Blue LED at 3.3V
in [Design]
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From: rich on 25 May 2010 13:18 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
From: Tim Wescott on 25 May 2010 13:22 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. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Jim Thompson on 25 May 2010 13:35 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. And some of us would brag about doing it right ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: John Larkin on 25 May 2010 14:00 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
From: Spehro Pefhany on 25 May 2010 14:06
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). |