LM27761: Input range to generate -5V output

Hi Rajesh,

Thanks for reaching out to us!

It is hard to suggest the tolerance for charge pump as the voltage inverts from the supply voltage and have internal LDO and it will have additional power dissipation.

I would suggest to set input voltage higher to regulate device to -5V and at overall temp range tolerance depend on external components, layout of device. Is voltage  regulating at -5% with the 250mA load on EVM?

Best regards,

Tanvee

Hi Tanvee,

Thanks for the response.

Could you please be specific on the required VIN range to get -5V out. Feedback resistors are 1.54M ohm and 500k Ohm.

In our design, we have two supply rails available - one is +5V with +/-5% tolerance and other one with +5.6V with +/-5% tolerance.

We understand that the LM27761 has LDO and charge pump arrangement to generate negative voltages output. In that case what do you think more usable range to get -5V output.

Other option we are considering, that if we provide +5V as an input to the LM27761, in that how could we estimate that what will be the output voltage, even though we set the feedback voltage accordingly.

We have not tested this using EVM board, however we just had a look in the EVM design, so not sure about EVM tolerances.

Awaiting your response....

Best Regards,

Rajesh 

Hi Rajesh,

Resistor divider for the device must higher than 50kOhm and your recommended value looks okay to me. Supply voltage above 5.4V will help to get the desire -5V output voltage.

However device normal operating input voltage range is 2.7V to 5.5V. One rail with +5.6V with tolerance is outside spec for device. could you please check if you can adjust the input voltage into given range?

Another question is regarding +/-5% tolerance of supply. How often are you expecting device supply go into tol band?  Because device abs max value for input range is 5.8V and voltage higher than this value will damage the device.

Best regards,

Tanvee

Hi Tanvee,

Thanks for coming back so quickly.

Yes, then there is an issue with 5.6V supply. In that case, we would like to drive with +5V supply which is readily available on board.

However, still wondering how to estimate the output voltage based on input supply voltage in LM27761 circuit? Now after providing +5V as input supply and having the same voltage divider, will we be able to get -5V typically? Please comment.

In fact 5.6V is required by other circuitry, so we can't be able to tune it down. In that case, we would like to use switching inverter regulator which can take 12V +/-5% as input and provide -5V output. Could you please suggest few economical and small form factor solutions to have as a back-up options.

Best regards,

Rajesh