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Original question:

LM27762: LM27762

LM27762: Negative LDO Dropout

Ryan.Bishop
Expert 3900 points
Part Number: LM27762
Other Parts Discussed in Thread: LM2776, TPS60400

Hello,

I noticed in this E2E post that the overall dropout for the negative output must be the sum of the LDO dropout and the charge pump voltage drop: https://e2e.ti.com/support/power-management/f/196/t/659023

Looking at Figure 4 of the datasheet, I see that at 1mA of load current the CP resistance could be as high as 300-Ohm. That would result in a 300mV drop. Does that mean for a -5V output, the input voltage would have to be 5V + 300mV =5.3V? (at very lower current the LDO dropout appears to be negligible)

In this application we have 6Vin, but I want to use the LM27762 to generate +/-5V. Since the max input voltage is 5.5V, I'm thinking of using a very low dropout LDO to step down the 6Vin to a safe intermediate voltage.

I don't want to get too close to the maximum of 5.5V, but is 5.4V my best bet? Could the negative dropout get even greater than 300mV under a no-load condition (<1mA)?

Thank you!

Regards,
Ryan B.

  • 0
    TI__Guru**** 150610 points

    Hi Ryan,

    That's basically correct.  There is essentially some minimum voltage drop across the charge pump, due to the PFM mode and losses.  You can see figures 5 and 8 in the LM2776 D/S.

    Your application probably doesn't need -5.0V.  -4.9V is likely ok.  Most would probably do an LDO to ~5.1V and then invert it with LM2776 or the TPS60400 family.