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LM27762: I want to generate +-5V power supply through LM27762+TPS7A39 is it feasible?

Part Number: LM27762
Other Parts Discussed in Thread: INA821, REF5050, REF3318, , TPS7A39

Based on my device it needs to be low noise between 0.1-20Hz. My +-5v is mainly for the ina821 instrumentation amplifier, which has a PSRR of 120db.

1.8VLDO(REF5050+REF3318) mainly provides voltage to two sensors(The total current of the two sensors is 300uA) to ensure that the same noise can be CMRR.

Would like to ask whether the circuit design needs to be improved or there is a defect?

The LM27762 says it supports up to +-5V output, if my +VOUT = 1.2 V × (R1 + R2) / R2 and -VOUT = –1.22 V × (R3 + R4) / R4 configuration exceeding +-5V will cause damage to the LM27762?

I can't perform temporal analysis because the computer can't figure it out for a long time.

  • Hi Leo,

    The schematic for LM27762 looks fine. The charge pump generating the -5V could be switch at low frequencies which are critical for you because of the low load current. In case you see a noise problem, you could add some resistive load to the -5V to increase the switching frequency to > 20Hz. In this case you need to check if you have enough margin between to VIN and VOUT to account for the voltage drop across the charge pump output resistance and LDO dropout voltage. 

    With the current feedback network you should get +5V and -5V assuming your load current is around 1mA.

    TPS7A39 belongs to a different product line. In case you have no further questions regarding LM27762, I can move the thread to the responsible product line for TPS7A39.

    Best regards,

    Varun

  • OK~ I understand. Thanks for the clarification.
    Please move to to TPS7A39.
    thank you very much

  • Hey Leo, 

    I reviewed your schematic and for the most part everything looks good however I cannot verify if your resistor dividers to set the output are set correctly or not since I don't see what the intended voltage is supposed to be that is supplying your amplifiers.

    Currently it seems like you have set the positive output voltage to 1.538V of and a negative output voltage of -0.122V. If that was not your intention then please see below for the relevant sections of the datasheet (for reference the typical Vfbp=1.88V and Vnr/ss=1.19V)

  • I need the TPS7A39 to output +-5V.
    I designed it based on the reference resistor given in the picture below. Isn't that right?

  • Hey Leo, 

    Is your component name for the bottom resistors (R2P and R2N in our schematic) pushed up against the value text? I read R110k as 110k and R410k as 410k, but if they are both 10k, then your resistor values are set correctly for +-5V. 

    That being said, for an LDO to operate properly the input voltage must higher than the output voltage. If you inputs to the LDO are only +-5V then the LDO will be in dropout which will result in the output being lower than 5V (closer to zero) and your PSRR will be 0dB. For a quick primer on LDO dropout voltage we have a video or text version depending on your preference for learning new material. It seems like you have a light load, but if you want decent PSRR I'd recommend Vin-Vout=300mV or more. Below is a quick graph which shows that the PSRR starts to get worse as you get closer to dropout (I don't have a graph for your exact use case of 1mV the general idea holds) . 

  • Sorry I should have marked it clearly. R1_10K, R4_10K

    Ok, thank you very much for your reply, that's exactly the answer I needed~.
    Best wishes.