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TPS61169: Trouble setting the current limit to 100mA

Mike Rankin
Prodigy 45 points
Part Number: TPS61169

I'm having trouble using part number TPS61169DCKR to drive just one IR diode part number VSMB294008G at 100mA. The IR led is a high powered one that can take 100mA  and my VIN is 3.3V. The data sheet says V FB is 204mV so 204mV/100mA = a 2.04 ohm resistor. With this resistor in place driving the LED I disconnected the one side and measure 250mA instead of 100mA going through it. Any idea why?

  • 0
    TI__Expert 4550 points
    Hi Mike,
    If you place the right resistor, 2.04 ohm, the current should be 100mA. When you "disconnect one side", do you mean you insert current probe to measure current? If so, do you happen to disconnect FB pin? When you see 250mA, could you please measure what the voltage of FB pin is?
  • 0 in reply to Caryss Li
    Prodigy 45 points
    Yes Caryss. When one side of the Led is removed and a multimeter inserted to measure current the FB pin is disconnected.

    Yesterday I removed the 2 ohm resistor and put a 200 ohm tim pot in its place. When it is set to 13 ohms I get exactly 100mA throught the led. This is perfect but does not follow the Rset calculation on the data sheet.
  • 0 in reply to Mike Rankin
    TI__Genius 9430 points
    Hi Mike,

    I believe what is causing the inconsistency is that the forward voltage of the IR diode is typically only 1.45V at 100mA. Since this is a boost LED driver and your Vin is 3.3V it doesn't need to boost to get to 1.45Vout. It has likely gone into a bypass mode and just passes the 3.3Vin through.

    Looking at 13 ohms, I believe this matches. Vin is 3.3V, LED drop is 1.45 so the voltage across the resistor is around 1.85V. 1.85V/13ohm = 142mA. This looks like it's within tolerance.

    If you only need to drive a single LED, I would recommend a buck LED driver. Or if you put more LEDs in series, the part will operate in boost mode and the current should match the equation in the datasheet.

    Please let us know if you have any follow up questions,
    Nate