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LMP7701 question

Other Parts Discussed in Thread: LMP7701

Team, can you please help with the questions below.

 

I am monitoring a input voltage supply point that can vary from 4V to 35V.  I am dividing the voltage by a factor of 200/1440, so that at 36V, the input to the op amp IN+ pin is 5V.  I am using the op-amp as a unity gain buffer feeding the input to an A/D.  Is this device OK to use in this application, or should I use something else?  The input voltage is DC, and I have a capacitor to create an anti-Aliasing filter at about 300Hz.

 

I have two questions:

1. Can the MP7701MF handle 5V on the Vin pin with the device being un-powered?

2. I see that Vin has a max differential voltage of +/- 300 mV, but the input pins are rated to Vdd +0.3V. What does this spec mean? Does this result in question #1 being a no?

 

Thanks

Viktorija

  • Hello Viktorija,

    Please see Figure 49 in the LMP7701 datasheet apps section.

    A #1. The limit is 300mV above V+ as the maximum voltage. The abs max input current is 10mA.

    With the power off (V+ = 0V), internal ESD diodes from each supply to the input pin will clamp the input pin to the V+ supply. If the current is limited to less than 1mA, then it should not be an issue. The series resistance of the divider resistors can be used to limit the current. Use a minimum of 1kohm per volt of over voltage. So 35V would mean a minimum series resistance of 35k.

    But really you want to keep that current as low as possible - below 1mA - so I would recommend somewhere around 100uA or less (350k series resistance - which is reasonable for a divider). If this condition is expected during normal operation, then an external Schottky diode is recommended to take the stresses off the internal ESD diode.

    A #2 - There is s diode clamp between the input pins (D1 and D2) to prevent the breakdown of the input stage B-E junctions under large (>5V) differential voltage (voltage BETWEEN the input pins). Under normal op-amp feedback operation, the inputs are always within millivolts of each other and the diodes have no effect. But during a large differential voltage, like in heavy slewing or an output error, the diodes will conduct and clamp the inputs together. These clamps should not be a contributing factor in your circuit...the ESD diodes will be taking the brunt of the current.

    So as long as the divider resistors are large enough, there should not be an issue.

    Regards,