LM27761: lm27761 Load issue

gopherfirm

Part Number: LM27761

Dear TI,

I am using LM27761 to supply negative 5V sensor. I have this issue that without the load I can see -4.86V at Vout but when I power up with the sensor negative pin attached to Vout then I started to see around 0.3V only but when I removed the sensor and placed back again then the voltage come back to -4.86V.

Sensor current is only 30mA and is required that it is always attached to Vout. May I know what is the reason for this load issue? I followed the same component values as indicated in the datasheet.

Thank you for your help.

Regards,

Leo

over 5 years ago

0 Febin over 5 years ago

TI__Mastermind 23860 points

Hi Leo,

Could you please share the schematic and layout for further evaluation?

Regards,

Febin

0 gopherfirm over 5 years ago in reply to Febin

Intellectual 270 points

Dear Febin,

Thanks for your reply.

Below is the screenshot of the schematic and the layout.

Regards,

Leo

0 Febin over 5 years ago in reply to gopherfirm

TI__Mastermind 23860 points

Hi Leo,

I cannot open any pictures. Could you please upload again or use some other format?

Regards,

Febin

0 gopherfirm over 5 years ago in reply to Febin

Intellectual 270 points

Dear Febin,

Please check the schematic and layout below. Please note that placing 4.7uF between pin 1 and ground has no effect.

Regards,

Leo

0 Febin over 5 years ago in reply to gopherfirm

TI__Mastermind 23860 points

Hi Leo,

- Are you applying a negative voltage on the EN pin? Please have a look at the datasheet for abs max ratings and voltage requirements of the EN pin.

- You cannot achieve 5V to -5V with this device. You may have to apply a higher input voltage to compensate for the losses.

- All the external capacitors mentioned in the d/s are required for the proper functioning of the device.

- Please choose a higher R2 (atleast 50k) and try again.

Regards,

Febin

0 gopherfirm over 5 years ago in reply to Febin

Intellectual 270 points

Hi Febin,

EN pin is the positive voltage from GPIO. The negative netlist sign is just to denote that is for the negative 5V supply.

I actually changed 4.7uF to 10uF and it works. You are right that it cannot get exactly negative 5V due to losses so I have to increase the input voltage.

Thank you.

Regards,

Leo

0 Febin over 5 years ago in reply to gopherfirm

TI__Mastermind 23860 points

Thanks for confirming!

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LM27761: lm27761 Load issue