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LM2903: Addition of resistor to VCC

Joseph DeWitt
Prodigy 190 points
Part Number: LM2903
Other Parts Discussed in Thread: LM393LV, TLV7211, LM339, , TLV1851, LM4040-N, TLV6703, TL331B

Hi,

I hope you are doing well. I would like to use Texas Instruments LM2903 IC for the purpose of monitoring the voltage level of my intrinsically safe battery assembly by performing a voltage comparison of the power supply voltage to the comparator voltage which will be set by a Zener Diode with a nominal voltage of 8.2 V.

Given that the nature of my application of instrinsic safety I need to limit the power that can faulted to the IC. One of the pathways is through the VCC connection. Therefore, I'd like to add a resistor to the track feeding the VCC connection. My battery assembly will have three 3.6 V cells in series resulting in a combined voltage of 10.8 V. Based on the datasheet family comparison table the IC consumes a maximum of 2.5 mA. Assuming a 1000 ohm resistor is installed on the VCC track this will result in a voltage drop of 2.5 V (2.5 mA x 1000 ohms = 2.5 V) and therefore 8.3 V will be supplied to the IC under this scenario (10.8 V - 2.5 V = 8.3 V) which is above the minimum supply voltage of (-0.3 V) according to section 6.1 Absolute Maximum Ratings Table.  

Can you please advise regarding whether this conclusion is accurate and I can proceed with installing the 1000 ohm resistor on the track feeding the VCC connection on your LM2903 IC.

Thanks in advance for your support.

Best regards,

Joe DeWitt

  • 0
    Prodigy 190 points

    Hi,

    In regards to the summary above I reviewed the datasheet again and noticed that according to 6.2 Recommended Operating Conditions the minimum recommended supply voltage is 2 V. Also, with three cells in series the minimum supply voltage will more likely be around 9 V once the cells become close to being fully discharged. Therefore, 9 V - 2.5 V = 6.5 V > 2 V.

    Therefore, I still believe the I should be alright with installing the 1000 ohm resistor on the track feeding the VCC input to the IC but would still like to confirm with you before proceeding.

    Best regards,

    Joe DeWitt

  • 0 in reply to Joseph DeWitt
    Guru 255530 points

    For the device to work correctly, you indeed need to respect the recommended operating conditions. Please note the input voltage range; with a 8.3 V supply, you cannot handle input voltages above 6.3 V.

    This application needs a comparator that can handle voltages near the positive supply, such as the TLV7211. If you have a low-voltage supply and divide down the battery voltage, you can use the cheaper LM393LV.

  • 0 in reply to Joseph DeWitt
    TI__Guru 51166 points

    Hello Joseph,

    I'm still not very clear on exactly what you are trying to do. Can you sketch it with the expected trip levels? Are the cells connected directly to the comparator inputs (that *definitely* should have a series current limiting resistor!). Are you using the comparator supply as a reference?

    As Clemens said, you need to be aware of the input voltage limitations. Most CMOS comparators have ESD cells between the inputs and supplies, so that would clamp the input to the VCC supply.

    The LM2903 does not have the ESD clamps to VCC, but it is limited to the maximum input voltage range of VCC-2V, or 6.3V max on a 8.3V supply (that's where Clemens got the 6.3V number).

    This is all explained in section 2.5 of the LM339 family application note:

    Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    To do what I think you are trying to do, you would be relying on the "feature" described in section 2.8, but you also could fall into the trap of output polarity flipping when both inputs are above the voltage limit. Not a good thing for an "Intrinsic Safety" design...

    The Mins/Maxes are fairly "padded" from limitations of 50 years ago when they were set. The typical current will be about 800uA for the modern dual.I would highly recommend using the LM2903B version, for the lower bias and supply currents.

    I would also not rely on the comparator supply current to provide a regulated, stable voltage. It will slightly change unit-to-unit, as well as change over temperature and actual supply voltage (see the supply current vs supply voltage graphs). A reference of some sort (Zener, shunt reference, etc.) would be needed to stabilize the voltage.

    You may also want to look at the new TLV1851/61, which has <1uA supply current and "failsafe, over-the-rail" inputs that can go up to 40V regardless of supply (min 1.8V), and still gives the proper output. I think it will do what you want to do. It also has reverse battery protection on the supply and was basically designed to monitor at or above the positive rail in battery applications.

  • 0 in reply to Paul Grohe
    Prodigy 190 points

    Hi Paul and Clemens,

    First allow me to say thank you for the detailed helpful explanations. The Texas instrument support team is by far the most responsive and helpful team of technical experts I've ever come across so I'm very grateful.

    To the topic at hand, attached please find an example schematic of what I'm hoping to achieve. The two tactile switch / LED / MOSFET circuits on the left side are intended to direct power to two separate circuits so they are functionally separate from what I'm trying to achieve with the LM2903. The circuit on the right hand side with the LM2903 is my representation of what I'm hoping to accomplish with respect to low voltage monitoring and triggering of the LED and micro buzzer once the low voltage condition (8.2 V) is reached. 

    Wonderful, thanks for the recommendation regarding the TLV1851/61. I'll definitely take a look at that IC. I also visited with my team and they are recommending a Texas Instrument LM4040-N as a voltage reference IC instead of the zener diode which I currently have shown in the schematic. 

    Thanks as always for your guidance and support.

    Best regards,

    Joe DeWitt

     LM2903 Circuit.pdf 

  • 0 in reply to Joseph DeWitt
    TI__Guru 51166 points

    Hi Joe,

    Ok..I see. So it looks like you want to have the buzzer/LED turn on (output low) when the supply drops below 8.2V (IN- > IN+). This is a simple "under voltage" detector.

    Instead of placing a resistor in the supply, place a resistor from pin 3 to GND to create a voltage divider on the negative input (instead of direct). Then set the divider tap voltage of the divider above the reference voltage. Be sure to set the threshold voltage to be at least 2V below your lowest expected supply voltage, so no inputs are ever above Vcc-2V.

    We do have some comparators with a built-in reference. See the TLV6703 (18V max supply, 400mV reference to -IN).

    The TL331B is the single channel version of the LM2903 dual - if you want to stick with the LM2903 family and avoid the floating inputs. The Appnote discusses what to do with the unused channel (section 5.1.2).