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TCA9539: 9.2.1.2 Minimizing ICC When I/Os Control LEDs

Howard Zou
Genius 12365 points
Part Number: TCA9539

Hi,

I've got question about datasheet's 9.2.1.2 part:

Because the LED acts as a diode, when the LED is off, the I/O VIN is about 1.2 V less than VCC.

My understanding is I/O is configured as input, and LED is off. But why VIN is about 1.2V less than VCC?

Vin is VCC-Vdiode， Vdiode should be less than 0.7V when off, 1.2V is almost on from my understanding.

  • 0
    TI__Mastermind 49040 points

    Hey Howard,

    The 1.2V comes the voltage drop across the LED due to leakage current. The 1.2V is an arbitrary number which will vary depending on the the LED you use. Some LEDs may have a 'Vdiode' less than 0.7V and other may have some larger than 1.2V. The point of this section in the datasheet is to highlight the effect of the leakage current into the input when a weak pull up resistor is not present.

    -Bobby