Powering many LEDs from one IC.

[WBrumble]

All,

I have a question for my project that I'm currently working on.

I don't need binary, I'm not making a cube or need to turn certain LEDs on at a time.

All the LEDs in the row will only be dimming exactly the same.

What's the best way to multiply the PWM output from a IC when more pins are needed?

Thank you.

 

[d123]

Maybe using a buffer IC similar to the 7407, which can be used to fanout each output into ten more (depending on your loads - you need to calculate that based on your circuit requirements), but perhaps it can't provide the current you need. Worth checking the fanout for your IC and another way is using transistors or a transistor array IC for the same purpose.

There are probably more efficient or simpler ways than my suggestions.

 

[betwixt]

It depends on the power requirements of the LEDs and the voltage available at the PWM output. The simplest method is probably a logic level MOSFET as the current switch. They draw negligible input current from the PWM and can often switch many Amps between drain and source pins. For small currents (less than ~250mA) or if the PWM voltage is very low it may be easier to use a bipolar transistor instead.

Brian.

 

[ted]

Usually when having many equal LEDs such as for illumination purposes one connects multiple LEDs in series. That usually simplifies the driving electronics, but requires also a higher feeding voltage, as all LED voltage drops in one string are added up. For instance for a white LED a rule-of-thumb forward voltage is 3V or thereabout. (you should consult the data sheet for the actual value). Therefore, if you series feed for example 10 LEDs, your power supply should be >30V. On the other hand, your driver would be handling only one LED current, and the driving would be simpler, and often the efficiency would be much higher.

A very efficient way of feeding LED strings is a switchmode converter. If your system doesn't have high enough voltage, you can also use a boost configured switcher to make the LED feed with pretty good efficiency. There are many chips and application notes helping to design quite simple solutions for that.

By-the-way, I suppose, that you know that LEDs are in general current-driven? In other words, you should control the LED current, as if you feed them with a low-impedance voltage supply, the current can be hard to control.

 

[WBrumble]

Thanks for the advice, I should have been in more detail to my project. But I was curious in general circuit design what's the most efficient way of doing it.

I could run three of the LEDs in series, but I'm building a modular light bar for my truck and want each one to be separate to the other.

Here's a photo of the schematic so you can see what I got going on, I just need to drive the PWM input pin of an LTM8042 constant current driver.



PWM input requirements:

High - min voltage - 1.5
Low - max voltage - 0.4
Bias Current - typ: 60uA max: 100uA

 

[KlausST]

Hi,

We need supply voltage, Led forward voltage, led forward current, led count.

Klaus

 

[WBrumble]

Klaus,

Will be 160 of the drivers pictured in my previous reply, in multiples of 40 for each color. Each LED will have it's own driver. I just need a way to multiply the PWM from the micro controller to accommodate for the amount of PWM inputs to the drivers.

Thinking of going with a Arduino compatible micro controller for the ease of programming and interfacing with Android. I want to control the light bar with my phone.

For the ATmega8 that would be 5volts for the PWM output.

 

[KlausST]

Hi,

Your application is still unclear to me.
There are informations like "three LEDs in series", "160 driver", "in multiples of 40 each"
In post 7 you now speak of "colors".
In post 1 you say "All the LEDs in the row will only be dimming exactly the same.".
--> Does that really mean you just need one PWM, or different PWMs for different rows, or -what makes more sense to me - different PWMs for different colors.

****
Your initial question "What's the best way to multiply the PWM output from a IC when more pins are needed?" should be correctely answered in post2.
--> My additional information: You only talk about the voltage levels of the PWM signal, but paralleling many inputs to one output is a problem of current. So you could yourself multiply the single input current with the count of inputs to get the total needed driver current.

***
If you need more information, please give the configuration of "one" module. No driver circuit needed so far.
A picture helps a lot.
Or something like that: just as an example...
* three connections per module (0V, 12V, PWM)
* 40 x 3LEDs (3 in series), all dimmed with one PWM signal.
...
Tell us about the color configuration..

Additionally tell us how many of those modules you decide to connect.

****
You show your driver is powered by 12V. Is it stabilized 12V, or just the vehicle supply with a voltage range of 11V ... 14.4V?
Also you talk of "truck". Isn't it powered by 24V?

Klaus

 

[WBrumble]

There seems to be some confusion, sorry for that, let me see if I can clear this up for you.

I need to take a PWM out with the following values and multiply it by 40 outputs:

Voltage: 0-5vdc
Max Current: 40mA

The input requires these values:

Voltage: min of 1.5 volts for a high signal
Current: typ: 60uA max: 100uA
It's also got an equivalent of a 50k resistor to ground internally.



It's main voltage supply from an alternator on a car, if your suggestion needs a regulated supply can pop a voltage regulator in there.

If you need my switching frequency it's 2Mhz (3us) with 9ms max on time for a 3000:1 dimming ratio.