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XTR111 - Voltage Regulator Pin Current Boosting

Rohit Ghatkar
Prodigy 50 points
Other Parts Discussed in Thread: XTR111

I want to used REGF & REGS pin as VOLTAGE REGULATOR(  5V ), As per datasheet they mention the voltage regulator provide only 5mA of current, and my circuit requirement of current is approximately 10-15mA. Also they mentioning there Voltage Regulator Current Boost Using a Standard NPN Transistor schematic  at page no.20 is that work for me ?.Or any suggestion regarding that ?

Thanking you,

Regards

Rohit Ghatkar ,

India

  • 0
    TI__Guru 62035 points

    Hello Rohit,

    You are correct and the VREG output will begin to decrease quickly when the output current exceeds 5mA.  The circuit you found in Figure 45 is the configuration you should use to increase the available output current of the regulator to support your 10mA - 15mA requirement.  I've pasted it below for reference.  Please let us know if you have any additional questions.

  • 0 in reply to Collin Wells
    Prodigy 50 points
    Thank you sir, i appreciate to your quick response.
  • 0 in reply to Collin Wells
    Prodigy 50 points

    TOPIC : XTR111 Current Transmitter.

    Hello sir,

    Please find the attachment of circuit diagram.

    As i shown that circuit, My Vin should be vary from 0.5 to 3V for 4-20mA. And my XTR111 supply should vary as per customer requirement.(8v / 12v / 24v / 40v ).

    I have completed that circuit on hardware and while testing, I get some problem.

    1.When i am changing my supply( +ve ) voltage that time i didn't get proper current output that i got in previous supply. so that every time while i am changing supply voltage i need to adjust R22 reg.

    2. Also when i was using 15 Ohms register on R16 and R22.That time i got 18mA offset current.So please tell me how to remove that offset.

    2. So i request you to suggest me correction so that i can used my current transmitter circuit on any supply without changing any component.xtr111c.pdf

  • 0 in reply to Rohit Ghatkar
    TI__Guru 62035 points

    Hello,

    I think I can identify the source of some of the issues you're experiencing.  First, the transfer function for the XTR111 is 10*Vin/Rset.  Therefore with a 1.5k RSET resistor and a 0.5V input source the output current should be 3.33mA, not 4mA as you stated in the other post which was the same as this one (which I deleted).

    Second, the R16 resistor sets the current limit trip point based on the Vbe voltage of the Q4 transistor.  With a 1k resistor installed for R16 the current limit will be set at 0.6V / 1k = 600uA, assuming a 0.6V Vbe voltage for Q4.  This resistor should be sized between 15 - 25 Ohms depending on the current limit level you desire.  22 Ohms yields roughly a 27mA trip point.

    Third, R16 and R22 are in series with the output current.  Based on Ohm's law the output current can not exceed the supply voltage divided by the output resistance (I = V/R).  Therefore if the resistive load is too high the output current can't flow.  This is described in detail in the following blog.  This combined with the reason above are why the output current is changing with supply voltage.

    Make the suggested changes and let us know the results.

  • 0 in reply to Collin Wells
    Prodigy 50 points

    Sir ,

    I have made changes that you suggest , now my R16 and R22 is 22ohms , and I am taking 0.6v Vin for 4mA.

    But still I am not getting proper current output. Now on12V ( +vcc ) supply, when i started to varying my Vin i got 19mA to 25mA current output.

    I am using TRANSISTOR( 3F-BC856 ) and MOSFET - BSP170P.

    Or should i used current limiting resistor on my Vin.

    Thanking you ,

    Regards,

    Rohit Ghatkar

  • 0 in reply to Rohit Ghatkar
    TI__Guru 62035 points

    Hi Rohit,

    I forgot to mention in my last post, but please also make sure that you have the connections on the transistor correct.  As you've drawn it you have the PMOS transistor connected backwards.  The current flows from source to drain so the source should be closer to the IS pin and the drain on the connection closer to the load. 

  • 0 in reply to Collin Wells
    Prodigy 50 points
    Hello sir ,
    I got output, After changing a my faulty MOSFET .

    Thank you sir, for help. and i appreciate your instant response.

    Regards ,
    Rohit Ghatkar.