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ISO7842: full # ISO7842FDWWR: Help to check internal Block on it.

Part Number: ISO7842

Hi TI Expert,

We now use ISO7842FDWWR, then find an issue on it about the power supply.

When we tested under the abnormal power supply of the chip, IC still could work. It is suspected that the EN2 pin and the VCC2 pin are internally connected by diodes. Please help to confirm the internal circuit, thank you.

Background: VCC1 and GND1 are normally supplied with 5V, while VCC2 and EN2 are normally supplied with 5V. However, when the 16PIN VCC2 pin is turnup, disconnected with PCB pad, IC can still work. The measurement shows that the voltage of 16PIN VCC2 is about 4.2v.

  • Why does it still work without power supply on VCC2? (Only EN2 have a 5V power supply)



  • All input and output pins indeed have ESD protection diodes from GND and to VCC. When you apply a voltage higher than VCC, a current will flow into VCC and can power up the chip. (This is why the absolute maximum ratings forbid such voltages.)

  • HI Cyan,

    Your guessed it right, there are ESD protection diodes between all the input/output and power supply pins. The below diagram shows the internal schematic of ISO7842FDWW input pins.

    When there is ESD voltage at these input pins, the ESD diodes clamp the input voltage to supply voltage thereby protecting the device. These diodes are only meant for ESD transients which are very short in duration (a few tens of nano seconds) hence, applying continuous voltage to the input pins when the power supply is not available is not recommended. This will reduce the life of ESD diodes and could damage them.

    The above operation also violates the absolute maximum voltage rating allowed on any input pins. The abs max rating of input pin is VCC+0.5V, this means that if VCC voltage is 0V then input voltage should be <0.5V otherwise it could lead to device damage.

    I hope that clarifies your doubt, let me know if you have any other questions. Thank you.

    Koteshwar Rao