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XTR111: maximum ratings on pins, when no power supply is connected

Marco Glietsch
Prodigy 20 points
Other Parts Discussed in Thread: XTR111

Hello,

I'm using the Voltage-to-Current Converter XTR111 together with a programmable Hall sensor MLX90251 as input. The MLX90251 is in normal mode powered by the REGF output of the XTR111. The XTR111 has a power supply of 24VDC. So far everything works fine.

When I want to program the Hall sensor with the according programing tool it sets the sensor in programming mode by rising the supply voltage of the MLX90251 from 5V up to 9V. The XTR111 in this case isn't powered by 24V. But the 9V are on the Pins REGF and over a voltage divider (5k6 / 8k2) on Pin REGS. In the datasheet of the XTR111 is written, that the absolute maximum rating for the Pins REGF and REGS can be up to Vsupply + 0.5V. In my case of programming the Hall sensor, I don't have a supply voltage for the XTR111. On Pin REGF I can place a small signal diode to block the 9V from the pin. But I can't do so on REGS. Does it harm the REGS pin, when there are 9V on the pin and there is no supply voltage for the XTR111

  • 0
    TI__Genius 12325 points

    Hi Marco,

    I made a simple graphic of the XTR111 internals to answer your question:

    Diodes D1-D4 represent the internal ESD protection diodes on the part. Notice that when the voltage on the REGF and REGS pins goes beyond the power supplies, either the upper or lower diode will become forward-biased and begin to conduct. In your case, if the +24V supply is disconnected, and therefore is a high impedance (NOT connected to ground), the upper diodes (D1, D3) will not conduct when you apply +9V to these pins. I also confirmed with a colleague that 9V would not exceed the reverse breakdown voltage of the lower ESD diodes (D2, D4). 

    If you wanted to be extra careful, you could use a resistor to limit the current into the REGS pin in the event of a diode breakdown. According to the datasheet, the absolute maximum current allowed to enter any pin of the device is 25mA. A 360 ohm resistor would limit the input current to the XTR111 to 25mA if the lower ESD diode were to conduct (9V/25mA = 360 ohms):

    This will introduce a small amount of error to the REGF voltage but I would not expect it to cause a drastic change.