Most suitable driver chip

I have a matrix of LED's, six rows and seven columns:

X X X X X X X

X X X X X X X

X X X X X X X

X X X X X X X

X X X X X X X

X X X X X X X

Across the rows, the LED's anodes are common, and each row has a single microcontroller pin.

Down the columns, the LED's cathodes are common, and each column has a single microcontroller pin.

The row pins have to source 25 mA, which isn't a problem for the =B5C. The column pins, however, need to sink 150 mA (150 mA =3D the current that will flow if every LED in a column is lit).

Because the column pins can only sink a maximum of 25 mA, I was going to use a transistor to provide the extra current. Since I have 7 column pins though, I'm going to need 7 transistors, which is why I'm thinking of going with a "transistor array".

I'm just wondering is there any "bread and butter" chip that people use for this? I've had a quick look at the ULN200X family and they look kind of suitable. If I was picking my own transistors, I'd use FET's as switches (instead of bi-polars), but I suppose if I the driver chip has a base resistor inside it, and if there's negligible current flow when it's turned off, then maybe a BJT array will be OK.

(Of course I'd be wise to switch the rows and columns so that I only need 6 transistors, but I'll get on to that later)

So what would you use as a "driver chip"? My microcontroller pins work at 5 V for high and 0 V for low.

Reply to
Tomás Ó hÉilidhe
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shows a good example of the 2003 used in LED configuration. I'll bet you will find heaps if you google uln2003 and leds

Cheers Don...

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Don McKenzie

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Reply to
Don McKenzie

You haven't considered limiting the individual LED currents.

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 [mail]: Chuck F (cbfalconer at maineline dot net) 
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Reply to
CBFalconer

The ULN2003 is the NE555 of the Driver market. Widely sourced, and 'cheap as chips'. Great if you can tolerate the Darlington characteristics.

-jg

Reply to
Jim Granville

=B5C pin ---> 150 ohm resistor ---> LED ---> transistor ---

Reply to
Tomás Ó hÉilidhe

Yup. How much current flows through the resistor? How much of that flows through each LED. We have known about Ohms law for some time.

You have trimmed too much, so other readers can't tell what this is about.

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Reply to
CBFalconer

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I see Abs. Max 25mA/port pin and 200mA max combined port A and C, and a prominent (not that it has to be) note:

"This is a stress rating only and functional operation of the device at those or any other conditions above those indicated in the operation listings of this specification is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability."

Now, let's look at "operation listings"-- parameter D080 is the highest rating (8.5mA sink). That would be a good number not to exceed. And D090 (-3.0mA source). There's another.

Of course if you don't care if device reliability is affected, then it doesn't much matter. In fact there's not much point in looking at the data sheet at all if you're going to only pay attention selectively. Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Where are you getting that number from? On page 127 (PDF page 129)

Abs. max Idd is 250mA Abs. max Iss is 300mA Abs. max current sunk by PORTA and C combined 200mA Abs. max current sourced by PORTA and C combined 200mA

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Yup, you win, although my suggested conservative numbers of 8.5mA/-3mA (IIRC) would be rather safe in either case (unless you're trying to sink 8.5mA from all 11 port pins at once, in which case it's a bit over).

Usually they add an "A" or "B" when they do a process shrink because some characteristics inevitably change a bit.

I think they discovered it dies REALLY fast at 200mA, and just revised the data sheet rather than revising the part! Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

=EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX

=EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX

=EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX

=EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX

=EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX

=EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX =EF=BF=BD =EF=BF=BDX

C.

First off 25mA is going to give you a dim display unless you are useing low current leds. ULN200x chips were designed to drive 7Seg led displays, so a perfect choice for you. PNP bjts are cheapest and simplest for the rows. You seem to have the rows/colomns mixed up.

Reply to
cbarn24050

I'm going to try get my hands on LED's that are bright in natural daylight. Do you think I'd get much performance out of any such LED's for 25 mA.

The rows are coming straight from the =B5C pins (the PIC16F684 pins can source and sink 25 mA).

Reply to
Tomás Ó hÉilidhe

Average current per LED is?

"Can" as in "absolute maximum rating", which is a rather foolish thing to aim for in a serious design.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

I don't know, I haven't picked the LED's yet. What I was asking about was whether 25 mA would be enough to light any LED bright enough to be seen in daylight (any LED on the market, that is)?

I don't understand your reasoning here. A spec sheet might say "Source and sink maximum current =3D 25 mA". If I'm not mistaken, this means that it's perfectly OK to put 25 mA thru them. If there was a need to undercut this figure by 10%, then surely it would make more sense if the spec sheets reduced their values by 10%.

Does anyone else see a problem with taking 25 mA from a microcontroller pin that's rated for 25 mA maximum?

Reply to
Tomás Ó hÉilidhe

I think you will find that's in the abs max section of the data sheet. Microchip (I remember that's who you are using) has the following note in that section "Exposure to maximum rating conditions for extended periods may affect device reliability." These are stress ratings not operating ratings. That is true for all manufacturers AFAIK although some are not as explicit about it.

In other words don't do that.

For driving a load a much prefer using something like a bss123 to do the actual drive and have the micro turn it on/off. That also isolates you from the load voltage to some extent.

Robert

Reply to
Robert Adsett

I read 25mA max per pin and 90mA max per chip. Namely, no more than 3 leds on at any time.

Reply to
linnix

I would look at the data sheet directly and see what is says.

Is this micro-controller a secret ?

Lets all take a look at the data sheet involved and see if this micro-controller would have a problem with this.

donald

Reply to
donald

And a 95mA max Idd and Iss, so you can't do more than 95mA absolutely.

Reply to
linnix

Up until now I had thought that there were two areas: The "Acceptable" area and the "Unacceptable" area.

For instance, if a =B5C pin had its maximum current listed as 25 mA, then I would expect anything below or equal to 25 mA to be "acceptable", and anything above that to be "unacceptable".

It seems now that they're introducing a third area, the "Kind of Acceptable" area or the "Acceptable just for a little while" area.

However, they don't mark the boundary between "Acceptable" and "Kind of Acceptable".

If a chip lists its maximum current as 25 mA, and then goes on to nonchalantly say "oh by the way you probably don't want to run it like that or the chip might go wonky", then how are we to know what maxium current to give it? 24 mA? 20 mA? I thought the whole point of having spec sheets was so that we can get a handle on these things.

Reply to
Tomás Ó hÉilidhe

Page 131 of

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That's strange. Are we looking at the same chip, PIC16F684?

Reply to
linnix

3

OK, your link is dated 2004:

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My link is dated 2007:

So, I won. This is progress (LOL). Probably process shrink.

Reply to
linnix

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