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LM2903-Q1: Operation at VCC power off

Hidekazu Someno
Expert 5130 points
Part Number: LM2903-Q1
Other Parts Discussed in Thread: LM2903, LM339, TLV7022

Hi, Team,

A customer observed the operation of the attached waveform when VCC was powered off on LM2903-Q1 application.

The waveform shows the output drops momentarily to low when vcc drops to about 2v.

Though the minimum recommended VCC voltage of IC is 2v,
- What are the possible causes for this behavior?
- Does LM2903-Q1 always work like this?

LM2903_VCC off operation.pdf

Best regards,

H.Someno

  • 0
    Guru 140200 points

    Hi Hidekazu,

    hard to answer without knowing the component values. Can you please add them to the schematic?

    By the way, please remove C2. A cap at the +input of a comparator with a positive feedback loop providing hysteresis is highly counter productive! Why? The hysteresis serves to speed up the toggling through the threshold region while the cap slows it down again... 

    Kai

  • 0
    TI__Mastermind 49946 points

    Hello Hidekazu,

    Even though the minimum supply voltage is 2V, the valid input voltage range is near zero.

    The upper valid input range limit is 2V below VCC. So as the supply voltage drops, so does the upper input voltage range.

    Please see the LM339 family application note, particularly section 2.4:

     https://www.ti.com/lit/an/snoaa35a/snoaa35a.pdf

    When the supply is below 2V, the inputs are essentially cut-off, and the output tends to go low when both inputs exceeds the upper voltage range limit. At less than 1.2V, the internal biasing fails and there is not enough voltage to drive the output transistor, so it releases.

    We do have devices that have power-on reset, that sets the output to a known condition when the supply is less than minimum to prevent this. See the TLV7022 family. The output will go high when below minimum supply voltage.