Other Parts Discussed in Thread: PCF8574
Hi,
I'm trying to make sense of the current ratings for the PCF8574 IC. The datasheet seems ambiguous in this regard, but I would like to know what the current rating per I/O pin so I can determine how to connect it to other devices.
Thanks.
Hello David,
Thanks for the question, I'll be glad to help you.
There are a few ratings you must be aware of with this device.
1) Recommended High-level and low level output current maximums (IOH = -1mA, IOL = 25mA)
2) There is an absolute maximum current rating through VCC/GND at +/- 100 mA, but this is an absolute maximum, which explains you will shorten the lifespan of the device by going over the recommended operation conditions.
To make the difference between IOH and IOL clear, I'll give some examples, explaining the difference.
IOH is the current that flows out of the PCF IO pin. Say for example you have P0 set to an output and have it high (high voltage) and connect it to a LED, which then connects to ground (Through a resistor). The current flows when P0 is high, and this would be a IOH current.
IOL is similar, but with P0 as an output, you set it low (about 0V). Most will connect a LED with the path to ground being through P0 (as seen in picture below from PCF8574 datasheet). Current that flows when the output state of a port is low or 0 is IOL.
Example of how to make sure you don't violate the port's operating currents:
It is fairly easy to make sure we limit 1 port to the IOH/IOL maximums of -1/25 mA. The part we really need to be concerned with is when we have multiple ports sinking or sourcing current at the same time.
Let's say we have 4 ports sinking 25mA of current when low at the same time. We've already hit our absolute maximum current through VCC/GND of 100 mA and this will shorten the life span of our device.
We need to find the worst case scenario of maximum possible current in our use case and make sure we stay well under the absolute max.
The PCF8574 (and other IO expanders) are not meant for sourcing a large amount of current. Typically they are used for tasks such as button presses, LED driving and being used to detect state changes. I would suggest trying to keep current through the device as low as possible (below 50 mA continuous or lower) to help prolong the device's lifespan, as higher currents will create more heat internally.
