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high side constant current regulator with a enable pin

gary davies
Prodigy 110 points
Other Parts Discussed in Thread: LM317, LM317M, TPS27081A

Hello,

I am after a high side constant current regulator with an enable pin like the lm317, should work up to 80V and over -40 to +125 C. 

Automotive safety application, so must be low cost and the solution to use a few as possible components.

The application is to test for defective electrolytic capacitors by charging them up with a constant current and then calculating capacitance 'C' by the voltage rise rate. Capacitor fail by low C, or high leakage current is detectable.

The constant current source should switch ON in a few microseconds. The attached schematic shows a solution using transistors, it is not very good with temperature and there is about a 7mA wasted current in forming the HSS. Capacitor A is charged up to 40V using a flyback transformer in <50mS

What part is suggested?both_14_for_document_report.pdf

  • 0
    TI__Mastermind 24485 points
    Hello Gary,

    How high current do you need? Would it be possible to use something like the LM317M and add a switch in series to disable. The regulator doesn't actually see the full 80V, so it can still be used in this application.
    A load switch like the TPS27081A would be suitable for enabling the path for current.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 110 points
    I don't understand how to use the proposed part TPS27081A? The maximum switching voltage of the proposed part was 8.0 Volts, I was after about 80 Volts.
    Does TI have a 80 Volt 3Amp High Side Switch that can be configured as a constant current source?
    I was after a 3 Amp part so the LM317M does not supply enough current anyway.
  • 0 in reply to gary davies
    TI__Mastermind 24485 points
    Hello Gary,

    Yes, that was my mistake with the TPS27081A.
    I looked through a number of our devices, including our eFuses: www.ti.com/.../integrated-fet-devices-products.page
    It seems you may be looking for something like this device, but with 80V tolerance: www.ti.com/.../tps25926
    I was not able to find any single IC to do this function, but maybe you can implement a current limit and a high side switch. Let me see if I can loop in someone more familiar with this area to help further.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 110 points

    I think that the latest proposed part family takes too long to switch on - 220uS - because it uses NMOS and a charge pump. I was after a device that turns on in a few microseconds that probably uses PMOS power element .Please let me know the contact of someone inside TI so I can submit from myself further documentation about my project. my email is lion@palmbeach.freeserve.co.uk Regards Gary Davies

    I quickly lashed together a circuit that works on a simulator, however there are too many components so it needs integrating onto a chip. (see attachment)

    The HSS works from 7 Volts to 80 volts, switches ON in < 1 microsecond, and OFF in < 6 microseconds.

    The results from the simulator shows that by knowing voltage, current, and time , Capacitance can be calculated.

    C = (Time_step / Voltage_change)*Charge_current

    Capacitor A = (70us / 0.177 Volts) * 0.796 Amps = 315 uF (actual value was 330uF)

    Capacitor B = (70us / 2.5 Volts) *0.796 Amps = 22.2 uF (actual value was 22uF)

    Fast PMOS HSS.pdf

    8738.Fast PMOS HSS.pdf