAND x VOUT_SCALE_LOOP and then rounding to the nearest LSB to set the voltages.  That is why you saw no change when you expected 2mV and 8mV change when you expected 4mV.   If you continue to raise the voltage, at +10mV it will jump to 16mV, the next available DAC LSB.

    In order to get the 1.953mV resolution for the output voltage, you are limited to the VOUT_SCALE_LOOP = 1 range from 0.5V to 1.5V.

    If you need higher resolution at higher voltages, see the TPS546D24A, which uses a 2^-12V/LSB reference DAC with a range from 0-0.75V.  In the VOUT_SCALE_LOOP = 0.125V range, it can achieve 1.953mV/LSB programming resolution.