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TPS40210 negative adjustable output control circuit

Other Parts Discussed in Thread: TPS40210

Hello;

My application is +15V input -1000V output. Output has to be adjustable with external 0- 3V ( 3V --1000V, 0V-0V). 

Can somebody suggest control circuit to convert output voltage to 0.7V feedback. It is non isolated circuit. 

Thanks

  • For feedback, you can use an inverting op-amp with a large input resistor to convert the -1000V to a small positive voltage, then you can use a resistor averaging circuit to average your control voltage with the inverted, scaled voltage and send that to the feedback circuit.

    For example, use an inverting op-amp with the non-inverting input connected to ground and the inverting input connected to your 0 to -1000V output through a 10 meg resistor and to the output of the op-amp with a 14k resistor. This will cause 0 to -1000V at your output to cause 0 to 1.4V at the op-amp output.

    Then use an op-amp to convert your 0 to 3V control voltage to a 1.4V to 0V output voltage. You can do this with an inverting amplifier with gain (-1.4/3.0) = (-0.4666667) and with the non-inverting input connected to (1.4*1/(1+0.4666667)) = (0.95454545) volts (derived from a voltage divider off of a reference voltage).

    Connect the feedback op-amp 0 to 1.4V output and the control op-amp 1.4 to 0V output to the 0.7V feedback pin with equal resistors of around 1k to 100k (for a resistor voltage averaging circuit).

    With a control voltage of 0V, 0V output will cause a feedback op-amp output of 0V, which is averaged with a control op-amp output of 1.4V for 0.7V at the feedback pin.

    With a control voltage of 1.5V, -500V output will cause a feedback op-amp output of 0.7V, which is averaged with a control op-amp output of 0.7V for 0.7V at the feedback pin.

    With a control voltage of 3V, -1000V output will cause a feedback op-amp output of 1.4V, which is averaged with a control op-amp output of 0V for 0.7V at the feedback pin.

    This circuit will likely be quite sensitive to component tolerances with such high voltages and ratios; be sure to test it thoroughly!
  • Morgan;

    Thanks for suggestion. The problem is it is high temperature application. So i am trying to avoid extra component (not sure they will work beyond 150C). So only ref voltage available is Vbp; which is according to datasheet 8V (but could be btn 7--9). Is there any other way to do this (convert 0 - 3V to 1.4V -- 0), without using any external ref voltage?
  • The TPS40210 only works up to absolute maximum junction temperature of 150C (recommended junction temperature 125C). It will also heat itself up from its own power dissipation, so your ambient needs to be lower than that. If you can get the TPS40210 cool enough to run, there are plenty of voltage references that don't mind the temperature.

    The Vbp voltage of 7-9V is far too inaccurate for this use; for example, if you set up your resistor divider for 8V but got 7V, 0V control voltage would give 1.225V output at the control op-amp, which would give 0.7V at the feedback pin with -125V output, so your minimum output voltage would be -125V! (That's what I meant about this circuit being very sensitive to tolerances.)

    Or, could you use a 3V-0V control signal instead of a 0V - 3V control signal? Then you could just use a resistor divider from your control input to your feedback pin (selecting resistors so that you include the parallel resistance of the divider when making the two averaging resistances equal).

    You could use Vbp of 7-9V if you include enough extra headroom so that 0V input causes 0V output even with worst-case tolerances. Then, with normal tolerances you would just have dead-bands near the end of the control range (so that 0V - 0.2V caused 0V output and 0.2V to 3V caused 0V to -1000V output, for example).