Window Comparator- LTC1042 for 12V window detection

I have implemented a comparator and LED indication circuit to detect 12V supply with-in range of +/- 300mV.

As in the valid window considered is 11.7V (lower limit) and 12.3V (upper limit). With this understanding I have implemented an separate LED individually on pin# 6 (ABOVE) and another on pin#1 (WITHIN).

The LED on pin#1 will glow when the detected voltage falls in Valid Window  (11.7V to 12.3V) and when the voltage is ABOVE12.3V the LED on pin#6 will glow. As a supply to this IC, on pin#8, 12V continuously supply from DC adapter is provided and on pin#3, the input is from power supply board, that generates 12V, is provided.

The problem is the LEDs on both the pin is not glow and indicating the proper margins. I simulated the circuit on LT spice, with the same values of resistors on IC pins and verified the high/ low levels (12V_EX in this case) on pin#1 and pin#6 when Vin was applied such that, 11.7 <= Vin <=12.3

But when I am measuring and testing on actual PCB, this circuit is not giving correct results. 

Please review it and let me know if I have missed out any implementation in it.

Thanks,

Dhara

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  • 0
    •  Analog Employees 
    on Nov 6, 2020 7:08 PM 3 months ago

    Hello Dhara.

    The LTC1042 output cannot drive directly the power MOSFET for the LEDs.

    Connect a 10k ohms resistor from the LTC1042 output to the MOSFET gate.

    In addition, replace the STP8N MOSFET with a 2N7000 series MOSFET.

    The 2N7000 series MOSFET is easier to drive.

    Best Regards

    Philip Karantzalis
    Senior Applications Engineer

  • Hi Philip,

    I have isolated the LED circuit and just checking the WITHIN and ABOVE pin status. But even if I do not apply any input on pin#3, then on pin#1 and pin#6 7V or 5V is observed..

    I checked by shorting pin#3 with pin#4 (providing 0V/ Ground), but still WITHIN and ABOVE pin reads as 5V or 7V.. Is this expected observations..??

    I have implemented the circuit on LT Spice and tested it with different Vin (pin#3) values and it is working as described in part# datasheet.

    Find the attached LT Spice snap for this circuit simulation and please advice to resolve this..

    Thankyou.

Reply
  • Hi Philip,

    I have isolated the LED circuit and just checking the WITHIN and ABOVE pin status. But even if I do not apply any input on pin#3, then on pin#1 and pin#6 7V or 5V is observed..

    I checked by shorting pin#3 with pin#4 (providing 0V/ Ground), but still WITHIN and ABOVE pin reads as 5V or 7V.. Is this expected observations..??

    I have implemented the circuit on LT Spice and tested it with different Vin (pin#3) values and it is working as described in part# datasheet.

    Find the attached LT Spice snap for this circuit simulation and please advice to resolve this..

    Thankyou.

Children
  • Hi Analog Devices Team,

    Please help to resolve this issues on urgent basis..

    Based on the simulation results, we were confident that this circuit implementation will work and so we already fabricated 3 boards.

    But now when we are testing the boards, we are not getting correct output at WITHIN pin and ABOVE pin.. The issue is already mentioned in my above post.

    We are not able to detect the valid window for which we used this part from Analog devices. Lets us know if at all this issue can be resolved and if there is any work around for this, else these 3 boards will not be useful and we will have to go for any other IC or alternate manufacturer for the window detection.