Re: Handling current through a 74HC595

On Sun, 02 Nov 2008 15:11:24 +0100, the renowned Jollino
<jollino@xxxxxxxxxxxxxxxxxxxx> wrote:

Hello,
apologies if this question is very stupid. :)
I have been toying with electronics for a little while, but I know I
lack some basic knowledge to wrap everything up together so... here is
my doubt.

I am designing some Christmas decoration with a bunch of LEDs driven by
an array of 74HC595s (controlled by an ATtiny45). The whole thing will be

....you kinda trailed off here..


I did something similar in the past, using three 595s to control three
7-segment LED displays, but I did it fairly randomly: I just connected
each output to each segment, with a resistor between the two.
Since this thing I'm planning to do is going to be simple yet much
bigger -- involving twelve 595s -- I'd like to do things properly.
The 96 LEDs could and probably will be all on at some point, so
considering a current draw of 15 mA for each LED,

Typically there's a limit of something like 70mA total, so you can't
get more than about half that and stay within the absolute maximum
specs. In practice you could probably push it for something junky like
an Xmas decoration, maybe to 15mA or even 20mA average since it
doesn't have to work for long, or in extreme environments, and maybe
all the LEDs on a single HC595 won't be on for very long-- but this is
a calculated risk since you're sauntering past the most severe kind of
warning in the data ***-- the electronic equivalent of a "lethal
force authorized" sign in the desert.

the whole thing would
draw a total of about 15 A, which is pretty much impossible to handle.
Correct?

Well, that's a fairly large power supply, but you do need that amount
of current if you want to have 15mA average through ~100 LEDs. Layout
would require some care (!). You can get a 5V 1.5A or 2A switching
wall plug supply pretty cheaply these days.

I will probably have to investigate charlieplexing to such a
big scale, or maybe make it so the 595s are working one by one at a fast
rate (by pushing a series of logic zeros down the data line).

There's no free lunch. Your average current will go down and the LEDs
will appear dimmer if you have less than ~1.5A. If you can live with
1mA/LED you can just use higher value resistors. If you used
Charlieplexing for the entire array you'd not be able to get more than
a couple hundred microamps average per LED, which is not going to give
much light except with the best (and most expensive) LEDs. In any
case, note that you cannot independently float outputs on a HC595.

In any case, let's assume for the sake of the example that I want to be
able to drive eight LEDs with a single 595. The data*** tells me that
the maximum current load on each output pin is 35 mA, and the current
load on the supply pins (Vcc and ground) is 70 mA. I am not sure how to
interpret this: does it mean that the sum of the output currents can't
be more than 70 mA?

Sort of. If you are driving some current in both directions, the sum
of the absolute value of the currents could be higher.

If that is the case, I suppose that I could use transistors. Would
connecting each output to the base of an NPN and the LED (with a series
resistor) between Vcc and the emitter work?

Sure. Although something like a ULN2803 per HC595 would be a lot
easier. And it would ease layout (but you'd still have to be a bit
careful with the high current paths).

Also, I'd like to finally understand whether it's better to source
current from a pin or sink current into a pin. I would think that
sourcing is "cleaner" because one doesn't "overdo" the chip -- if the
current is not enough, it simply doesn't work -- but I'm not sure about
that at all, because on many schematics I have seen LEDs sinking into
MCUs' ports.

Typically CMOS outputs sink current quite a bit better (ie. lower
voltage drop) than source so that's how we usually do it. The circuit
is the mirror image in either case-- an LED in series with a
transistor across the supply-- so the difference are 2nd order (the
n-channel transistors work a bit better than the p-channel kind).

Thank you in advance, and apologies again if I sound like an airhead. :D
When it comes to these things, well... I am!


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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