LED driver recommendation - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2017, 04:10:33 pm
content: I'm thinking of working on a project that lights a lot of LED's. They don't require a lot of current, maybe 3-5mA, but they will be driven from various pins from the same microcontroller used on the Arduino Due and some of its pins are 3mA and others are 15mA so their drive strengths are not consistent I guess. Someone already did a project like this and he is using a base resistor/npn/current limiting resistor for each of the 36 LED's. I'm planning on adding 16 more LED's than that. So power requirements are not huge (3-5mA), but each LED needs to be driven directly from a 3.3v weak logic signal. It is also something that might be touched by human fingers so ESD is a factor which probably makes a transistor a better choice than the gate of a mosfet. I am looking for a SMD part. Any recommendations?

author: Brutte
author_rank: Frequent Contributor
timestamp: November 11, 2017, 04:37:12 pm
content: Starchips SCT2024, 16 constant current drivers in one package, SPI input. Ok down to 3.3V.

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2017, 04:47:15 pm
content: That IC looks like a good solution.A MOSFET is a transistor, but I think you're talking about a BJT. One thing to note about MOSFETs and ESD is, it's the gate which is the sensitive part of the transistor. The drain-source will avalanche over quite safely, without damaging the transistor, as long as the energy of the pulse is limited to a safe level. As far as ESD is concerned, once the circuit is assembled, BJTs will be just as vulnerable to ESD as MOSFETs and vice versa.

author: phil from seattle
author_rank: Super Contributor
timestamp: November 11, 2017, 05:02:34 pm
content: What's your source voltage? USB powered?Also, do you want to turn all the LEDs on and off at the same time or some level of individual control?What is the Vf of your LEDs? If it's >2.9V the Starchips ic will need to have >3.3V Vdd. Running that chip at 5V will need voltage translation because it's Vih min is .7 Vdd.

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2017, 06:02:22 pm
content: Sorry, no interface driving like spi or i2c. Direct pin drive. I've got a 3.3V pin coming from the microcontroller for each LED. Each LED needs to be on/off with individual control.>once the circuit is assembled, BJTs will be just as vulnerable to ESD as MOSFETs and vice versa.I've always thought about this, but wouldn't a BJT gate be much less sensitive to ESD because there is someplace for it to go as a mosfet gate is isolated? I know it would be better in circuit with another path, but even then, wouldn't a mosfet still be easier to damage?Not sure of the power. I usually use the APT3216CGCK / APT3216SURCK for LED's because they are low power and decent brightness.

author: ocset
author_rank: Super Contributor
timestamp: November 11, 2017, 06:13:50 pm
content: yes BJTs are less sensitive to esd damage than fetshttp://www.edaboard.com/showthread.php?t=369302&p=1581311#post1581311a bjt junction clamps itself in one direction...as its a diode....also there is more leakage in a bjt junction in the n->p direction , than in a fet gate-source.

author: phil from seattle
author_rank: Super Contributor
timestamp: November 11, 2017, 07:05:43 pm
content: Yes but in a circuit, the gate is protected. If wired to a microcontroller, the clamp diodes will take care of esd, at least up to a point (where other things get fried first).OK, to the original point.Those LEDs have a low enough Vf to be powered off of 3.3V. If you go for white, though, you'll need higher.You'll have to use transistors and resistors to directly drive them. You can avoid individual transistors, though. Driver chips will do that. I've used 74HC595s to drive LEDs to a reasonably low power level (say 8 mA). There's a higher power version of that (the part num escapes me, tho). The good news is, you only need 2 pins to control any length of string. Just chain them together in increments of 8 LEDs. Bad news - you'll still need resistors.Another approach that might work is to use something like the WS2812 LEDs. While it's possible overkill (RGB) they are pretty easy to use and there are libraries for arduino. You will need to run them off of 5V (which is available from the Due). Only 1 pin required for control of any length string. No resistors needed though they recommend bypass caps for some percentage of the LEDs.

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2017, 09:24:59 pm
content: Yes, once the MOSFET's gate is connected to the MCU output pin, it will be protected against ESD and be no more sensitive than a BJT.If the forward voltage is unknown, then connect the LED to a 5V supply, via a 1k resistor and measure the voltage across the LED.It's possible to drive white and blue LEDs from an MCU, run off 3.3V with no series resistor, as the internal resistance of the I/O port will be high enough to limit the current to a safe level.Is there a higher supply voltage than 3.3V?If the LEDs need 5V, then how about a shift register such as the 74HCT595? Its inputs are compatible with 3.3V logic and will run off 5V.https://assets.nexperia.com/documents/data-sheet/74HC_HCT595.pdfFor higher voltages, up to 20V, there's the CD4504 level shifter, but it does have fairly high resistance outputs, although it should be fine for 3mA to 5mA LEDs.http://www.ti.com/lit/ds/symlink/cd4504b-mil.pdf

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2017, 09:47:06 pm
content: Maybe there isn't a constant current LED driver that does what I want. I tried to find one at Digikey using their search engine and had no luck. What I'm looking for is a constant current LED driver that would take 7 or 8 logic level inputs and then output to 7 or 8 LED's at a constant current without a current limiting resistor. No spi/i2c/etc.Is what I'm looking for more of a buffer? Are there buffers that would do something like this? They would probably need a current limiting resistor though...

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2017, 10:40:13 pm
content: Quote from: alank2 on November 11, 2017, 09:47:06 pmIt's difficult to suggest anything, other than what I've posted previously, without any more information: LED forward voltage, whether it's just 3.3V only or you've got a higher voltage such as 5V or 12V handy.If the forward voltage of the LED is close to the supply voltage, then a resistor may be not necessary because the output resistance of the transistors will be enough to limit the current to a safe level. For example, the CD4504 has an output resistance of 400R, when the supply voltage is 5V, calculated R = VCC-VOUT/I = (5-4.6)/0.001 = 0.4/0.001 = 400. If the forward voltage of the LED is 3.5V and the supply voltage is 5V, then the LED current will be I = (5-3.5)/400 = 1.5/400 = 0.00375 = 3.75mA.

author: alank2
author_rank: Super Contributor
timestamp: November 12, 2017, 03:15:51 pm
content: Maybe there is nothing better than a bjt/mosfet for each LED. Let's go back and explore the idea that mosfet's are just as protected from ESD as BJT's are once they are in circuit? I get that when in circuit there is another pathway that can protect the gate, but does that mean the gate (mosfet) is just as safe as a bjt gate? I wonder. What do you guys think?