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Original question:

LP8860-Q1: Protection nFETs and SQW output

LP8860-Q1: Protection nFETs and SQW output

Newton Cardoso
Intellectual 745 points
Part Number: LP8860-Q1

Hi,

Could you evaluate if the following connections made in protection FET of OUTx pin of LP8860 are suitable?

Best wishes,

Newton Bomeisel Cardoso
Engenheiro Sênior Desenvolvedor de HW
PULLUP SOLUÇÕES EM SISTEMAS ELETRÔNICOS LTDA
Av. Professor Lineu Prestes, 2242
CIETEC/IPEN - sala 210
Cidade Universitária - USP - Butantã
São Paulo/SP - CEP: 05508-000
(11) 3039 8311

,

  • 0
    TI__Expert 5475 points
    Hi,

    Would you mind describing a little bit more about your problem? Sorry but I did not understand what your problem is. Where do LED1 and DRIVER1_OUT1 connected to and what is the function of this circuit?
  • 0 in reply to Shirley He
    Intellectual 745 points

    Hi,

    Instead of using SQW output to bias the gate of an N-channel protection MOSFET, and as 5V isn't available in my design, I used 3.3V connected to the gate of SI2392, an N-channel MOSFET with Vgs threshold smaller than 3 V.

    My circuit is depicted below:  

    Si2392 safe operating area is:

    I'd appreciate if you could check  my design of the overvoltage protection for OUTx pins of LP8860, I am not sure if SI2392 is suitable for it.

    Best wishes,  

  • 0 in reply to Newton Cardoso
    TI__Expert 5475 points
    Hi,

    I'm afraid 3.3V gate voltage is not high enough. Considering the LED current sink headroom voltage, the LED comparator hysteresis, and voltage variations of the four LED strings, what is the maximum possible voltage at OUTx pins? You have to make sure Vgs is always higher than the Vgs_th for normal operation. Why don't use the SQW pin to get a voltage twice of VDD?
  • 0 in reply to Shirley He
    Intellectual 745 points

    Hi,

     The voltage on OUTx pins when current source of LP8860 is on is 1V (maximum). We are trying to avoid layout changes in our design as it is in ramp up phase, that is why we are trying to avoid using SQW pin.

    Changing the voltage in SQ2398 gate to 4V, all 4 LED strings presented very similar luminosity, that is the condition we were looking for. As SI2392's Vgs(th) is lower than SQ2398's , I am optimistic that SI2392 will work well, but to be sure I have to wait one week more until this P/N arrives.

    Best wishes,

  • 0 in reply to Newton Cardoso
    TI__Expert 5475 points
    Hi ,

    If the maximum voltage on OUTx pins is 1V, I think the SI2392 should work.