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LM2903: LM2903 Output state

Part Number: LM2903

Tool/software:

I am using Lm2903 as a comparator for my short-circuit protection.

I am setting a threshold of 0.2V on the inverting pin and non inverting pin is connected current sensing IC whose output varies from 0 to 3.3V (1.25V for 0A)

However, for sleep mode implementation supply of Current sensing output is turned off during idle state which makes it output float. 

How will LM2903 will behave that time? Output will be still 5V or 0V?

  • Part Number: LM2903

    Tool/software:

    I am using Lm2903 as a comparator for my short-circuit protection.

    I am setting a threshold of 0.2V on the inverting pin and non inverting pin is connected current sensing IC whose output varies from 0 to 3.3V (1.25V for 0A)

    However, for sleep mode implementation supply of Current sensing output is turned off during idle state which makes it output float. 

    How will LM2903 will behave that time? Output will be still 5V or 0V?

  • Hello Monica,

    5V

    Assuming LM903 supply voltages are 5V and GND, IN- is 0.2V, and IN+ is floating, input bias current flows out of IN+ until IN+ voltage is roughly 5V - 1.3V = 3.7V. With IN+ near 3.7V and IN- at 0.2V, the output is high impedance. Therefore a pull up resistor at output will pull output high.

    If IN+ isn't truly floating, then things may be different. IIB is a small current.   

  • Monica,

    If output low (and input low) is preferred, place a 200k or less resistor from IN+ to ground. The maximum 500nA IIB passing though 200k ohms would only generate 0.1V ; so output (and input) voltage will be low.

  • Adding to Ron's comments, if the current sensing IC being turn off means that its supply is actually at the grounded (and not floating), ESD1 would provide the path for the LM2903 input bias current clamping its voltage close to ground.  In such case, the output of LM2903 would be driven low (see below) without the need of adding the resistor between IN+ and ground.