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.

tps61040 step-up boost converter smps led driver

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.

tps61040 pcb layout artwork

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.

One Reply to “TPS61040”

  1. These integrated controllers are great, I used two similiar LM2576 (easy to use but pricy) in this piece here, I used one for red, and another for green/blue Vf for the LEDs:
    The LM2576 devices retail for around $AU14 here is OZ….thanks for demonstrating a cheaper…more useful chip, it’s not every day you need to put in a 15W LED supply!
    For the picture frames, as a suggestion, why not use perspex or thick glass for the front panel and laser etch a design….then edge light underneath the frame. With white behind, a bold yellow or green would look great as a highlight.
    I’ve done a number of frame lights with RGB cycling, great christmas presents (my family is sick of LED presents by now). One thing you may need to look at is diffusion of the light. I’ve used crushed glass, tracing paper and other methods. One thing to try that is cheap and remarkably nice looking is plain simple bubble wrap. Choose the smaller bubble bubble wrap fo best effect. If you want to get fancy, some BEF films or holographic films (as in the Bell Tree light above) are also good if you want an even glow…though often, particularly with RGB lighting, specular highlights from the bubble wrap or glass fragments can actually enhance the final work. A final suggestion is to use textured glass or plastic for the front panel. This usually has a clear side and the textured side. Put the textured side facing the LEDs, and the clear side on the outside. If you can diffuse or cover the clear side with diffuser, you get caustic shadows being projected……it looks very cool with dynamic lighting.

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