Other Parts Discussed in Thread: ULN2003A, , ULN2003LV
I am optimizing the first iteration of a design that uses an SPI interface to two Maxim MCP23S17 devices to drive 20 reed relays.
Object is to drive the reed relays from the uC GPIO port pins, through either a TPL7407LA or ULN2003A or ULN2803A.
My design uses 5VDC reed relays and I wish to interface to a CMOS uC with a VDD = 3.3V and GPIO Voh of the uC is 2.9VDC min.
The TPL7407LA datasheet states the following regarding a Vcom < 6.5V:
"the part still functions with a reduced COM voltage that has a reduced gate drive voltage and a resulting higher Rdson."
What does this statement mean? How to quantify RDSon for 5VDC Vcom?
Alternatively, is the ULN2003A a better choice for this application? I am concerned, here, with whether the 2.9V Voh is high enough
to turn on the device input.
Another alternative - ULN2803A. The ULN2803A datasheet is very confusing as to whether it will interface to 3.3V logic input.
This because the first datasheet page states "the ULN2803A has a 2.7 kΩ input resistor for 5 V TTL and CMOS".
Not sure if this refers to 4000 CMOS powered by 5VDC.
So, for this application, which of these three devices is the best fit? Or should I look for something else?
Thank You,
-mark
