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TPS61196 FBP resistor setting

Dan Andersen
Genius 11460 points
Other Parts Discussed in Thread: TPS61196

What is the proper method for determining the FBP resistor setting?  The datasheet gives equation 5 to calculate the resistor, but it is a little unclear as to what Vled_short actually represents, and what that should be for any given design.  Can we give more detail on this?

Thanks,

Dan

  • 0
    TI__Intellectual 1780 points

    Hi Dan,

    TPS61196's Boost output voltage is constantly adjusted to account for any changes in the LED forward voltages. 

    The expected LED current is set by R11 using equation 3. To sink the set current, a minimum headroom voltage at the IFB pins is required. TPS61196 also regulates the minimum IFB voltage to the required voltage set by the resistor R10 at the IFBV pin using equation 4. 

    If a string has one or more LEDs shorted, the corresponding IFB pin voltage rises but continues to sink the LED current. The headroom voltage increases, this leads to more heat on the device. If there is no short voltage threshold (or if it very high), the device can turn off due to Thermal shutdown. 

    We have VLED_Short (LED short threshold voltage). If the voltage at any IFB pin exceeds this threshold, the device turns off the corresponding current sink and removes the corresponding IFB pin from the output voltage regulation loop instead of turning off the entire device. Current regulation of remaining IFB pins is not affected.

    The choice of VLED_Short depends on the application and the customer. If they want to detect 1 LED short, they want to set accordingly considering mismatch of the strings. Usually it is set ~6V (by the customers I'm working with)... so if there are about 2 LEDs shorted, the channel/string turns off.   

    Hope this answers your question. Please feel free to ask if you've any further questions.

    Best,

    -Harini