LM27761: design review

lei zhou1

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?

Hope your reply, thank you very much.

over 1 year ago

0 Varun John over 1 year ago

TI__Genius 13090 points

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

0 lei zhou1 over 1 year ago in reply to Varun John

Prodigy 190 points

Hi Varun

Thank you for your help.

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

0 Varun John over 1 year ago in reply to lei zhou1

TI__Genius 13090 points

Hi Lei,

Yes that should be the maximum.

Best regards,

Varun

0 lei zhou1 over 1 year ago in reply to Varun John

Prodigy 190 points

Hi Varun

It's really helpful for me, thank you.

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LM27761: design review