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TI Fusion Digital Power Designer IOUT_OC_WARN Limit on TPS544B20

Rudy Garcia
Intellectual 380 points
Other Parts Discussed in Thread: TPS544B20

The following PMBUS registers are set in my 5V to 1.2V TPS544B20.

IOUT_CAL_OFFSET (0x39) = 0xE000 (0A)
IOUT_OC_FAULT_LIMIT (0x46) = 0xF834 (26A)
IOUT_OC_FAULT_RESPONSE (0x47) = 0x3F (Hiccup mode)
IOUT_OC_WARN_LIMIT (0x4A) = 0xF80A (5A)

I running some load testing on my output and noticed that I have been getting an OC warning even though the electronic load is only set to 3A. I have measured this on my scope and also logged the VOUT and IOUT using the Fusion PMBUS logging tool.

Any ideas on why this is OC warning alert is generated?

I am not exactly sure how the IOUT_CAL_OFFSET is used and I am wondering does this default of 0A need to be changed?

Thanks,

Rudy

  • 0
    TI__Genius 11095 points

    Hi Rudy-- 

    There is a degree of inaccuracy in how well the actual over-current warning threshold will match the output DC current. TPS544x20 devices use a sensefet current sensing architecture to determine the average current through the low-side FET. The low-side FET itself is a 2mOhm device (designed for efficiency at 20A), so the signal level becomes very small at relatively low output currents like 3A. You'll want to add some headroom to avoid getting OC warnings at low output current. 

    The IOUT_CAL_OFFSET command can be used to apply an offset to the current measurements in units of amperes. For instance, if you set the e-load to 3A DC, and READ_IOUT = 2A, you can set IOUT_CAL_OFFSET to +1A to make the two match. The same is also true for OC Warning level. It's worth noting though, that the offset from READ_IOUT to real DC current may not be consistent across devices, so the most useful purpose  of IOUT_CAL_OFFSET would be for "calibration"

  • 0 in reply to Matt Schurmann
    Intellectual 380 points

    Matt,


    Thanks for your answer. This makes sense.

    Other than measuring the output current on actual boards is there a rule of thumb that I can follow for setting the OC warning for lower output current loads?

    Rudy