Selecting and driving a P Channel MOSFET

[Macka]

Hi,

I'm wanting to drive some LEDs in an 8x32 LED matrix. The LED characteristics are:
If = 20mA
Vf = 3.4V
Vcc = 5V

I'm planning to arrange the LEDs in common anode configuration and will drive the low side with TPIC6B595Ns (high current 8-bit shift register). If a full row was ever on, it'd pull a current of 640mA.

My understanding is that if I use a logic level P channel MOSFET, I need to pull the gate low to activate it.

I was thinking I could use one of the TPIC6B595Ns (since I need a shift register anyway) to drive the gate. eg putting a 1 into the the LSB of a TPIC6B595N would pull the output low and hence turn on the MOSFET.

I've looked around on the AU element14 website and I selected a IRFD9120PBF MOSFET, because it has a Gate-Source threshold voltage of -4V (max) and a continuous current rating of 1A.

So, my questions are:

1) Can I drive the MOSFET like this?
2) Will this MOSFET do the job? - If not, why? and how do I go about selecting the correct MOSFET for the job?

Thanks

 

[MikeMl]

Since the shift reg is open-drain, there is no pull-up, so you will have to add one.

 

[Macka]

ok, so other than the need for a pull up, say 1kΩ, everything checks out?

 

[ronsimpson]

looks good to me.
You can use /G and clear to power up with all LEDs off. (and mosfet)

 

[Macka]

Thanks guys