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LMP8603: Saturated output operational mode

Part Number: LMP8603

Hello,

I'm going to use the LMP8603 ( i.e. nominal gain of 100 ) with external feedback resistor to increase the gain up to a total of x500. I'm using 5V supply.
The nominal current to be detected will provide a voltage output between 3.5V and 5V. Their will be input current producing output voltage theoretically well above the 5V rail and I expect that it will be clamped to rail ( i.e. 5V ).

In order to ensure proper use of the device, I would like to know if such use of output saturation is OK with this device and how large the differential input voltage can be when the device is saturated at its output. I don't see any specification in the device datasheet.

Many thanks and best regards,

Jonathan Bonnevie

  • Hi Jonathan,

    Thank you for using the E2E forums.
    I can confirm the device will not be damaged while operating in saturation. The maximum output voltage will be clamped at approximately VCC-300mV, so we can only guarantee correct operation up to 4.7V from a 5V supply.

    The differential input doesn't change with the operational voltage, however, would you be able to provide us with some more details such as a simplified schematic or the actual differential values at which you plan to have the part operating, so that I'm better able to advise?

  • Dear Carlos,

    Thank you for your quick answer.

    You will find below a simplified schematic that a I'm using for design simulation validation. The goal is to detect the presence of the RS485 termination resistor R2. The switch network between R2 and R6 simulate the connector interfaces that can be hot plugged/unplugged. When this interface is unplugged, no current will flow through sensing resistors R1 and R6 so that we can easily detect it using U2 and U3 as near 0V output. When connectors are plugged ( i.e. with the second end of RS485 line and its termination resistor R2 ), a biasing current will flow through R1 and R6 because of the RS485 resistor biasing network ( i.e. R3, R4 and R5 together with connected R2 ). This will be detected with U2 ( U3 has negative current and will output near 0V ).

    The tricky part comes when I need to take RS485 communication signalling into account to avoid false disconnection detection. This is handled by using U2 together with U3 to detect larger RS485 current that can flow in both directions with larger magnitude than the biasing current. I fixed the overall gain to a value such that communication signalling produce saturated output and biasing current produce a value between 3.5V and 5V at output of U2. I use filters to reduce noise as much as possible before and after amplification. The simulated differential voltage that appear at the inputs Vdiff = IN+ - IN- is around +/-150mV with actual configuration.

    I hope you have enough information with that to validate the proper use of the LMP device in such configuration.

    Best regards,

    Jonathan.

  • Hi Carlos,

    Did you get expected info from the schematics and the description of my expected use of the LMP8603 ?

    Many thanks and best regards,

    Jonathan.

  • Hi Jonathan,

    Thank you for bearing with me while I checked this.
    I can confirm, in the intended application and with the indicated signal amplitudes no damage will be caused to our device, however I cannot validate whenever the circuit itself will work.

    One thing I'm concerned about are transients. I see no input ESD protection. Given the nature of the RS485 protocol itself, it would be desirable to have some sort of filtering/limiting/ESD on the input, should the lines be subjected to abnormal conditions.

  • Dear Carlos,

    Thank you for your confirmation. I simulated this implementation and it seem to work. I'll validate it with actual prototypes very soon.

    Regarding the RS485 transients, not all parts has been placed for simulation. On complete schematics however, I already placed some TVS devices for EFT, ESD and surge protection to clamp the transient at voltage below the maximum common mode voltage of the LMP8603 device.

    In practice, the RS485 cable will be only 2 meters maximum and it is used for single point to point connection so it should not have too large transients by the way.

    Thank you for your advices.

    Best regards,

    Jonathan.