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LM2901: Input Common Mode Range

Brian Wang0928
Genius 12500 points
Part Number: LM2901
Other Parts Discussed in Thread: , LM339LV

Customer facing a noise issue and wondering to know what happened. pls see below description and schematic/waveform.

We found output of LM2901 is fault trigger (CH5) during noise coupling on DC bus of the ac inverter, you can see there is a drop out voltage as red color. REF voltage is set at 4.54V. But it looks that the Input of the comparator (pin 6) is not reaching the 4.54V. 

Trying to increase the C5 bigger, it could solve the issue.

1. One more question, what if the IN- (pin6) is lower than -0.3V (Single bias power +5V to 0V) during operation? If so, why input range is over range that could cause the fault trigger?

2. I find the common mode input range is different between LM2901 and LM2901B, right? Is LM2901 able to use dual bias power? how about LM2901B?

Application: AC inverter

Signal Frequency: 60KHz

Bias power of LM2901: +5V to 0V

Regards

Brian

  • 0
    Guru 269710 points

    The description says that the comparator is supplied with 5 V, but the schematic shows 16 V.

    If the supply is 5 V, then the input voltage indeed is outside the valid common-mode range; the LM2901 needs a headroom of 2 V to the positive supply. It is not guaranteed to work correctly.

    1. If the input voltage is below V−, then the device is not guaranteed to work correctly. If the input voltage exceeds the absolute maximum rating, then the device might get damaged. For what exactly happens, see section 2.6 and 2.7 of the LM339 family application report. To avoid those currents, try adding a Schottky diode clamp at the input.

    2. The LM2901 and LM2901B have the same common-mode range. Both would be able to use dual supplies.

    If you want this circuit to work with a 5 V supply, then use a low-voltage comparator with rail-to-rail inputs like the LM339LV.

  • 0 in reply to Clemens Ladisch
    TI__Genius 11701 points

    Thank you Clemens for the explanation.

    Hi Brian,

    Yes with a 5V supply you would be operating outside the common mode range (-0.3V to VCC-2V). Pleass reference the app note that Clemens linked. You can use a split voltage divider and diode clamp configuration if you expect to have large negative voltages.  This will go for both LM2901 and LM2901B.

    As our team is out on holiday this week, please expect some delay and we will get back to you shortly. Thank you for your patience.

  • 0 in reply to Chi Nguyen1
    TI__Genius 12500 points

    Thanks. One more question.

    If supply bias power is +15 and -15V on LM2901B, the input common mode range should be -15V to +13V, correct? and what is the absolute max. Input range?

    In the 7.1 Absolute Max. Ratings Table, the Input pins (IN+, IN-) is -0.3V to 38V. I'm a little confused about Input range max. rating when +/-15V supply power is used.  

    Regards

    Brian 

  • 0 in reply to Brian Wang0928
    Guru 269710 points

    −15 V to +13 V is correct.

    Voltages are measured relative to the GND pin, so ±15 V is seen as 0 V … 30 V by the device. The absolute maximum rating is independent of the supply voltage, so the device will not be damaged by voltages higher than VCC. For what actually happens when you go above VCC − 2 V, see sections 2.4 and 2.5 of the LM339 family application report.

  • 0 in reply to Clemens Ladisch
    TI__Genius 11701 points

    Thanks Clemens for your support.

    Brian,

    Yes, the recommended input common mode range would be -15V to +13V since it is relative to the VEE and VCC pins. We just cannot warranty behavior if both inputs are outside of this range as the output would then be considered indeterminate. The comparator wouldn't be damaged up to 38V however, as Clemens mentioned. Please take a look at those sections in the app note. .