I'm looking for a driver chip to run several "high-current" (20-30mA) LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, but the current exceeds the MCU ratings for direct connection. There's no shortage of advertisements for LED driver chips like Maxim's MAX6966, which could do the job (via SPI). I've got to think there's a simple multi-channel transistor package for this purpose, but Digikey searches on "buffer" or "driver" turn up too many hits to filter through. What generic part number should I be searching for (and how does one find these generic numbers in the first place)? Also, is there any trick to driving blue & white LEDs, or just additional current and higher forward voltage? Thanks, Richard
Simple "LED driver" chip?
Started by ●April 6, 2006
Reply by ●April 6, 20062006-04-06
On Thu, 06 Apr 2006 07:47:28 -0700, in comp.arch.embedded "Richard H." <rh86@no.spam> wrote:>I'm looking for a driver chip to run several "high-current" (20-30mA) >LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, >but the current exceeds the MCU ratings for direct connection. > >There's no shortage of advertisements for LED driver chips like Maxim's >MAX6966, which could do the job (via SPI). I've got to think there's a >simple multi-channel transistor package for this purpose, but Digikey >searches on "buffer" or "driver" turn up too many hits to filter through. > >What generic part number should I be searching for (and how does one >find these generic numbers in the first place)? > >Also, is there any trick to driving blue & white LEDs, or just >additional current and higher forward voltage? > >Thanks, >Richarduln2803 martin
Reply by ●April 6, 20062006-04-06
"Richard H." <rh86@no.spam> wrote in message news:IR9Zf.12763$kT4.6195@fed1read02...> I'm looking for a driver chip to run several "high-current" (20-30mA) LEDs > from a single MCU. I've got enough MCU pins for a 1:1 solution, but the > current exceeds the MCU ratings for direct connection. > > There's no shortage of advertisements for LED driver chips like Maxim's > MAX6966, which could do the job (via SPI). I've got to think there's a > simple multi-channel transistor package for this purpose, but Digikey > searches on "buffer" or "driver" turn up too many hits to filter through.Devices like the ULN2803/4 spring to mind... perhaps a bit old in the tooth these days. Steve http://www.fivetrees.com
Reply by ●April 6, 20062006-04-06
On Thu, 06 Apr 2006 07:47:28 -0700, the renowned "Richard H." <rh86@no.spam> wrote:>I'm looking for a driver chip to run several "high-current" (20-30mA) >LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, >but the current exceeds the MCU ratings for direct connection. > >There's no shortage of advertisements for LED driver chips like Maxim's >MAX6966, which could do the job (via SPI). I've got to think there's a >simple multi-channel transistor package for this purpose, but Digikey >searches on "buffer" or "driver" turn up too many hits to filter through.If it's several, as in 2 or 3, just use transistors, BJT or MOSFET. Darlington arrays as others have suggested would be better if you have more than four... keep in mind that they have significant foward drop on a 3-5V circuit (which may be a factor if you're using blue or white LEDs). You might also be able to use a HCMOS shift register.>What generic part number should I be searching for (and how does one >find these generic numbers in the first place)?There are hundreds of possible solutions.>Also, is there any trick to driving blue & white LEDs, or just >additional current and higher forward voltage? > >Thanks, >RichardPretty much. If you're running small LEDs at that high a current you may well have reliability and lifetime problems, particularly when the ambient temperature gets high. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●April 6, 20062006-04-06
Richard H. wrote:> I'm looking for a driver chip to run several "high-current" (20-30mA) > LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, > but the current exceeds the MCU ratings for direct connection. > > There's no shortage of advertisements for LED driver chips like > Maxim's MAX6966, which could do the job (via SPI). I've got to think > there's a simple multi-channel transistor package for this purpose, > but Digikey searches on "buffer" or "driver" turn up too many hits to > filter through. > > What generic part number should I be searching for (and how does one > find these generic numbers in the first place)? > > Also, is there any trick to driving blue & white LEDs, or just > additional current and higher forward voltage? > > Thanks, > Richardyou can use a 74HC4094 shift reg to drive 8 led's with 1 ea 470ohm resistor. rw --
Reply by ●April 6, 20062006-04-06
