• Resolved

Voltage-controlled, Current-limited switcher circuit

Hello,

My product requires a voltage-controlled, current-limited power supply and for a number of years I have used various switcher chips with an op-amp to feed back a current sense voltage to the feedback pin. For the last couple of years I have been using an LM22680, but I seem to get variable results board to board. (Output voltage not consistent)

I supply my circuit with an off the shelf 24 VDC switching power supply and the idea is to drive a variable load at a maximum of 1 amp.  I have used WebBench for the different circuits that I have used.

In my application, the output voltage can be anywhere between 9 and 21 volts. The key is to drop the output voltage to limit the current to1 amp.

Three questions: 1) Am I taking the right approach to meet my application parameters?

2) If so, why are the LM22680's providing varying output voltages from board to board with the same load conditions?

3) Is there a better solution I should pursue? I have used an LM25010 in the past, but the 1.25 amps proved to be too much current. Would it work in conjunction with the op-amp feedback circuit? (According to the datasheet, their is a ripple voltage requirement at the feedback pin, that may not be provided by the op-amp.)

Thank you!

Craig

  • Hello Craig,

    could you please share your schematic? we would like to review it before say anything.

    Regards,

    Giuseppe

  • In reply to Giuseppe Pinto:

    Hello Giuseppe,

    Thank you for your assistance.

    Attached is my circuit schematic.

    My application can only tolerate up to 1 amp of current, so I need the power supply to limit the current to 1 amp. It does that by dropping the voltage.

    The boards are exactly the same. The only differences are the components and the solder connections. Some boards will generate 21 volts at 1 amp and others will only generate 18 volts at 1 amp, with the same 20 ohm load.

    Will the LM25010 work with the op-amp feedback circuit? I never had any problems using it without the op-amp circuit, so I feel better going back to it.

    Thanks again!

    Craig

  • In reply to Craig Combes:

    Hello Craig,

    does the voltage drop happen only at full load? 

    Do the boards have the same output voltage when there is little or no load?

    -Giuseppe

  • In reply to Giuseppe Pinto:

    The boards do have the same voltage unloaded. So, I suppose it could be the variation in resistance values of the op-amp feedback circuit, but those components are all from the same run, so it seems unlikely they would vary significantly.

    Please advise me as to whether the LM25010 would work in this configuration.

    Thank you!

  • In reply to Craig Combes:

    Something I forgot to mention: I have also seen significant variations in ripple voltage present on the output from board to board under the same load conditions.

    I have also had a couple of boards come back that had swollen Cout capacitors. Even though I was using a cap that WebBench recommended.

    I think it's safe to say, in general, I have had issues with National/TI switcher chips in the past that were "unofficially" acknowledged by National/TI support staff, who suggested I migrate to other devices. Being a small company that can't afford to make a "market mistake" by delivering an unreliable product, I need accurate guidance on what device and configuration will result in a consistent and reliable product.

    I'm sure you can appreciate my position.

    Thank you.

  • The problems you are having may be coming from the high loop gain.  The error amplifier

    of a regulator will have a high gain, when combining that with the high gain of the exteranl opamp, you will

    get into stability problems.  I would try to find parts that have a compensation pin, so you can bypass the regulator

    error amplifier.

    F.D.

     

  • In reply to Frank De Stasi:

    Hello Frank and thank you for your reply.

    When you refer to the "regulator", is that the switcher chip?

    If so, it seems you're saying I should find a switcher chip with a "compensation" pin.

    Are there any you can recommend?

    Thank you!

  • In reply to Craig Combes:

    Yes; devices such as the LM5575 and LM25574 have compensation pins.

    F.D.