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TPS2660: TPS2660: IMON and RTN during Normal operation and Reverse Polarity

Alexandre Pozzer
Prodigy 10 points
Part Number: TPS2660

I plan to use TPS2660 in a 24V application. The design includes a microcontroller referenced to GND to measure the load - IMON voltage is measured with R(IMON).
I have some doubts regarding the IMON pin behavior:
    - During normal operation, the IMON voltage is referenced to RTN. But the microcontroller ADC is referenced to GND. During normal operation, is there any voltage difference between RTN and GND?
    - In a reverse input polarity, I could not find how the IMON pin works. What is the voltage / current on this pin in such condition? I understand i have to add a series resistor (100k), but I want to understand the reasoning and voltage / current values involved.

  • 0
    TI__Genius 16305 points
    Hello Alexandre,

    During normal operation, there is no voltage difference between RTN and GND. They are shorted internally.

    But during reverse polarity, RTN and GND are open to protect from reverse input damaging the IC. In this case RTN gets pulled to IN and IMON also gets pulled to IN due to the R(IMON) and ESD diode from RTN to IN.

    For -24V input, so IN will be at -14V and IMON will be close to -24V. Micro controller input will be close to 0V (due to ESD diode from GND to MCU pin).
    In this case, huge current can flow through:
    1. GND to ESD of MCU (IMON)
    2. MCU(IMON) to R(IMON) to RTN
    3. RTN to IN through ESD diode of TPS2660.

    This current is now only limited by R(IMON). To reduce the current to a safe level, 100k in series between IMON and MCU is recommended.
    Please refer to the application note " www.ti.com/.../slva934"
    This note covers fault handling using TPS2660 during reverse polarity condition in detail, however the current flow path and limiting the current are similar to IMON as well. IMON in addition has R(IMON) to RTN.

    Regards,
    Kari.