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LMZ31506: PWRGD not enabling LMZ31503/LMZ31506/TPS7A8801RTJ devices via INH/EN

Shane Augustine
Prodigy 140 points
Part Number: LMZ31506

We're using an LMZ31506RUQ to generate 1.0V from 5.0V - which it is doing - and then using the PWRGD output of that device to enable 2 LMZ31503RUQ's,1 LMZ31503RUQ, and 1 TPS7A8801RTJ. However, the outputs of these sequenced devices are a fraction of what is expected and the PWRGD output from the first LMZ31506RUQ is measuring 0.6V - not high enough to enable the devices. We're using a 68K pull-up resistor to 5V. There is one other device connected to this fanout of 4 devices - a 74LVC4245APW - which is configured as an input (although relies on the previously mentioned power circuitry to turn on). If connecting the PWRGD output to INH inputs that already contain pull up resistors, is adding an external pull up causing any potential issues? Any other thoughts? Perhaps the 74LVC4245APW is causing a voltage divider (5V side on, 3.3V side off), or there's defective device in the chain?  Thank you

  • +1
    TI__Expert 3548 points

    Hi Shane,

    We would need the schematic of this circuit to understand better what is going on. When you are measuring 0.6V at PWRGD pin, is that when Vout is good, meaning it's within the range of the desired output? Is 74LVC4245APW connected before or after the 68k pullup resistor? What are the values for Rset and Rrt?

    There is a chance that the sum of all internal pull up resistors of all the other devices connected are affecting the output. If there is too much pullup resistance, then the PWRGD might going out of its parameters. I would suggest if you could disconnect all the other devices (LMZ31503RUQ, TPS7A8801RTJ, 74LVC4245APW) and re-measure Vout and PWRGD pins, keeping the 68kohm pullup resistor. If after disconnecting, PWRGD goes up, then it means that the LMZ31506RUQ was too loaded. If nothing changes after disconnecting, the there's a chance that the problem might be related to LMZ31506RUQ.

    Regards,

    Eileen Hernandez

  • 0
    Prodigy 140 points

    Thank you. The issue had to do with an unpowered 74LVC4245APW that was loading down the PWRGD signal.