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LMZ31520: Pull up INH more strongly

Kazuki Itoh
Genius 9175 points
Part Number: LMZ31520

Hi team,

From the functional block diagram, INH of LMZ31520 has internal pull-up and down resistors.

I recognize they will pull-up the INH pin to VIN * 0.4.

In my customer's system, VIN=5V.

My customer would like to pull-up INH pin more strongly because the INH pin is connected to PG of prior converter, and the PG voltage is externally monitored with 4.4V threshold.

So I'd like to pull-up the INH to >4.4V to set pull-up voltage above the threshold.

For this reason, is it allowed to connect external 1kOhm pull-up resistor to INH pin?

Regards,

Itoh

  • 0
    TI__Expert 5310 points

    Hi Itoh,

    The maximum allowable voltage applied to the INH pin will be 0.4*14.5 = 5.8V. This is calculated by taking the maximum input voltage of the LM31520, which is 14.5V and multiplying that by 0.4.

    That said, although INH is recommended for use as an open drain/collector pin, the INH pin may be pulled up to an external voltage greater than 4.4V so long as the voltage seen by the INH pin is below the 5.8V threshold outlined by the datasheet spec. For example, if INH is pulled up externally to 4.4V and the VIN=5V on your customer's application, I went through the calculations and determined the inhibit voltage would be approximately 3.918V when the external PG FET is off. Another caution is to ensure the PG pin of the external converter can sink the current when the FET is turned on. In the case of your customer's system, I would recommend setting the external pullup voltage such that the INH pin and PG pin of the prior converter see 4.4V-4.5V if possible and ensure the PG pin can sink the current when the PG FET turns on. In order to set the INH/PG voltage to 4.5V with the configuration you described this would require an external pullup voltage of 5.12V approximately.

    Regards,

    Alec Biesterfeld