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lm339: Prevent comparator output from pulled high when comparator turns on in the beginning.

Jose Peeterson20
Prodigy 160 points
Part Number: LM339

Hello,

I have an overvoltage protection (OVP) in a 380V power supply. Overvoltage protection is at 420V. This power supply does not have a 380V discharge circuit so the output still discharges slowly when the output is turned off.

I am using a LM339 comparator to compare a 2.5V reference with a the voltage divided 380V output.

We are doing repeated turn on/off of the power supply quickly using the mains AC switch. The circuit is shown below.

When the AC is turned ON quickly after turning off the spike of 2.5V is coming at the comparator output. Is there a way to slow down or delay the compactor output? the spike lasts for about 500uS. The comparator is actually starting up due to 15V at the instant the AC power is turned on. Can it falsely produce output if it is starting up and the output is pulled-up to 15V?

Because I also saw scope wave forms where the OVP is below 2.5V reference and the comparator output goes up to 2.5V and the comparator quickly pulled it low.

Is it possible to null the comparator for the first 500uS and allow it to operate after that?

Maybe the comparator is working correctly then can I know how to remove this spike in the beginning? 

Cheers,

Peeterson.

  • 0
    TI__Prodigy 600 points
    You could try to swap the 0.1 uF (far left) with 0.22uF (far right) so to make 2.5V ramp up faster than ovp. Following the same concept, increase the cap at ovp higher as long as output meets the protection timing.

    BTW, you could use a greater value for the 1 k resistors as to not over drive the lm339 input ESD diode. 100k perhaps a better choice.

    -Jian
  • 0
    TI__Guru 53826 points
    Hello Peeterson,

    500us is too long for the LM339 comparator start-up delay. What is the ramp rate of the comparator's power supply (I assume the 15V)?

    It is possible for the comparator to output an incorrect value when the supply voltage is less than 2V and/or if both inputs are greater than V+ - 1.5V.

    What's missing is what the comparator power supply is doing at the time. Below minimum supply, the output is generally high impedance (no guarantees). So the pulled-up output will initially track the 15V supply ramp until the comparator pulls the output low. You may be seeing the initial 15V supply ramp up to 2.5V.

    At 2.5V supply, BOTH inputs would need to less than 1V for a correct output. See section 9.2.2.1 in the apps section of the LM339 datasheet for more info - but keep in mind this is only with a valid minimum supply voltage (>2V).

    Can you provide a scope photo showing both input pins, V+ pin and the output pin at the same time during the "glitch". I think this will give us a clearer explanation of what is going on. We need to see the inputs in relation to the comparator supply ramp. Even a cellphone snapshot of the scope screen is helpful...

    Do you have a AC_ON signal line? The output is open collector, so it may be possible to parallel another comparator, or MOSFET, with RC time constant to "mute" (hold low) the line for a period of time after a power fail.

    What you really want to do is increase the hold-up time of the 15V supply to the reference and comparator. Is it possible to feed the 15V from the 15V isolation diode directly to the comparator (lifting a pin & adding a large bypass storage cap). You need to keep the 2.5V reference up and the comparator supply at least 4.5V.
  • 0 in reply to Jian Zhang5
    Prodigy 160 points
    Thanks Jian,

    Reducing the capacitor is better because i am repeatedly turning on/off so you have to mind the capacitor discharging ASAP so that it is as close to 0V as possible when 2.5V starts.

    100K instead of 1K. Will it affect hysteresis? I put 220k for hysteresis Resistor.
  • 0 in reply to Paul Grohe
    Prodigy 160 points

    Thanks a lot Paul.

    15V takes about 30mS to reach 15V.

    Like you said, I also feel just a RC filter isolation between 15V going to Vcc and output of comparator will delay the output enough only during startup.

    I will share photos soon.

  • 0 in reply to Jose Peeterson20
    TI__Prodigy 600 points

    Yes, using a 100k (say R1) at the non-inverting input will affect the hysteresis. The 220k (say R2) resistor need to be adjusted to match your original hysteresis values as follows:

    When the output is low, the following equation holds:

    (vOVP / (R1 + R2) ) * R2 = vRef (2.5V)

    Using R1 = 100k, given vRef and vOVP to solve for R2

    If you think the replies answered your question, please mark it Resolved.

    Best.

    -Jian