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LM2903: During power-on, what is the output status

Part Number: LM2903
Other Parts Discussed in Thread: TL431, LM393LV

Dear Teams,

My customer is using LM2903. They are wondering what is the output status (H or L) during the power on? I do see that TLV704X has a POR circuit to determine the output status (H or L) during the power on. Thus, LM2903 output status will be floating because it does not have internal POR circuit, right? Also, if we add a pull-low resistor at the output end, then the output status will be L during the power on, am I right?

Thanks!

Dylan 

  • A POR circuit ensures that the output has a defined state (high or low or high-impedance, depending on how it is designed) during power up. Without a POR circuit, there is no guarantee.

    But in the case of the LM2903, the internals are well documented; see the Application Design Guidelines for LM339, LM393, TL331 Family Comparators:

    When VCC is too low for three VBE voltage drops (at about 2 V), both I1 and I2 are cut off, so Q7 is off, and Q8 is on. But when VCC is too low for the bias circuit (about two VBEs), Q8 is off.

    The LM2903 needs a pull-up resistor at the output; an additional pull-down resistor would not make sense.

  • Hi Ladisch,

    Customer found there is a step change after power on. Please check the waveform below. Yellow one is the power supply, and the blue one is the OUT1. They have connected 10K pull-down resistor at the IN1, then to GND. IN+ connected to 1.25 Vref which comes from TL431. At the OUT1, they do connect a 10K resistor to Vcc. Could you help give some suggestions here?

    Thanks!

    Dylan

  • The LM2903 does not have a POR. If this behaviour at startup is a problem, use a comparator with POR.

  • Thanks for your question Dylan.  This sort of need for a known startup condition is becoming more common.

    As Clemens mentioned, it would be best to use a device with a POR circuit included.  I also noticed that your operating voltage appears to be in the 3V range.  We have a new device, LM393LV, which is the 5V version of the LM2903.  This device has POR integrated in addition to having better specs in nearly all categories.

    Chuck

  • Hi Sins,

    Good to know that we have such powerful new device. I have recommended it to my customer, and see whether we can push them to change it.

    Appreicated!

    Dylan

  • You're very welcome.  It is an exciting new family of products for us and has been well received so far.  Thanks for your support of our comparators :)

    Chuck