Simple "LED driver" chip?

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

Reply to
Richard H.
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uln2803

martin

Reply to
martin griffith

Devices like the ULN2803/4 spring to mind... perhaps a bit old in the tooth these days.

Steve

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Reply to
Steve at fivetrees

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.

There are hundreds of possible solutions.

Pretty 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

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Reply to
Spehro Pefhany

you can use a 74HC4094 shift reg to drive 8 led's with 1 ea

470ohm resistor. rw
Reply to
Ryan Weihl

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 to
Mark Borgerson

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

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Reply to
Spehro Pefhany

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 to
Jonathan Kirwan

Careful. 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

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Reply to
Steve at fivetrees

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.
Reply to
Roberto Waltman

When powered up the pins are pseudo-inputs (outputs set to "high"), which means the weak pullups are on. That's enough to drive (or, perhaps worse, partially drive) a BJT.

Something like this (you drive the pin LOW to turn the LED on). The weak pullup holds the PNP transistor 'off', at least on modern CMOS parts, when the part is reset.

Vled + | VCC .-. + | | | | | Port pin | '-' ___ |< | o----|___|--| | |\ V LED | - | | .-. | | | | | | | '-' | | |/ +-----| .-. |>

| | | | | | '-' | | | +-------- | | GND

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Hello Richard,

Check out the Philips PCA9532, around $2 and Digikey has them. But abs max is 25mA per output and watch the grand total limit. It has a nice PWM scheme to adjust all the LEDs and you can control it via I2C.

Regards, Joerg

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Reply to
Joerg

Thanks, that's dead clear.

Or assuming that +V >= Vcc+1V or so, then something like:

C1 and R3 just being an oft-useless, but sometimes useful, speed up to overcome Q1's capacitance. D1 just to provide a path for C1 charge when 'going the other way' (or pushing instead of pulling.) So it really can be reduced to a much simpler:

One less resistor. The pin still has to handle enough current for driving Q1 as needed, though, so your circuit is better that way.

Thanks for the information. I just couldn't remember the details about the 8051 pins.

Jon

Reply to
Jonathan Kirwan

"Richard H." schrieb im Newsbeitrag news:IR9Zf.12763$kT4.6195@fed1read02...

This one is a good one, but not very popular at the moment:

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and:
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MIKE

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Reply to
M.Randelzhofer

Richard H. scrobe on the papyrus:

What about the old faithful MM5450 with an SPI compatible interface. It will drive 35 LEDs @ 15mA each and as the outputs are constant current, they can be paralleled up for higher drive currents.

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John B
Reply to
John B

... snip ..

Consider a current mirror, which can be fabricated from a matched set of transistors. The thing that counts is the ratio of emitter areas. Note that it is a three terminal device, and that all the dissipation is in the device. One input control resistor sets the output current delivered.

Vcc Current mirror, shown 4x gain | +---+----+----+----. | | | | | >| >| >| >| >| |---------|----|----|-+ /| /| /| /| /| | | | | | | | +----+----+----' | | | +---+ | | | | | | | .-. V LED | | - | | | '-' | | /| | -O< |- === \| GND Open collector drive (created by AACircuit v1.28.6 beta 04/19/05

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Reply to
CBFalconer

Sounds like the venerable, (but still very cheap & widely sourced,) ULN2003, and the ULN2803 darlington arrays. These will give a sizable boost in current drive.

The forward voltage needed seems to vary by supplier.

-jg

Reply to
Jim Granville

Depending on where you are buying stuff you'll find these chips are crazily expensive. You can get a serial in parrallel out shift register for 10c along with 4c transistors and resistors and you are set. Blue and white just have different forward voltage. Blue typically switches alot slower if it matters, you cant get a good signal above about 10khz on most of them.

Reply to
DAC

Thanks to everyone for the quick comments!

The need here is very simple, so I'm reluctant to use a feature-rich / pricey part (we'll be making a lot of these; the forward voltage drop would also be an issue). We'll probably do 4-5 LEDs per MCU.

The flexibility of discrete transistors sounds good (and possibly very cheap); since I'm doing SMT, we could get them pretty small too. Hmmm.

I'll do some AoE reading and look at the discrete designs offered here.

That brings me back to the question about the generic part numbers - in this case, a general-purpose transistor. Is there a book (website?) that offers a good reference for these part numbers and their characteristics? (E.g., Lancaster's TTL and CMOS cookbooks have about

100 pages of reference for logic chips, but it seems there's a lot more, and there are newer processes.)

No problem here. They're not normal LEDs; they're high intensity (several hundred mcd, but not the multi-candela ones), and I'll be running them well below rating.

Thanks! Richard

Reply to
Richard H.

I assume you mean the driver chips.

That would be my take. I prefer discrete to specialized drivers that almost always cost a fair piece and may not always be around when you need them.

But I don't understand why that would be an inherent property of the blue LED. I don't think there are any physical processes going on there that are that slow.

So can you expand on the details you were using when you observed this limitation and what you meant by "good signal"?

Jon

Reply to
Jonathan Kirwan

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