Most of the applications for LEDs I’ve run into require a lower than supply bias voltage for the LEDs. This situation is fairly easy to deal with, you can use a current limiting resistor, a linear regulator, or in high power applications, a step-down or buck converter.
However, occasionally, it may be beneficial to have a rather high LED bias voltage, even with a low supply voltage. For example, in small battery powered applications, rather than run your LEDs in largely parallel configurations (which has many drawbacks), you can string the LEDs together in a large series string, and then parallel those strings if need be.
Enter the TPS61040 from Texas Instruments. The TPS61040 is an integerated high efficiency step-up or boost converter. It is a integerated converter rather than a controller, because it contains both the power switch and the feedback circuitry. This means a very simple design is all that’s required to make it work – and in my opinion, the smaller the part count, the better.
There is my schematic, which provides for a variable voltage constant current “LED Driver”. LED current is programmed by resistors R1 and R2, which are connected in parallel. Inductor L1 is a small 10uH inductor, and D1 is a ‘standard’ schottky diode. C1 and C2 are low ESR ceramic capacitors, with an X7R rated dielectric. The chip itself, IC1 is an amazingly small SOT23-5 package surface mount IC. The rest of the components are also surface mount, both for space savings and laziness, as I hate drilling holes. I was out of SMT schottky diodes at the time I drew this, so D1 is a 2.8mm by 7mm DO41-7 package. C1 can be a 10 or 16 volt cap, C2 should be rated at the output voltage plus a safety factor (for dealing with ripple). L1 should be sized to handle the current demands of the circuit – I just went with 1.6 amps since it was cheap.
The layout is about 20mm square. My intended application is for lights inside a “shadowbox” style picture frame. I want a small string of white LEDs powered by some cheap AA batteries. I’m out of PCB developer right now, so no PCB fab this weekend. Once I get some more in, I’ll share the finished product with everyone, as well as some pictures of my cheezy art project.
If anyone should want full resolution layout or pcb artwork, just hit the contact justdiy button, over in the right hand sidebar.