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 __________________________________
LED Drivers
Started by ●April 26, 2004
Reply by ●April 26, 20042004-04-26
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
Reply by ●April 26, 20042004-04-26
--- 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
Reply by ●April 26, 20042004-04-26
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... > > >
Reply by ●April 26, 20042004-04-26
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
>
>
>
>
> __________________________________
>
Reply by ●April 26, 20042004-04-26
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
Reply by ●April 27, 20042004-04-27
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... > > > __________________________________