In article <IR9Zf.12763$kT4.6195@fed1read02>, rh86@no.spam says...> I'm looking for a driver chip to run several "high-current" (20-30mA) > LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, > but the current exceeds the MCU ratings for direct connection. > > There's no shortage of advertisements for LED driver chips like Maxim's > MAX6966, which could do the job (via SPI). I've got to think there's a > simple multi-channel transistor package for this purpose, but Digikey > searches on "buffer" or "driver" turn up too many hits to filter through. > > What generic part number should I be searching for (and how does one > find these generic numbers in the first place)? > > Also, is there any trick to driving blue & white LEDs, or just > additional current and higher forward voltage? >I have been using 74LPT245 chips from Pericom for LED driving and general IO usage. The '245 chips are octal buffers. These are tri-state chips, and sometimes it is handy to have control over all the outputs so that you don't get glitches at power-up or reset. If your driver inputs float because the microcontroller pins are inputs until your codes intializes them, you can get odd things happening at startup. This is may not be a problem with LEDs, but it has bitten me when controlling external power for other devices. If this is a problem, consider using pull-down resistors. The 74LVC2245 also works OK for this purpose. I suppose that you could also use an SOT-23 logic-level n-channel FET for each LED. This solution allows you to use an LED drive voltage higher than the microcontroller pin voltage limit. A lot of the LED or peripheral driver chips are basically multiple open-collector or open drain transistors in a single package. You can avoid startup glitches by using a simple NPN transistor driver instead of an FET. This requires a base current limiting resistor, but you don't get a glitch if the MCU pin floats, as the transistor responds to current, not voltage. Mark Borgerson
Reply by ●April 6, 20062006-04-06
On Thu, 6 Apr 2006 09:44:34 -0700, the renowned Mark Borgerson <mborgerson.at.comcast.net> wrote:>You can avoid startup glitches by using a simple NPN transistor >driver instead of an FET. This requires a base current limiting >resistor, but you don't get a glitch if the MCU pin floats, as >the transistor responds to current, not voltage.Unless you are using one of those execrable pseudo-bidirectional ports, such as on some 8051 variants, that too-closely mimic the original. In such a case, two transistors and four resistors per LED might be required to allow the higher voltage positive supply and glitch-free operation. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●April 6, 20062006-04-06
On Thu, 06 Apr 2006 13:05:38 -0400, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Thu, 6 Apr 2006 09:44:34 -0700, the renowned Mark Borgerson ><mborgerson.at.comcast.net> wrote: > >>You can avoid startup glitches by using a simple NPN transistor >>driver instead of an FET. This requires a base current limiting >>resistor, but you don't get a glitch if the MCU pin floats, as >>the transistor responds to current, not voltage. > >Unless you are using one of those execrable pseudo-bidirectional >ports, such as on some 8051 variants, that too-closely mimic the >original. In such a case, two transistors and four resistors per LED >might be required to allow the higher voltage positive supply and >glitch-free operation.I remember you saying that the 8051 port uses a strong pullup with a weak pullup and that the strong pullup is switched on briefly when the pin is driven high. But can you expand on this a little more? In the case of a higher V+ for the LED, how does the old 8051 port pin require two BJTs for glitch free operation, perhaps in the context of your earlier point about the pullups? Jon
Reply by ●April 6, 20062006-04-06
"Ryan Weihl" <nixnam@noname.net> wrote in message news:e13ep9$jlk$1@emma.aioe.org...> Richard H. wrote: > >> I'm looking for a driver chip to run several "high-current" (20-30mA) >> LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, >> but the current exceeds the MCU ratings for direct connection. > > you can use a 74HC4094 shift reg to drive 8 led's with 1 ea > 470ohm resistor. > rwCareful. While the 74HC4094 can sink/source 25mA, the datasheet suggests a max total ICC of 50mA. In any case, be sure the total dissipation of the device (Iled * Vsat * 8) is within limits. Also a 470R resistor (assuming 5V) would give nowhere near the 20-30mA the OP is looking for. Summary: there are better ways. Steve http://www.fivetrees.com
Reply by ●April 6, 20062006-04-06
"Ryan Weihl" wrote:>Richard H. wrote:>> I'm looking for a driver chip to run several "high-current" (20-30mA) >> LEDs from a single MCU. I've got enough MCU pins for a 1:1 solution, >> but the current exceeds the MCU ratings for direct connection.>you can use a 74HC4094 shift reg to drive 8 led's with 1 ea >470ohm resistor.For "conventional" low current LED's yes, but the OP specified "High Current" LED's. Can the 74HC4094 drive 1/4 Amp? (8 x 30 mAmp). Also at 30 mAmp, a 470 ohm current limiting resistor would require a 15 Volt power supply.