Microcontrollable adjustable and switchable constant current source for driving LED's

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calltronics

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Hi,
I am looking for a suitable solution for driving LED strings with constant current.
As depicted below I need to do some precise timed switching on/off of the LED’s from a microcontroller.
Pretty standard microcontroller (Ti) with DIO, AD and DA.

Could anyone help in pointing me in the best direction for the switchable constant current source circuit solution.
As depicted in the attached diagram?
  • Range = (10mA to 150mA)
  • The microprocessor needs to adjust the constant current to enable a set current dependent on which of the various LED strings are attached.
  • The constant current requires on/off switching from a digital port on Micro, not fast (0.1sec on/off).
Anyone got any good starting suggestions?
 
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Your diagram shows four LEDs with a supply voltage of 4.5V. Most LEDs have a forward voltage of ~2V so ~8V will be the minimum required to get any current to flow. Unless your solution is some kind of switching regulator.

Mike.
Edit, maybe something like the UC1842 range but with an enable pin.
 
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Hi Sorry mind was elsewhere, changed diagram to more accurate representation
 
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Assuming appropriate supply for the LEDs:

I'd set the LEDs to the maximum wanted brightness using a current regulator or voltage regulator plus low value limiting resistor(s).

Then use PWM to switch the LED chain on and off, with varying duty cycle to control the brightness.
 
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Yes that's what I was thinking. But how do I adjust the current regulator using the microprocessor when I switch to different LED strings?
 
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If it's a constant current source then differing strings of LEDs will just work. Voltage allowing. A NPN transistor (or MOSFET in the ground connection of each of the strings would switch them on and off.

Why do you need varying currents?

Mike.
 
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Pommie said:
If it's a constant current source then differing strings of LEDs will just work. Voltage allowing. A NPN transistor (or MOSFET in the ground connection of each of the strings would switch them on and off.

Why do you need varying currents?

Mike.
Click to expand...
I need varying currents to connect to varying LED strings and LED lights. Same solution just varying confgurations without having to change resistors etc.
 
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You need to tell us more about what you are actually trying to do. There is no need for component change unless you're changing the LEDs (which are components).

Mike.
 
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Pommie said:
You need to tell us more about what you are actually trying to do. There is no need for component change unless you're changing the LEDs (which are components).

Mike.
Click to expand...
Yes, that is the point I am changing LEDs and that is why I need the software controlled configurable option. One solution will drive many types of external LED strings and displays.
 
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What current range do you need for the LEDs ?

You want each string leds in series (best as they all get same current) or
parallel ?

How many leds in each string, and what color ?

Does current source need to be high or low side drive ?

Wireless or wired control ?

Resolution of current control circuit, in bits, 8, 10, 12 bits......?


Regards, Dana.
 
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Pommie said:
How will you know which LED string is attached and hence, how much current is required?

Mike.
Click to expand...
As you state each type of LED string and Lamp has a fixed current requirement. The type of LED fitted to each solution is read from an I2C interface seperate from this diagram and part of the configurable LED fixture.
Hence we know what current is required for each connected LED fixture. We just need to configure the current to match through the Microprocessor.
 
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How many different strings and what is the maximum voltage required for any string?

Mike.
Edit, and maximum current (150mA)?
 
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Hi Dana,
As per the diagram 10mA to 150ma.
The strings vary in numbers and colours, hence the need to have a configurable current.
High or Low not really bothered at this stage.
Wireless or wired???? - not sure why you ask this concidering the question and diagram.
 
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Pommie said:
How many different strings and what is the maximum voltage required for any string?

Mike.
Edit, and maximum current?
Click to expand...
I really don't want to limit the string types. But currently they are 26 different types of strings, 16 displays and 31 single LED lamps.
Max voltage is 4.5V but normally the drive voltage is between 1.2V to 2.2V depending on the LED solution fitted.
Max current so far is 150mA. Not sure if this may increase as LED fixtures expand. But it is a good starting point.
 
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As you have a micro you could setup a 4 bit current source to give you 0 to 160mA in 10mA stages ( 1 more bit will give 0 to 320mA). With only (max) 3.5V to drop fairly small transistors could be used (~300mW max dissipation) - more if 5 bit.

mike.
 
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