Hi,
What is the sink current capability of the ALL_DONE pin on the DS80PCI800? We would like to connect a LED to indicate that the IC has successfully loaded its EEPROM's contents. The only mention I could find in the datasheet is a test condition of 4mA for the Vol spec. Can it sink more?
Thank you,
Regards,
Cedrick
Hi Cedrick,
The device should be able to sink more current, though it may come at the expense of a lower VOH or higher VOL level than what is ensured in the datasheet. How much current are you expecting to sink through the ALL_DONE pin?
Most LEDs that are specified with a forward current around 20 mA should work. By design, we can limit the current based on requirements. If it is of any help, I recommend using the same LED that we have been able to verify works correctly with the DS80PCI800EVK and a 220-ohm series resistor and keeping the sink current around 4 mA. This LED we use is the ROHM SML-P12PTT86. I believe that there is now an updated version of this called the P13PT in the datasheet for these parts.
The green LED's forward voltage v. forward current, i.e. sink current is shown in the figure below (see SML-P13PT specs):
With some back-of-the-envelope estimates, we can design for a forward current of 4 mA across the LED. We deduce that 4 mA forward current results in forward voltage of about 1.95 V (see the green line above). If we assume a typical drop of 1.95 V across the LED with a 2.5 V VDD supply, and assuming that VOL = 0 V (typ) at the device pin when DONE# = Low, then the current will be limited by the series resistor. With (2.5-1.95)/R <= 4 mA, we find that R > 138-ohms will suffice to keep the current around 4 mA or less. The maximum limit for R is approximately 600 ohms to ensure that the LED turns on with at least 1 mA of current ((2.5-1.9)/R >= 1 mA).
I recommend striking a balance in the middle with a typical value of 220-ohms series resistor to accompany the LED (assuming 20 mA-rated) you use.
Thanks,
Michael
