Q: Driving a lot of leds

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What do you guys do when you have to drive quite a few leds? ie 4
segented led displays and 20 standalone? 8 bit shift registers?

Re: Q: Driving a lot of leds

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shiftregs are a pretty good way. You don't need to use latched ones (595 etc.)
as you can kill the
common drive while shifting new data in.
 

Re: Q: Driving a lot of leds

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Maxim (Dallas Semiconductor) makes a line of LED controllers.  I've
used the MAX7219 with success.

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Re: Q: Driving a lot of leds
On Thu, 25 Aug 2005 17:52:59 -0400, the renowned Chris

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Multiplexed, in a very straightforward way, you can do that with 7 + 8
port pins (assuming a 7-segment internally multiplexed display with
decimal points and 20 discrete LEDs). For example, if you use a common
anode configuration you could have 7 source drivers and 8 sink
drivers. If you can live with an average current of 1mA for each 'on'
LED (say you have good LEDs), that's a maximum of 52mA total. Each
sink driver will have to be able to sink 7mA at about 100% duty cycle
and each source driver will have to be able to source 56mA with a 1:4
duty cycle.

In many cases, you can do that with a micro port plus one source
driver chip (or a few dual p-channel MOSFETs). Plus a couple quad
resistors.

If you want 15 or 20mA average per LED (either because you need
daylight visibility or because you want to buy Asian street-sweepings
for LEDs), you'll probably have to go with static drive and things get
a lot more complex. Using regular logic chips as LED drivers breaks
down above a certain current because of maximum per Vdd line or GND
line limits (read the data sheet carefully) and you have to go with
more parts again.

In between at perhaps 5mA per LED, you maybe can get away with HC
logic for some of the drivers, and can certainly use multiplexing.


Best regards,
Spehro Pefhany
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Re: Q: Driving a lot of leds
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Multiplex.

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Re: Driving a lot of leds
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Multiplexed or with a LED driver IC.

Interestingly, before the specific LED driver IC's we used a printer head
driver. It was just a big shift register with 24 high current sink outputs
and an output strobe we we used to control the brightness.

Ross



Re: Driving a lot of leds

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I normally use the M5450 from ST: serial data input, no external resistors!
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Re: Q: Driving a lot of leds

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Look at Maxim. They have quote a few new devices designed to drive
LEDs in low power systems. Each LED's current can be programmed in
16 steps. One also do not need a resitor in series with the LED, so
save quite a lot of power that way.  Something like the MAX6957, which
are available from Digikey.

Regards
  Anton Erasmus

Re: Q: Driving a lot of leds
On Fri, 26 Aug 2005 16:56:16 +0200, the renowned Anton Erasmus

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How does dissipating the wasted power in the driver chip rather than
resistors "save quite a lot of power"?


Best regards,
Spehro Pefhany
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Re: Q: Driving a lot of leds
On Fri, 26 Aug 2005 12:34:46 -0400, Spehro Pefhany

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You are correct, I was not thinking. One would need some sort of
switch mode current control to improve the efficiency.

Regards
  Anton Erasmus


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