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# LM27761: design review

Part Number: LM27761

Hi TI

I am preparing use LM27761DSGT to design LDO circuit, so I have some question about LM27761DSGT:

1. LM27761DSGT  first use a charge pump convert Vin to -Vin, and then use a LDO convert -Vin to Vout?

2. In my design, Vin=3.3V, Vout=-3V, Vout max output current Imax=220mA, the circuit is below, please help to review the design is or not OK?

• Hi Lei,

I don't think it would be possible to get -3V from +3.3V with 220mA of output current. The reason is that there is a voltage drop across the charge pump output impedance and the LDO also has a dropout voltage. The charge pump output impedance will be around 2Ohm and with 220mA of output current, the voltage drop will be 440mV. The drop out voltage of the LDO is 70mV at this load current (figure12 in datasheet). So ideally your input should be at least 550 to 600mV higher than 3V.

Also make sure L7 has low DC impedance as there will be a voltage drop across this also.

Best regards,

Varun

• Hi Varun

So if the Vin=3.3V, Vout=-3V, the max output current is about 120mA?

• Hi Lei,

Yes that should be the maximum.

Best regards,

Varun

• Hi Varun

It's really helpful for me, thank you.