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LP38513 for constant current source

Other Parts Discussed in Thread: LP38513-ADJ, LP38513

Dear Sir,

My customer develope a contact resistance measurement instrument with 4wires configuration. that is to say exicate currrent source and output signal is seperated.  LP38513-ADJ is recommended to build constant current source.

It seems LP38513 output is instable because when it is enabled, 3X-5X inrush current at input may occurs. I think it is caused by incorrent loop compensation, but I don't know how to fix. appreciate your comments.

More details information could be found from the file i uploaded here.3808.LP38513_currentLoop.pptx

Again appreciate your comments.

regards

Hawk Tong 

  • Most likely this is in-rush current for charging of the output capacitor. This could be as much as 5.6A when the output capacitor is completely discharged (Vout=0V).

    Check for stability by checking the output voltage for oscillations when enabled.

    Output voltage should be at, or very close to, final value, and stable, in about 200us from enable pin going high.

  • Jones,

    I think insatblity may happen when LP38513 is enabled because too much ringing over-shot is found. inrush current is caused by output capacitor by somedegree. Can I serial a resistor with the output capacitor ? maybe it make loop stability worse.

    Where to download spice model of this part? I want to do simulation test and find a walk-around.

    Also appreciate your comments how to do the patch at right siuation.

    regards

    Hawk Tong

  • Yes, I agree that adding a series resistor to Cout (C7/10uF) to simulate ESR should be helpful if the basic problem is lack of adequate phase margin.

    I would suggest starting with 0.1 ohm resitor in series with Cout to see if there is any improvement. The 0.1 ohm and 10uF in series will create an fz of approximately 160kHz.

          fz = (1 / (2 x pi x Cout x Resr) )

    I would also suggest that Cff (C6/47uF) be eliminated.

    The f(zero) from adding a series R to Cout will create a well controlled zero in the control loop, while the fz created with the Cff will change with the resistance between the Vout and ADJ pins.

    Making Cout larger will lower the load pole, and that might be helpful.

     

  • Jones,

    thank you for your suggestion. I will try it ASAP.

    pls forward me spice model of this part or your simulation test result if it is avaible in your hand.

    many thanks.

    regards

    Hawk Tong

  • Sorry, no spice model is available.

    Even if we had one, it would be 'behavioral' and would not accurately support phase margin simulation.