Meaning of Vce(sat)?

The main device... but the driver device saturates. ...Jim Thompson

I was thinking more about the strength of the fet if it is on the high side of the threshold spec. Just something to consider.

Looking more carefully, I think I might need P-Channel MOSFETS, and then I'd need to invert my logic driver (replace the 74HC238 with a 74HC138). If I used an N-Channel, I don't think I could get the Vgs to be high enough.

I was thinking this part:

Then I would have the source connected to +5v, the drain connected to to my load, and the gate connected to the output of the 1-of-8.

The load is some parallel LEDs (each in series with a constant-current-sink), so the drain voltage could vary between +1.8v up to +5.0v, The current can vary from 10mA to 1200mA. Depending on how many parallel LEDs are enabled.

Does this sound like it would work?

Thanks, Daniel.

On 11/03/14 19.58, Daniel Pitts wrote: ...

You could control the constant-current-sink directly?

E.g. by sorting the gate/source or base/emitter by a small MOSFET (2N7002)?

Double 2N7002:

/Glenn

Ok, looking at that part it appears it would do exactly what you need.

But I've noticed they have only 1 in stock? Sometimes when you see low stock counts like that, it's a sign of stay away... ;)

Jamie

I'm multiplexing 24*8 LEDs.

The "current sink" is 3 8-bit shift-register constant-current-sink (TLC5196) which power between 0 to 24 LEDs in parallel (depending). The MOSFET is a "row enable" to choose between 1 of 8 sets of 24 LEDS. So in total, I'm powering 196 LEDs, just 24 at a time.

Ah, didn't notice that, thanks...

Here's a "DUAL P-CHANNEL ENHANCEMENT MODE FIELD EFFECT TRANSISTOR" in stock.

About .1? Rds(on) @ 4.5v, which is good for me.

The pinouts also look like they'll be convenient for my application. More than the single-fet packages.

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just remember that now that you have turned things up side down the Vgs is you supply - Vol of your controller

So if you supply your LEDs with 1.8V it'll be at most 1.8V

-Lasse

-Lasse

As I wrote earlier:

Now, my source is VCC(+5v) always, The Vgs will be 0v (off) when the gate voltage is VCC(+5v). The Vgs will be -5v (on) when my gate voltage is at GND. The 74HC138 will select exactly one of these, the rest will be near VCC. The one that is "logic-enabled" will be near GND.

My schematic:

My board layout: