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TPS92691-Q1: what is the purpose of Q4 in fig 26 of the data sheet

ajayt
Genius 9555 points
Part Number: TPS92691-Q1
Other Parts Discussed in Thread: TPS92691

Hi Team,  Could not the function just be done with resistor divider in this diagram ( fig 26 Q4) ?   Also just wondered if there was an undervoltage circuit for this part?

  • 0
    TI__Expert 3590 points

    Hi

    You need Q4 to level shift and convert the differential voltage between LED+ and LED- to single ended voltage referenced to ground. The resistor divider would only give you the LED+ voltage

    -Sumeet.

  • 0 in reply to Shashank Kulkarni
    TI__Genius 9555 points

    Many thanks, I guess what I was also trying to understand why is it important to measure the overvoltage across the led terminals,  is not the failure condition that you should be most worried about an open string and Vled+ going too high?  If there was a led that shorted short then the voltage between the led terminals would decrease?

  • 0 in reply to ajayt
    TI__Mastermind 18890 points

    Hi Ajayt,

    The feedback to the OV pin is only use to protect an over-voltage condition and not a one LED short out of a string of LED's.  The transistor level shift gives the actual differential LED+ and LED- voltage and if there is one LED short out of a string then the voltage would decrease but the TPS92691 will not be able to detect this one LED short failure from a string of LED's. 

    Thanks Tuan