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Adjustable power supply


Advanced Member level 3
Nov 3, 2018
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We are looking for adjustable power supply that can be controlled by FPGA/Microcontroller. The output voltage range is 0V to -3V. The input is 18V to 24V. The output current can be up to 500 mA but 300 mA will also work. The components we need to add in out custom PCB design. Any idea how to achieve it ?
A polarity-inverting charge pump is quick and easy. Input +18V, get -3V referenced to 0v gnd. A half-bridge can drive it. Since you bias it at a small voltage, you'll need to contrive a transistor or two to apply proper bias.
You are trying to control the Vout of the supply, correct ? With FPGA pin ranging in
V from 0 - 3 V. If so what range of controlled supply is its Vout ? 0 - 3V also ?

Regards, Dana.


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Clarification on controlled by FPGA/Microcontroller and adjustable.

I am thinking to use Digital to Analog (DAC) with I2C interface on the digital side. This way I can drive and vary the positive Analog voltage controlled by FPGA/Microcontroller and then I need to feed to voltage inverter.

The output voltage range, load current and the input available is the same as mentioned in post 1.

"A polarity-inverting charge pump is quick and easy".
I have not used charge pump before. Kindly let me know if that can be used to invert the polarity. But I think this will have fixed output for a fixed input. The input Analog voltage I need to control by the FPGA/Microcontroller.
A charge pump would be used to generate a - supply V for an OpAmp. The
OpAmp then used as inverter, unity gain (or whatever is needed) to invert
its 0 - 3V input to whatever you want as final generated control V.

You can also feed a PWM to a pin, and using diodes generate a - V for OpAmp
inverter. Filter with a CAP, and rely on OpAmp PSRR to reject ripple. Use schottky
small signal diodes and low power OpAmp. Many other considerations, latency
to power up, PWM switching frequency, cam sizes....simulate for answers.


Regards, Dana.
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The problem specification suggests an inverting buck (actually inverting buck-boost) or SEPIC converter, not a charge pump.
Did not see the current requirement, so post #7 not applicable, ignore.

What is the accuracy of the translation you need, input to output V accuracy ?

Regards, Dana.
Thanks for your comments. I will read some related documents.

If someone can suggest devices here that can work in this problem that would be best.
Topology suggested by TI Power Stage Designer


--- Updated ---

You can use a popular integrated buck converter IC in inverted configuration, set output to 3 V and have a control current source that injects a current into Vfb node, trimming the output voltage down.


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