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Original question:

LT1054: Creating a single power op amp for a video signal

LT1054: creating a single power supply for video signal

Brittany Cohen
Prodigy 190 points
Part Number: LT1054

I am using an AD8170 (www.analog.com/.../AD8170_8174.pdf  ) to create a 2:1 switch for HD-TVI cameras.  I need to create a single power supply for this chip to allow it to operate on our PCB. I came across this forum (electronics.stackexchange.com/.../generating-plus-and-minus-voltages-for-op-amp   ) where I saw this solution for a single supply using the LT1054 (see below) I wanted to get someone's opinion if they thought this would be a good solution. 

  • 0
    TI__Guru* 76785 points
    Hi Brittany,

    Thank you for your interests. The LT1054 is a charge pump circuit able to generate -Vin from a +Vin without using an inductor. The schematic shown above can produce roughly +9V and -5V from a +5V input. If you don't need +9V but +5V and -5V are sufficient for your circuit, you can delete D1, D2, C7 and C9. You +5V rail can be your positive bias, and the VA- (=~ -5V) can be your negative bias.

    Hope this clarifies.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 190 points
    Hi Youhao, thank you much for the help! out of curiosity which do you think is the best choice for me to create a plus and minus signal
    1) use a voltage regulator to get 5V then send that voltage into the LT1054
    2) use a voltage regulator then connect that to a virtual gnd?
  • 0 in reply to Youhao Xi
    Prodigy 190 points

    Hi Youhao,

    So I tried the schematic below (see picture attached) and I am getting VA- to be about a .2V drop from the input 5V (any suggestions on fixing this)  also am I suppose to be taking VA+ from where I have it drawn? If so then VA+ and VA- aren't the same voltage. 

  • 0 in reply to Brittany Cohen
    TI__Guru* 76785 points
    Hi Brittany,

    The 0.2V drop at the VA- is caused by the internal circuit during charging and discharging the switched capacitor. Yes I meant to obtain the VA+ directly from your 5V input. If you want to drop the voltage a little from VA+ you can insert a small Schottky diode between the 5V input and VA+.

    For Op-Amp bipolar bias, the + and - supply do not need to be perfectly matched, this is where the simple charge pump LT1054 fits the best. However, if you really want precisely regulated and matched +/- voltage rail from a 5V input, more complex will be with the LT1054, but I don't think you really need such complication.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Prodigy 190 points

    Hi Youhao, that is great to know! do the op-amp bipolar bias not need to perfectly match because of this section (see picture attached)?

  • 0 in reply to Brittany Cohen
    TI__Guru* 76785 points
    The bias supply voltage usually is beyond the your signal dynamic range. For instance in your case, I doubt the signal that you are processing are close to +/-4.5V. As long as as your bias supply covers this range, your won'd see performance difference.