LVCMOS -> Darlington Array -> 8 20mA LEDs (parallel)

Discussion in 'General Electronics Chat' started by mbird, Mar 12, 2012.

  1. mbird

    Thread Starter Member

    Jan 29, 2009
    Hi --

    I want to find a transistor array so I can drive a bank of 8 20mA LEDs from my Spartan 3 FPGA board. The FPGA output will be LVCMOS at 3.3V. I will drive one LED per LVCMOS output (so parallel). I see the ULN2803 would be ideal but the datasheet says: The ULN2803A has a 2.7-kΩ series base resistor for each Darlington pair for operation directly with TTL or 5-V CMOS devices. I can't see in the datasheet if 3.3 LVCMOS would be acceptable. Do you have suggestion for transistor array drivers for this application using 3.3 LVCMOS input to the bases. I can do source or sink for the LEDs so either type of drivers is OK.

    If guess it was just a single transistor in each spot in the driver and we knew the hFE, BE vDrop, etc. we could calculate it and then know if 3.3 V was enough -- but with this array we don't have all that info so I can't calculate what base voltage would saturate the pair given the 2.7K base resistor.