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TLV767: Constant current source with TLV767

Philipp123
Prodigy 170 points
Part Number: TLV767

Hi,

I want to build a constant current source with the TLV767. The reason for choosing this part is the enable input, input voltage range, package size and the integrated output pulldown.

My design of the circuit keeps the voltage over the R_SENSE resistor constant. Thus a constant current flows through R_LOAD.

1. What do you think of this design, is it reasonable? It works in simulator.

2. Can you say something about the stability?

3. Is there another circuit example for a constant current source with TLV767 where the load resistor R_LOAD can be connected to GND?


Thank you very much.

  • 0
    TI__Guru 55940 points

    Hi Philipp,

    TLV767 is designed to regulate the output voltage as set by the feedback resistor network.  The voltage at the FB pin is one input to the internal error amp.  The other input is the internal reference of 0.8 V.  As such, the TLV767 will drive the output rail so that the voltage at FB will be equal to 0.8 V.

    In order to set a constant current through a load resistor (Rl) use the following application:

    Choose your output voltage and load resistor such that Iload = Vout / Rl.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 170 points

    Hello Ryan,

    thanks for the straightforward answer, but that wasn't the point that interested me. The principle of a voltage regulator is well known to me.


    My application requires a constant current source (switchable) for a slowly changing load (in the range of 20..40 Ohm). Is this possible with the TLV767?

    Regards,

    Philipp

  • 0 in reply to Philipp123
    TI__Guru 55940 points

    Hi Philipp,

    Okay since your load will be changing in resistance, you are correct that my recommended location for the load will not yield a constant current.  Since Rload is changing, your schematic is the only location that will yield a constant current through Rload as long as the current through Rsense >> Ifb and you have adequate headroom (Vin - Vout) to keep the TLV767 out of dropout.

    As long as your load is resistive, I do not see an issue from a stability point of view.  For stability inductance in the feedback loop must be kept to a minimum.

    Potentially another method to set a constant current would be to place the Rload on the input side of the LDO and set a resistive load for the output.  This is similar to the concept used below for an LED Driver application.  This method would eliminate the stability concern as your load is no longer in the feedback loop. 

    We do always recommend prototyping applications so that you can see the actual performance that you can expect before committing to a design.  The TLV767EVM-014 can be used to help speed up this early prototyping.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 170 points

    Hi Ryan,

    I will give the TLV767 a chance and build prototypes with both topologies. Once with constant load on the output (dynamic load on the input) and further with dynamic load in the feedback path. Finally, the performance as a pulsed constant current source determines the suitability. Your suggestion was helpful. Thank you.

    Regards,

    Philipp