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TPS92690-Q1: Current sense question

Mark Sun50
Prodigy 110 points
Part Number: TPS92690-Q1
Other Parts Discussed in Thread: TPS92690

I am looking into LED driver TPS92690 for new product design and I am interested in cuk topology in Figure 19 of datasheet. I've question about LED current sense. In Fig. 19, the current is sense by Rcs, which is diode D1 current and not LED current. How does this circuit to keep the LED current constant when input line and current change? If you sense MOSFET current by Rds-on, the accuracy over temp. is not good.

  • 0
    TI__Mastermind 25500 points

    Hello Mark,

    I can try and get you the specifics if you would like tomorrow if needed, but basically if you do the math the average diode current in a Cuk is exactly the same as the average LED current. That is one of the reasons it is attractive, particularly for automotive. It allows you to sense locally and have the positive terminal of the LED string connected to system ground for a potentially one wire system. It regulates very well. There is a Cuk reference design for this device is you are interested, I believe there are some line regulation graphs included. 

    By the way, there are two sense elements here since you mention the MOSFET. That current sense path is for current mode control and current limit. The RCS is for the actual LED string drive current. But you are correct in the sense that the current limit setting will not be extremely accurate using Rds(on) sensing. If you need that to be accurate you use a resistor from the source of the FET to ground and sense its voltage rather than the FET Rds(on) voltage.

    Regards,

    Clint