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LP3910

Dave Murphy
Prodigy 20 points
Other Parts Discussed in Thread: LP3910

On the LP3910, the datasheet talks about using a blocking diode which has been implemented. When the part is connected to a wall adapter that is reverse connected, the blocking diode seems to have no effect and the unit takes 1A of current.

Has anyone come across this. Should the Charge Detect pin be either connected through a resistor or to the cathode of the diode instead.

Many thanks,

Dave

  • 0
    TI__Intellectual 1955 points

    Hi Dave,

    Can you offer me some more detail on the implementation, type of diode, description rating of the adapter voltage using, and where is the 1A current went....

    Also  s the LP3910 loaded or not.  I will go review the DS in the mean time.  Thanks  Kern

  • 0
    TI__Intellectual 1955 points

    Hi Dave ,

    Thought I should include the page in the DS on power routing and like to see if you could help pinout what I/V are being observed on the nodes and the net as shown in the diagram below.   I am thinking that the example shown is probably not for reverse polarity polarity protection.  It is for protecting the battery from discharging back into the wall adapter (e.g. if a lower potential exists such as a defective device or accidental or whatever might have caused the adapter being shorted or heavily loaded such as its output goes below the battery voltage. 

    It appears when adapter polarity was reverse there is a path from the CHG_DET returns to the adapter from ground.  Check if this is a path where the ESD and other normally reverse biased junction are turned on by the polarity swap.   Hope this helps.  Let me know if you have further questions.

    Regards,

    Kern

  • 0 in reply to Kern Wong
    Prodigy 10 points

    Hi Kern

    Dave originally posted this after we asked him the question, but we never really got an answer to the real question here though, which is...

    How do we implement reverse supply protection with the LP3910?

    If we follow the datasheet and connect the 5V AC-adaptor backwards, lots of current flows through the CHG_DET pin of the chip and it gets destroyed. An obvious answer would be to put a series resistor between the AC-adaptor input and the CHG_DET pin, but we don't know if that will affect the normal operation. Can we add a series resistor, and if so what value would you recommend? If not, is there another way to implenet the protection we need?

    Cheers

    Andrew

  • 0 in reply to Andrew Hodges1
    TI__Intellectual 1955 points

    Hi Andrew,

    Since the CHG_DET has spec of 4.5V +/-100mV so a Sch_diode can't do.  A current limiting resistor as you suggested is viable.  Normally for a compare input, which is relatively high impedance the resistance may not be critical unless it influences the hysteresis thresholds.  You can put a series resistor starting say 100ohms and observe the current drawn and voltage drop, then adjust for minimum value that does the protection.   Check with Richard Dye, the PE and defecto apps person for comments on this.  Will send him a message and copy you.

    Kern