Q: Driving a lot of leds

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.

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

-- ======================================================================== Michael Kesti | "And like, one and one don't make | two, one and one make one." mrkesti at comcast dot net | - The Who, Bargain

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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Multiplex.

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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

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I normally use the M5450 from ST: serial data input, no external resistors!

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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

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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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

Regards Anton Erasmus