Blue LED at 3.3V
in [Design]
|
From: John Larkin on 25 May 2010 14:10 On Tue, 25 May 2010 10:22:59 -0700, Tim Wescott <tim(a)seemywebsite.now> wrote: >On 05/25/2010 10:18 AM, rich wrote: >> I need to drive a blue led from 3.3V. Most of the SMD blue leds I >> find have a Vf equal to or greater than 3.3V. >> >> I am curious how others are dealing with this. > >That pretty much demands a voltage boost of some sort. Depending on how >many lights you have, how much power you're willing to waste, how much >design time you want to spend and how expensive you want the final >product to be, your choices sort of boil down to a switcher with >inductors and diodes and all that, or a current pump. > >Most of us would solve this problem by looking for a suitable IC. >_Some_ of us would do it with two transistors, an inductor, and a cap, >then brag about only needing one $.001 resistor instead of three. One resistor: ftp://jjlarkin.lmi.net/LED_boost.JPG John
From: Jan Panteltje on 25 May 2010 14:16 On a sunny day (Tue, 25 May 2010 11:00:42 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <b93ov5hd2c0t71ma2adoaejiop1o81615j(a)4ax.com>: >On Tue, 25 May 2010 10:18:11 -0700 (PDT), rich ><rsoennichsen(a)gmail.com> wrote: > >>I need to drive a blue led from 3.3V. Most of the SMD blue leds I >>find have a Vf equal to or greater than 3.3V. >> >>I am curious how others are dealing with this. >> >>Thanks >> >>Rich > >I have some nice Osram blues that are OK at 1 mA and 2.65 volts, >bright at 10 mA, 3.1 volts. So you could just get by with a resistor >or current limiter from 3.3. You could use one of my famous beta >limiter circuits. > >I sometimes make my "3.3" volt supplies actually 3.5 or 3.6. Most >other parts don't mind. > >Or use a booster. > >John Yup, my blue one drops 2.66 V Extremely bright at 3.5 mA. ftp://panteltje.com/pub/low_current_LEDs_img_1964.jpg in that picture it is at 10% PWM with 180 Ohm in series fro ma 3.3V PIC output.
From: Tim Wescott on 25 May 2010 14:26 On 05/25/2010 11:10 AM, John Larkin wrote: > On Tue, 25 May 2010 10:22:59 -0700, Tim Wescott<tim(a)seemywebsite.now> > wrote: > >> On 05/25/2010 10:18 AM, rich wrote: >>> I need to drive a blue led from 3.3V. Most of the SMD blue leds I >>> find have a Vf equal to or greater than 3.3V. >>> >>> I am curious how others are dealing with this. >> >> That pretty much demands a voltage boost of some sort. Depending on how >> many lights you have, how much power you're willing to waste, how much >> design time you want to spend and how expensive you want the final >> product to be, your choices sort of boil down to a switcher with >> inductors and diodes and all that, or a current pump. >> >> Most of us would solve this problem by looking for a suitable IC. >> _Some_ of us would do it with two transistors, an inductor, and a cap, >> then brag about only needing one $.001 resistor instead of three. > > One resistor: > > ftp://jjlarkin.lmi.net/LED_boost.JPG > > John > tim(a)servo:~$ ftp jjlarkin.lmi.net Connected to jjlarkin.lmi.net. 421 Service not available, remote server has closed connection ftp> -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Tim Wescott on 25 May 2010 14:27 On 05/25/2010 11:00 AM, John Larkin wrote: > On Tue, 25 May 2010 10:18:11 -0700 (PDT), rich > <rsoennichsen(a)gmail.com> wrote: > >> I need to drive a blue led from 3.3V. Most of the SMD blue leds I >> find have a Vf equal to or greater than 3.3V. >> >> I am curious how others are dealing with this. >> >> Thanks >> >> Rich > > I have some nice Osram blues that are OK at 1 mA and 2.65 volts, > bright at 10 mA, 3.1 volts. So you could just get by with a resistor > or current limiter from 3.3. You could use one of my famous beta > limiter circuits. > > I sometimes make my "3.3" volt supplies actually 3.5 or 3.6. Most > other parts don't mind. How is it at cold, though? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Tim Wescott on 25 May 2010 14:29
On 05/25/2010 11:06 AM, Spehro Pefhany wrote: > On Tue, 25 May 2010 10:18:11 -0700 (PDT), rich > <rsoennichsen(a)gmail.com> wrote: > >> I need to drive a blue led from 3.3V. Most of the SMD blue leds I >> find have a Vf equal to or greater than 3.3V. >> >> I am curious how others are dealing with this. >> >> Thanks >> >> Rich > > Probably in a lot of applications where people choose a blue LED there > is a higher voltage (eg. 5V) supply present as well. > > Here is one way to do with about 2-3 cents worth of parts (3 tiny SMT > jellybean parts, no inductors) if you have a microcontroller doing the > driving: > > > +3.3V > > | > | > | > .--|--. > | | | > Cs | V | > Rs | - | > || ___ | | | eg. BAV99 > -||--|___|--|- + | > || | | | > Port pin | V | > | - | > '--|--' > | > | > V LED (Blue or White only) > - > | > | > === > GND > > AC on the port pin => ON, either level of DC => OFF > > > Or you could search on, say, LTC's website and find a $5 chip which > will be designed for the purpose (blue LEDs are electrically the same > as white LEDs in most cases, so all those white LED drivers will > typically work equally well with blue LEDs). AFAIK a white LED is just a blue LED chip with some fluorescent material in the package that makes enough "whatever else" to make it look white. They look exceedingly blue to me -- I don't know if that's because they are, or because I'm color deficient in green and don't see them the same as other people do. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |