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TINA/Spice/TL431: TL431 as current source

Ella Zhang
Prodigy 330 points
Part Number: TL431
Other Parts Discussed in Thread: TINA-TI, , TLV431

Tool/software: TINA-TI or Spice Models

Dear All

My customer want to use the TL431 as constant current source with 2~3V input.

But the current will change, when the load change.

Vin=16V,  change R2 from 1 to 10. The current remain unchanged.

But Vin=5V, the current will change from 156.77mA to 225.42mA

I wonder why the current will change. Thanks a lot for your help.

  • 0
    TI__Mastermind 19580 points
    Hi Ella,

    At 5V you do not have enough headroom for R2.
    The current set is by Vref / R1 + Ika = 2.495V / 10 Ohms + Ika = ~0.25 A.
    If R1 and R2 = 10 Ohms then then Voltage drop between those 2 resistors is already 5V.
    Aside from that there is a voltage drop VCE across the 2N2221 which is ~1.1V based on the Ic of 250mA.
    Then there is the voltage drop across R3 that is based on the bias current of the TL431.


    If you need to create a 250mA constant current source based on a TL431 then you need more headroom. If you want this to work at 5V then switch the TL431 for a lower VREF device such as the TLV431 and scale the resistor down. A TLV431 will lower the headroom requirement but it will still have its limits.

    -Marcoo