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TPS544B20 BPEXT issues

Mark Guastaferro
Genius 11645 points
Other Parts Discussed in Thread: TPS544B20

We observe some unexpected behavior related to the BPEXT input.

1. An external 5V is connected to the BPEXT pin, and a 12V input supplied to converter. device is enabled with CNTL pin, everything is fine.  If device is disabled by de-asserting CNTL pin, about 500 mV is observed on the output. 12V Vin and 5V BPEXT still applied.  If this is repeated with no external 5V on BPEXT and BPEXT grounde dor floating, output drops to 0V as expected.

2. If operating the converter with BPEXT grounded (or floating), an overcurrent alarm is noted. This did not occur with BPEXT tied to 5V. Overcurrent warning set to 20A OC fault set to 26A. Fault is not being tripped. Max load current for rail is about 8A but typically 3A. Another odd thing is when doing a current read back, current reads expected 3A when external 5V connected to BPEXT, but value reads closer to 9A when BPEXT is grounded.

  • 0
    TI__Prodigy 580 points

    Hi, Mark,

    1. From the description, looks like there is a leakage path from BPEXT to output. Could you please send me the schematic and layout for a check? This leakage current is not expected as normal.
    2. The inaccurate current reading and OCP discrepancy can be improved by layout modification. Please check the latest datasheet, VDD decoupling cap is recommended to be grounded to GND instead of AGND; meanwhile, the AGND is recommended to be connected to AGNDSNS through a dedicated copper trace. I intend to think the VDD cap is grounded to AGND in customer design.

    Thanks.

    Best regards,

    Rui Chen

    SWIFT Application

    r-chen@ti.com

  • 0 in reply to Ray Chen
    TI__Genius 11645 points
    Schematic sent through email.

    Thanks for pointing out the change in layout recommendaiton. This could be part of the problem with the current monitoring accuracy.

    We determined that the output leakage issue was not due to the TPS544B20 (issue 1), and thus were able to reconnect the external 5V to BPEXT. Using BPEXT corrects issue 2 in this application.

    We can close this post