[Noisebridge-discuss] 3.3v serial-parallel shift register with open-collector output?

[Jake]

for fast switching of FETs (P-channel included) i recommend a nice mosfet 
driver like the IXDD604PI.  Propogation time is 19ns.  Of course, they're 
not shift registers, but you can hook them up to shift registers.

each 8-pin chip has two separate FET drivers.  You can use it with a 
system voltage up to 35 volts.  If you are driving P-FETs on the high 
side, your voltage has to be within the maximum gate swing voltage of your 
P-FETs because when you turn them "on" your FET driver will be pulling 
their gate to ground.  The PFETs i use have a maximum Vgs of +/-30V.  Most 
FETs have a range of +/-20V but each part is unique and you gotta check.

Of course, if you're driving P-FETs your FET driver is inverted.  But 
there are inverted FET drivers if it matters so..

Now i'm actually reading what you wrote, having a system voltage of 4.2v 
or so and I realize you don't need FET drivers with a minimum Vcc of 4.5v.

You can probably just hook your GPIO pins to the P-FETs as-is.  If your 
GPIO goes to 3.2V in its "high" state and your battery is 4.2V max, your 
P-FET will see 1.0V from gate to source.  For a lot of P-FETs this is 
still solidly in the OFF state.  You can certainly choose a P-FET that 
will work with you on this.

You can also couple from the GPIO pin to the gate through a capacitor (and 
resistors) if you need less Vgs in the "off" state.  But these tricks 
won't help if your GPIO pins aren't strong enough to charge/discharge your 
gate fast enough by themselves.. or whatever shift registers you're using.

lemme know if you want more of my ideas on this.

-jake

Anders wrote:
Heyoo,

I've become rather exasperated in my search for an 8-bit serial-parallel 
shift register with open collector outputs. See, I'm trying to switch the 
gates of some P-FETs that act as high-side current switches for the rows 
in a scanned LED matrix. The LED bus voltage is 4.2v and all the ICs run 
on 3.3v.

I found the SN74LS596, but it's an ancient part that is not only enormous
but runs at 4.5v-5.5v which won't work in my system.

I could use a microcontroller and just toggle the tri-state status of the
pins but I worry about how fast the device will respond - It will have to
blast out the I/O change within a blanking cycle lasting 250ns. That
requires between 4 and 8 MIPS which is not too powerful but this is a
low-cost product. =/

Any suggestions are most appreciated!
--
Anders Nelson

+1 (517) 775-6129

www.erogear.com