HC908AP LED driver question

[erokc]

I'm not understanding the LED driver on port A.  Vol = 2V with Vcc = 3V leaves one volt to drive the LED.  I don't have any 1 volt LEDs.  Is this a pull down pin or a source pin?  Anyone using it?

Dave

[bigmac]

Hello Dave,

The specification you refer to is the port pin sink current (via the N-channel MOSFET).  A higher voltage across the LED means a lower voltage at the pin, and therefore less current through the LED.

The output characteristic of the MOSFET, when conducting, may be represented by a resistance to the negative rail.  For 3 volt supply, the calculated resistance value may vary between approximately 130 ohms and 670 ohms (unit-to-unit).  A red LED would have a voltage drop in the vicinity of 1.8 volts, so the current through the LED may vary between about 2mA and 9mA, with no external series resistance.  If this current is insufficient, or the variation of current is too wide, an external buffer transistor or MOSFET would be necessary (including a resistor in series with the LED).

For a 5 volt supply, the internal N-channel variation would be approximately 120 to 330 ohms.

Regards,
Mac

Message Edited by bigmac on 2006-06-17 01:03 AM

[erokc]

Mac, thanks for the clearification.  I could not understand how to apply the driver.  Brightness is not an issue for me as this will be used for diagnostics, not normal product indicators.

Dave