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
