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LM3409: Using external current sense amplifier as Rsense

Part Number: LM3409
Other Parts Discussed in Thread: INA250

Hello,

I am designing a simple programmable current source around LM3409. One of the application requirements is load current reporting.

As I will most likely be using INA250 for current measurement, is it possible to employ its built-in high precision sense resistor as Rsense?

Regards

Tim

  • Hello Tim,

    The LM3409 regulates peak current so the INA250 would not work for supply current sense information.

    What could be done is the INA250 output could be used with Viadj of the LM3409 to improve accuracy though it would require additional circuitry to do this.

    Best Regards,

  • Hello Irwin,

    Thank you very much for instant reply.

    My primary goal was to skip external sense resistor whatsoever. Apart from current reporting feature, are there other drawbacks?

    So if I understand it right, and the INA250 would report the peak current, then perhaps adding a simple LPF would be enough?

    Routing INA250 output back to LM3409 looks attractive, I am only afraid of the consequences as that'd add another control loop on top of LM3409 built-in one, making current regulation less efficient.

    Regards

    Tim

  • Hello Tim,

    The LM3409 doesn't have a control loop, it's hysteretic control.  This is why the INA250 won't work it has limited bandwidth.  The LM3409 needs to see the real time inductor current to operate correct.  Other parts with control loops could use the INA250 output as long as the bandwidth of the LED driver is low enough to negate the delays of the INA250.

    Routing the INA250 current information back to Viadj would be done with an external error amplifier.  This also depends on if you are dimming, PWM or analog.  This would complicate it.

    Best Regards,

  • Hello Irwin,

    Thank you. INA250's bandwidth of 50KHz appears to be too limiting. I'll give it a try as a shunt opamp on its own.

    In my application, the LM3409 is powered from separate supply, and load current source can be either DC or PWM up to 1KHz.

    Regards

    Tim

  • Hello Tim,

    The LM3409 being hysteretic is good with PWM dimming.  Other parts that have control loops have circuitry that allows the feedback to be disconnected when PWM is low to prevent keep the compensation capacitor from changing during PMW low.  This would make an external loop difficult.  Using it as you last statement is a good way to go.

    Good luck,