LED Drivers

Started by Raghavendra Bulusu April 26, 2004
Are there any Constant Current LED driver that
regulates the current through the LED independent of
external factors, such as variations in LED Vf, power
supply drift, etc that can be interfaced to a
microcontroller.I am trying to drive a string of LEDs
at 28 v.

Thanks,
Raghu 


	
		
__________________________________


Beginning Microcontrollers with the MSP430

Raghavendra Bulusu wrote:

>Are there any Constant Current LED driver that
>regulates the current through the LED independent of
>external factors, such as variations in LED Vf, power
>supply drift, etc that can be interfaced to a
>microcontroller.I am trying to drive a string of LEDs
>at 28 v.
>
>Thanks,
>Raghu 
>
>  
>
A single supply ampop plus a power mosfet to increase current capability 
plus a shunt resistor to measure the current can do the trick (but it's 
power hungry). If you have a good digital vcc, you can even use a 
PWM+filter to modulate the current at low freqs.

                            LEDS
                  |\         |
pwm  ----RRR------|+\        D
             |    |  \______G
             C    |  /       S
             C  --|-/        |
             C  | |/         |
             |  |            |
             |  -------------|
            GND              R
                             R
                             R
                             |
                            GND

Ricardo


--- In msp430@msp4..., Raghavendra Bulusu <codemsp430@y...>
wrote:
> Are there any Constant Current LED driver that
> regulates the current through the LED independent of
> external factors, such as variations in LED Vf, power
> supply drift, etc that can be interfaced to a
> microcontroller.I am trying to drive a string of LEDs
> at 28 v.

To do it with discretes, it only requires a single-supply op-amp, a
BJT or MOSFET, and (by my count) two resistors, one cap, a reference
voltage and a large, low-value resistor that serves as the feedback
point for the op-amp.

In a design I did where LEDs were used to illuminate a keypad, with
the power ranging from 8 to 12V, I had the op-amp maintain the NPN's
emitter at +0.5V, while passing 200MA to feed 7 columns of LEDs. I
used a 0.1Ohm resistor in each LED column to ensure good
current-sharing across the columns. The whole thing worked very
nicely, and was easy to implement because I already had a quad op-amp
on board for other purposes.

--Andrew


Check out:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/2198/ln/en

-Sumukh


--- Raghavendra Bulusu <codemsp430@code...> wrote:
> Are there any Constant Current LED driver that
> regulates the current through the LED independent of
> external factors, such as variations in LED Vf,
> power
> supply drift, etc that can be interfaced to a
> microcontroller.I am trying to drive a string of
> LEDs
> at 28 v.
> 
> Thanks,
> Raghu 
> 
> 
> 	
> 		
> __________________________________
> 
> 
> 
> .
> 
>  
> Yahoo! Groups Links
> 
> 
>      msp430-unsubscribe@msp4...
> 
>  
> 


Hi Raghu,

That depends on the supply voltage you have.
If it's less than than the LED string voltage, you need a step-up 
converter.

Here's a list of LED string drivers with internal power switch:

Catalyst CAT32
Elantec EL7513
Fairchild FAN 5608
GMT G5111
Supertex HV9903
Linear Technology:
 LT1932
 LT1937
 LT1618
 LT3465
 LT3466
Microsemi LX1992, LX1993
Maxim:
 MAX1553/4
 MAX1561
 MAX1582
 MAX1848
 MAX1984-MAX1986
Micrel MIC2287
MPS MP1521, MP1523
ON Semi NCP1403, NCP5008
JRC NJU6050
Sipex SP6690
Toshiba TB62731FU
Toko TK11851L
Texas Instruments:
 TPS61040 (slea004.pdf)
 TPS61041
 TPS61042
 TPS61043

Of these I've used LT1932, LT1937, LT3465.

This list may be incomplete (and some parts obesolete), and there are 
many more drivers with external power switch (e.g. Zetex).

Basically, you can use any step-up converter if you put the high-side 
of a current-sense resistor to the feedback input (usually around 
1.25V). There will be power loss in this resistor, so it's better to 
use ICs with lower feedback voltage, or use a trick like in 
slea004.pdf, or a feedback voltage amplifier (e.g. OP with fixed 
gain).

Hope this helped 
Wolfgang


--- In msp430@msp4..., Raghavendra Bulusu <codemsp430@y...> 
wrote:
> Are there any Constant Current LED driver that
> regulates the current through the LED independent of
> external factors, such as variations in LED Vf, power
> supply drift, etc that can be interfaced to a
> microcontroller.I am trying to drive a string of LEDs
> at 28 v.
> 
> Thanks,
> Raghu 
> 
> 
> 	
> 		
> __________________________________
> 


You don't actually need an op-amp to do it. You can do a high sides
constant 
current source quite easily with a PNP transistor a couple of resistors and a 
voltage reference. 

If you put the reference voltage between the base and +V, the voltage
between 
the emitter and +V is Vref - Vbe. You can set the current by the resistor 
between the emitter and Vbe by (Vref-Vbe)/R. Things will vary slightly with 
Vbe and Vref tolerances, but is easily accurate enough for LEDs.

This doesn't work so well if the amount of LEDs in the chain varies a lot. You 
then need to start looking at a switching circuit. 

Personally, I would try and steer clear of special LED constant current ICs as 
you're at the mercy of the manufacturers pulling the plug or going on long 
lead times. With discretes you can usually get equivalents...

Cheers,

Rob




--- In msp430@msp4..., Raghavendra Bulusu <codemsp430@y...>
wrote:
> Are there any Constant Current LED driver that
> regulates the current through the LED independent of
> external factors, such as variations in LED Vf, power
> supply drift, etc that can be interfaced to a
> microcontroller.I am trying to drive a string of LEDs
> at 28 v.

To do it with discretes, it only requires a single-supply op-amp, a
BJT or MOSFET, and (by my count) two resistors, one cap, a reference
voltage and a large, low-value resistor that serves as the feedback
point for the op-amp.

In a design I did where LEDs were used to illuminate a keypad, with
the power ranging from 8 to 12V, I had the op-amp maintain the NPN's
emitter at +0.5V, while passing 200MA to feed 7 columns of LEDs. I
used a 0.1Ohm resistor in each LED column to ensure good
current-sharing across the columns. The whole thing worked very
nicely, and was easy to implement because I already had a quad op-amp
on board for other purposes.

--Andrew




.

 
Yahoo! Groups Links



 




Raghu,
I did use a constant current Diode for one of my
application where I did design a power supply for an
accelerometers and they need constant current.
http://www.centralsemi.com/leadedpdf/1N5283-5314.PDF

The following link is a led driver
http://www.infineon.com/cmc_upload/documents/025/572/AN_066_RevD.pdf
Thanks
Prakash
--- Raghavendra Bulusu <codemsp430@code...> wrote:
> Are there any Constant Current LED driver that
> regulates the current through the LED independent of
> external factors, such as variations in LED Vf,
> power
> supply drift, etc that can be interfaced to a
> microcontroller.I am trying to drive a string of
> LEDs
> at 28 v.
> 
> Thanks,
> Raghu 
> 
> 
> 	
> 		
> __________________________________
> 
> 
> 
> .
> 
>  
> Yahoo! Groups Links
> 
> 
>      msp430-unsubscribe@msp4...
> 
>  
> 



	
		
__________________________________