I have a question about driving an LED from an IC. NXP in the datasheet of their PCAL6416A (section 9.1) says that a resistor parallel to the LED should be used in order to minimize power consumption in the off-state, see image. Why is that needed?
In their datasheet they say that the voltage at the pin Pn of the IC should be at or higher than VDD when the LED is off. However, this can be achieved by setting pin Pn as output at high level, which should be VDD. In that case the voltage (with reference to GND) before the LED and behind the LED is both VDD, so no current should be flowing even in the absence of the 100k resistor.
Additionally, I could set Pn as input pin and thus disable current flow through the LED, again without the 100k resistor. So why is the 100k resistor needed?
More confusing is the other suggestion, supplying the LED with a higher voltage then VDD, see second picture. In this case, if Pn is configured as output, it can be high (3.3V) or low (0V). Now let's use a red LED with a voltage drop of 1.5V and supplied from 5V. In this case, I will get a current through the LED regardless of whether I set Pn to high or low. Why is this design suggested as a solution to reduce power consumption?