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LT1054: Output voltage lower than expected

Allen Du
Prodigy 220 points
Part Number: LT1054
Other Parts Discussed in Thread: TPS65133, TPS65132

Hi Team,

I am using LT1054IP to generate -5V from 5V for op-amp power supply.

The circuit I built is same as datasheet typical aplicaiton, R2 is 102.6k ohm.

The problem is output always lower than input around 0.2V.

when input +5V, out put is around- 4.8V,

input = 5.5V, output =- 5.3V

input =4.7V, output=-4.5V
(Havn't connected load, just placed a LED and 5kohm resister in serial on output)

Could you help advice is this normal? how to get a stable -5V output, thanks in advance!

I  

  • 0
    TI__Guru 58215 points

    Hello Allen,

    This is normal behavior for this device, as there are losses in the internal MOSFETs to invert the voltage. With a charge pump inverter the output voltage will be close to the negative of the input voltage, but it will not be exactly at the input voltage because of the losses in the system.

    Normally when using ceramic capacitors you can reduce the impedance in the whole system a bit, so it is somewhat less voltage drop between input and output, but there will be always some drop.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 220 points

    Hi Brigitte, 

    Thanks for your immediate reply, I wonder is the LT1054 suitable for high speed op-amp power supply (>100MHz)?

    I also look for other solution for -5V, TPS65133 seems a option but no stock, could you advice other available options.

  • 0 in reply to Allen Du
    TI__Guru 58215 points

    Hello Allen,

    The AC supply current for a high-speed Op-Amp normally needs to be buffered through a low impedance capacitor on the supply lines.

    So almost any DC/DC converter capable of the necessary DC current to supply the circuit is suitable.

    Dependent on the system requirements for accuracy, you might need to make sure that the voltages on the positive and negative rails stay within a certain range. For example, if you supply the negative rail with a charge pump from the positive rail, you might need to introduce the same drop on the positive rail. But this is dependent on your system requirements.

    TPS65133 could be an option and it seems that it is in stock again. Another possibility could be TPS65132, if the current requirement is lower.

    Best regards,
    Brigitte