AL5801W6-7 LED driver

[NXTreme]

I recently bought some high power LEDs to augment the counter-top lighting in the kitchen. When I bought the LEDs I looked for something that was fairly bright, was the right colour (I got some white warm, and some white neutral) and that was fairly cheap. I ended up buying Cree MX-3 LEDs, depending on the colour of the LED they are about ~$1.3 US apiece, put out ~90 lm apiece and have a viewing angle of 120°. I bought seven of each, which I think should be enough for about 5.3 m2 of counter/sink/stove space. I want the warm coloured LEDs over the stove-top and the neutral LEDs over the sink, with mixed colours over the actual tiled counter.

I had originally thought of using a LM317 paired with each LED as a constant current driver, along with a high power transistor attached to a Picaxe for PWM control but I soon found out that they warmed up quite a bit, even with a substantial heat sink. While I think I could still use the LM317 and high power transistor as a driver and for PWM-ing the LED, I was looking for a better way of driving the LEDs, preferably with one chip with the driving and PWM-ing circuitry all built in. I found what I think is a pretty good driver, but I want to make sure that it is suitable for my intentions. It is the AL5801W6-7 and costs about $0.50 US apiece in quantities of over ten on Digikey [http://www.digikey.com/product-detail/en/AL5801W6-7/AL5801W6-7DICT-ND/3481118].

There is one main question I have about the chip however. The Cree MX-3 LEDs [http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampMX3.pdf] are rated for a maximum of 400 mA at about 3.8 V but as they are quite bright I want to power them at a more modest current, around 300 mA or so. On page six of the datasheet [http://www.diodes.com/datasheets/AL5801.pdf] Figure 12 shows the relationship between Output Current and VOUT. I'm not quite sure what exactly that chart means, but I am thinking that it may be a chart of the maximum current output at a certain voltage and vice-versa. In this case, if I wanted to run them at 300 mA there wouldn't be enough voltage, and again vice-versa. Am I misinterpreting this graph or is this a correct assumption?

One more small question. The datasheet for the AL5801W6-7 shows two circuits that can be used to PWM the LED. I think I would go for the one that uses a 2N7002 MOSFET because of the smaller quantity of parts. However, it doesn't mention any specifics for the MOSFET and looking up '2N7002' on Digikey returns a myriad of results. What general sort of specifications should I be looking for on the MOSFET or does it not make much of a difference?

Lastly, if anyone has suggestions for other cheap, small and simple LED drivers that are PWM-able, I would love to hear them. I'm used to using a resistor, and this is my first foray into high-power LEDS .

 

[inglewoodpete]

Before I attempt to answer your question about figure 12, perhaps you need a briefing on the concept of the device.

Best described here, using discrete components, the circuit is a simple analogue constant current device: it tries to deliver the designed current through the LED or chain of LEDs. Like all regulators, there are limitations to that capability. One limitation is the maximum voltage available to drive the LED or chain of LEDs: is there enough voltage available to generate the required current through the LED/s?

Figure 12 appears to describe the second limitation of the circuit: the higher the voltage across the MOSFET, the greater the I2R heat loss in the driver chip. The three curves appear to represent the heat limitations given the heatsinking capabilities of the copper on the PCB, since the device is a SMD.

The AL5801W6-7 may well be a good option for driving your LEDs. There is a lot to consider here. Depending on the voltage you use, all unused energy is wasted as heat. You may want to look at switchmode LED drivers.

 

[NXTreme]

Before I attempt to answer your question about figure 12, perhaps you need a briefing on the concept of the device.

Indeed . Thank you for the explanation, it clears up my main question. Seeing as I want the circuit boards as small as humanely possible, the heat sinking capabilities will not be that great. As far as voltage goes, I don't have a power supply for the whole setup yet, as I was thinking of trying to find a power supply that suited the current and voltage requirements. I am thinking a laptop computer power supply might work, there are some for smaller computers that put out 12 V and up to 3 A. I would prefer to drive the LEDs in parallel though, so a lower voltage might be better. I could probably find a lower voltage supply that could still handle the current requirements.

The AL5801W6-7 may well be a good option for driving your LEDs. There is a lot to consider here. Depending on the voltage you use, all unused energy is wasted as heat. You may want to look at switchmode LED drivers.

Even though I could tailor the input voltage to the chip's liking to some degree, a more efficient LED driver would be much better. If you have any suggestions for switchmode LED drivers or at least some pointers about what to look for, they would be greatly appreciated.

 

[inglewoodpete]

There are several options open to you. Unless you have your heart set on a specific design of light fitting, look at some of the options.

There are many types of 12v DC LED lights available. They already have some form of current regulation built into them and they often fit directly into industry standard halogen light fittings. Note that many of the high-current halogen lighting switchmode power supplies do not start very well with LEDs due to the vastly reduced current draw. So you would need to use a more suitable supply, available from many (hobby) electronics suppliers. They often have exposed 230v terminals, so need to be enclosed in another box for safety.

If you want to continue down the AL5801 path, you will see from the heatsink graph (Fig. 12) that the higher the voltage drop across the chip, the lower the maximum current has to be. The voltage across the chip will be roughly Vchip = Vsupply - (Vled + Vrset). From the various datasheets, Vled @ 300mA is about 3.6V and Vrset = 0.56V (roughly 0.6v) so, for a 12v supply, Vchip = 12 - (3.6 + 0.6) = 7.8v That would allow a maximum of 60 to 70 mA current: not enough! Alternatively, you could daisy-chain 3 lights together: Vchip = 12 - ((3.6*3) + 0.6) = 0.6 (barely enough) - you would have to do some experimenting.

If you drop the supply to (a regulated) 6v, then the formula looks like Vchip = 6 - (3.6 + 0.6) = 1.8v You can get your required 300mA with that. Note that you would need the 6v power supply within a few metres of the lights, otherwise the resistance of the wiring would eat into the small voltage headroom. The power supply could be a lot simpler too - a 3 Amp, 6v switchmode plugpack ("wall wart") is a lot easier to buy and fit than an open-frame SMPS.

My current LED project? I'm using RGB LED lighting to (subtly) light up the side of a 5 storey building. And yes, it will be controlled by a single PICAXE.