LM27762: Negative LDO is not working for my design. I keep seeing 0.3 V on the -V output.

anand2623

Part Number: LM27762

I have designed for +-1.8 V for a 3V input from 2 AA bats.

I see 1.783 V at the +1.8V output and 0.315V at the negative LDO. The PGOOD signal goes high.

I see the same voltages when I don't give the EN-. However, the PGOOD signal is low as it should be.

I have tested rest of the circuit with the EVM to see if the load is creating some issue. The rest of the circuit worked fine with the EVM.

PFB my schematic and layout. Please do let me know how to solve this.

over 6 years ago

0 Chris Glaser over 6 years ago

TI__Guru*** 142730 points

Hi Sai,

Thanks for sharing your layout.

Does the negative output work correctly when only it is enabled?

Are you testing with batteries or a lab supply?

1 big improvement to your layout would be to connect all the capacitor GNDs directly back to the IC without viaing down first. This lower impedance is important.

0 anand2623 over 6 years ago in reply to Chris Glaser

Prodigy 180 points

Hi Chris,

I am testing with batteries.
Nope the negative output does not work when only EN- is active.

I have a ground plane on the bottom layer shouldnt that create the low impedance?

thanks
sai

0 Chris Glaser over 6 years ago in reply to anand2623

TI__Guru*** 142730 points

Hi Sai,

I recommend testing with a DC lab supply, before using batteries, to overcome and debug any issues that are not caused by the batteries' internal impedance.

Can you post a waveform of Vin, OUT-, CPout, and OUT+? Does the negative voltage recover when you remove your load? You can also compare those same signals on your board and the EVM, when you start (apply power to) each of them.

Viaing down to a GND plane creates a much higher impedance compared to just pouring a short copper plane to return the currents to the IC's GND. Vias are very inductive.

0 Chris Glaser over 5 years ago in reply to Chris Glaser

TI__Guru*** 142730 points

Hi Sai,

Do you have any update on your testing?

0 anand2623 over 5 years ago in reply to Chris Glaser

Prodigy 180 points

HI Chris,

Sorry for the delay.
I had another board on which I was testing and both the voltages are coming out fine. (+1.789 V and -1.81 V respectively)
So I am assuming that the first board had some solder or IC issue.
I couldn't retest on the first board anymore as I had cut some traces during my testing.

Thanks,
Sai

0 Chris Glaser over 5 years ago in reply to anand2623

TI__Guru*** 142730 points

Thanks for confirming.

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LM27762: Negative LDO is not working for my design. I keep seeing 0.3 V on the -V output.