TPS61040 Constant Current Driver

I’m already up past my bedtime, so just a few pictures for now. Write-up coming soon!

Specs: Input 2v to 6v DC … Output constant current 50mA up to 28v DC
Efficiency: Initial measurements, somewhere around 75%
Chip Texas Instruments TPS61040

tps61040 boost converter constant current led driver
size comparison, american quarter dollar piece

circuit detail – design is one-sided PCB with two through-hole jumper wires and the diode, everything else is smt

twin 1uF tantalum capacitors … the output capacitor I had originally selected was limited to 16v, so this was the best I could come up with on a Sunday. Note the top of the coil is missing – these things are fragile!

PCB Photolithography and Inkjet Printers

I may be easily amused, but I’m not easily impressed.

Everything I have read about pcb prototyping using photolithography claims I needed to use a laser printer and some sort of transparent or translucent medium. Armed with this knowledge, I used acetate, or overhead projector transparencies. The film claims to be designed for laser printers and copiers, but it still distorts some when printed on. Anyway, the problem with my laser printer is toner pin-holes. For whatever reason, the output would have these little holes everywhere, and these little holes would swiss-cheese my traces and ground plane pours. This forced me to use thick traces, which would still get swiss cheesed, but generally retained enough composure to be electrically conductive. The pin hole problem seems to be getting worse … the last test I performed on my printer as an exposure test of varying width lines and different sized holes and pay layouts. In general, the performance was pretty bad and was not acceptable for more advanced designed I wanted to make, involving very small SMT components.

So, I tried printing the same exposure test, on paper, to my inkjet printer. The results were stunning. The lines were sharper, the holes clearer, the pads better defined. What a difference it makes going from a 600 dpi laser to a 4800(?) dpi inkjet. So, I felt it worth the risk, and decided to run my exposure test with the inkjet. Using some inkjet transparencies (they’re kinda coated with some type of fiber?), I printed my patterns. I exposed my board, and then developed it… the results were WOW! Of course, I messed up a few things with this first run, mostly I let the light cook for too long. So in the best un-scientific manner possible, I changed a bunch of variables at the same time, and tried another pass.

photolithography inkjet pcb

That is the result! … Please ignore the greasy thumbprint on the left side of the board – that was acquired after etching, and has no effect on anything aside from mild embarrassment on my part. My setup was fairly simple. I printed my design at best quality, monochrome mode onto a transparency. Next, in my darkroom that doubles as a furnace room, I laid down my PCB, emulsion up, then the transparency, then a sheet of 1/4″ plate glass. About 4″ above that, I have two 15 watt under cabinet lights, each loaded with a GE Daylight bulb. The lights are connected to an extension cord, so I can turn them on and off together. After making sure everything is lined up, I start my stopwatch and plug in the lights. After letting it cook for eight minutes, I turned off the lights and slid the pcb into a waiting bath of developer. The developer had been sitting for about a day, so it was a little slow. I left the board sit for about 2 min, before turning on the room lights. As the emulsion started to dissolve, I could see the results were good, real good! With the room lights on, I stirred the developer and lightly brushed the pcb using a foam brush (as recommended by the manuf.) My image grew sharper and sharper.

After I was sure all the emulsion that needed to be gone was gone, I rinsed the board and slid it into a waiting bath of ferric chloride etchant. A few min later, I pulled the pcb out, to make sure all the copper had turned pink. If the copper is not pink, it means some emulsion remains, and its time for another trip to the developer. Fortunately all the copper was pink, no problems here. I let the pcb soak for about 30 min while I had some lunch. After lunch, and without washing my hands i might add, I extracted the PCB from the etchant and rinsed it off. The results are excellent.

So, no longer will I shy away from tiny SMTs, since I can now lay down traces as thin as 8 mils without issue (determined by the earlier exposure test). Granted there were three flaws I had to fix on this board, I suspect they were caused by either dust on the transparency or those fibers that are embedded in the plastic. A quick touch-up with the sharpie solved them without a hitch.

Next stop, de-panelize with the PCB “suicide” saw.

pcb saw depanelize

Audio Matrix Switch

I’ve had some down time project wise lately, most of my SMPS research is on hold, since I need to order even more parts (namely diodes and switches)… so, I’ve taken the time to draw.

One of my ‘back burner’ projects is a whole-house audio distribution system. So, this past week or so, I set out to design the heart of the system, a matrix switch. Originally I had tried to go “too big”, designing a system with four inputs and eight outputs. However, routing the signal lines and control lines was too much of a challenge, even using a two layer board. So, scaling things back, I wound up with a 4×4 matrix that was barely manageable.

audio matrix switch 4x4 74hc4052 74hc595 schematic

There is the schematic for the latest ‘stable’ version. I call it a stable version, because it passes all of the DRC tests and all the nets are routed. I have another version in design, which adds ESD protection to the external audio connections in the form of high speed schottky diodes which will clip any voltage coming in that rises above or falls below Vcc and Vee.

Control will be provided by either a serial / parallel connection to the audio server PC, or from a “controller board” powered by a microcontroller. I haven’t really figured out how complex I want to make the interface yet. Control of the switch itself is fairly straight forward. Two 74HC595 latching registers are daisy chained together, forming a sixteen bit register. This register is used to provide the eight bits which tell the multiplexers which connections to make. Each multiplexer has a two bit interface, A and B, which selects one of the four IO channels to be connected to the COM channel. I selected the 4052 multiplexer which is internally divided into two sets of four channels each, ideal for stereo I thought. So the first eight bits of my 595 register are connected to the eight control lines on the 4052’s. Second, to provide a “hardware mute” or “output enable” feature, another four bits of my 595 register are connected to the /Inhibit (enable) control lines on the 4052’s. This allows the output of each 4052 to be electrically disconnected from all the sources. The final four bits of the register are used to power four “general purpose outputs”, perhaps to control relays, external indicators, whatever. The control lines connecting to the GPO header have inline resistors, to limit current draw to a safe level, so not to damage the 595 in the event of a short or connection to an unsuitable load. The four bits connected to the inhibit lines are also connected to four LEDs, so I can see which zones are active.

The 74HC4052 is an “analog” multiplexer, in that it allows voltages other than digital 1’s and 0’s to pass through. It is also a bidirectional multiplexer, allowing current to flow in either direction. This allows the multiplexer to handle the “AC” nature of an audio signal. In order to allow a true bipolar sine wave to pass, the 4052 requires a dual rail voltage supply. For simplicity, I will be using a five volt supply with positive and negative outputs.

two layer pcb layout audio matrix switch

Here is the ‘bare’ pcb layout, without the top and bottom pours rendered. This is a composite, showing both the top layer and bottom layer at the same time. It’s easier to follow the traces this way I think.

two layer pcb layout audio matrix switch

Adding in the ground planes or copper pours, it makes the board look mostly purple. I tried to maximize the size of the pours, to help minimize any interference or crosstalk. Although, at such low frequencies, I don’t really expect much trouble.

pcb parts layout silkscreen audio matrix switch

Finally, the board with just the silkscreen or layout layer selected. This again, is a composite, showing parts on both the top and bottom of the board, which makes it look like some of the parts are overlapped.

There is a lot more to come on this subject, but it will be a while. Overall, this was just a fun project to draw, and hopefully one day it will actually get fabricated. I need to figure out how complex I want the interface to be, and what options I have for interfacing with the audio server.