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Some time ago I ordered samples of TI’s TPS61085 step-Up DC-DC converter. I have been looking for a DC-DC converter capable of more than the usual 5-12 V output, and the TPS61085 looked like a perfect match. This great little chip (MSOP and TSSOP packages) can run off as few as 2.3 V and output up to 18.5 V. It integrates an 2.0 A, 0.13 Ohm power switch and offers a selectable 650 kHz and 1.2 MHz switching frequency. The high switching frequency makes it possible to use very small inductors and capacitors, thus reducing the needed PCB area.

After the parts had been lying around for some time I finally had the time to layout a PCB to try them. The PCB layout is based on the layout of the TPS61085EVM evaluation module made by TI and the hints from the IC’s datasheet. The resulting PCB is just 29.21 x 25.40 mm in size, although it could still be made smaller.

I had two of these PCBs made and built one up for an output voltage of 15 V. As I didn’t have the exact resistor values for the feedback divider, the output voltage ended up to be 15.7 V instead of 15 V. I then ran some tests.

Output ripple at low load current.

Output ripple at high load current.

I originally planed to use this IC in a headphone amplifier, so I was curious about the transient response of the circuit. I replicated the measurement pictured in the datasheet to be able to compare the results. Two paralleled resistors were connected as load, one of them through a NPN transistor. A Teensy board was programmed to switch the transistor at 1 kHz and 50 % duty-cycle. The results are shown below.

Transient response.

Of course I also determined the efficiency of the circuit. At a load current of 44 mA the efficiency turned out to be 82.75 %. Not too bad for a first revision. The complete data is shown below.

The TPS61085 is a very capable little boost converter IC with many possible applications. As I want to use it in an audio circuit, low ripple at high frequencies is key. It also uses very little PCB area and matching inductors are tiny and very cheap. From a 5 V USB power supply this chip, together with an inverter can supply +/- 15 V for precision OpAmps.
With an output ripple between 17 mV and 33 mV the circuit as is would probably need additional regulation if used to power a tube or Class A circuit. In this case the high ripple frequency is a problem, as most common regulator ICs have pretty low attenuation at high frequencies. A simple LC filter might suffice to attenuate the ripple, but I haven’t tested that yet.
As I want a power supply with a positive and negative rail, I am right now looking at the TPS65130, which integrates two converters in one IC. Building an evaluation PCB for this chip is next on the list.

Im Moment arbeite ich im Rahmen einer Hiwi-Stelle an einem DC-DC Wandler Design. Das besondere bei der Schaltung ist die geforderte Ausgangsspannung 220 V bei einem Strom von 150 mA. Das sind mehr als 30 Watt und damit wesentlich mehr als bei anderen Entwürfen die ich bisher gemacht habe. Dabei ist mir wieder eine Schaltung eingefallen, die ich das erste Mal auf circuitsathome.com gesehen habe: Jim Williams’ Trigger Probe Amplifier (siehe Linear Technology Application Note 70). Die unten abgebildete Schaltung ermöglicht es mit einer in der Nähe der Spule des Wandlers montierten zweiten Spule ein Trigger Signal zu generieren. Der Aufbau ist dadurch galavanisch von der zu untersuchenden Schaltung getrennt, was die Messung unter Umständen stark vereinfacht. Außerdem kann auch der Stromverlauf durch die Spule auf dem Oszilloskop dargestellt werden.

Leider ist die Schaltung schon etwas betagt, das CA3036 Transistor Array ist nicht mehr zu bekommen und die zwei OpAmps und der Komparator sind alles andere als günstig. Daher habe ich mir einen MyLinear Account eingerichtet und die drei Bauteile als Sample geordert. Große Hoffnungen hatte ich nicht, laut Google ist Linear Technology bei Samples nicht sonderlich freigiebig (im Gegensatz zu Texas Instruments).

Knapp zwei Wochen später dann die Überraschung: in meinem Briefkasten liegt ein Umschlag von Linear Technology. Und tatsächlich, LT hat mir die drei bestellten Bauteile wirklich zugeschickt. Kostenlos und ohne Rückfragen. Im Sample Formular hatte ich genau das angegeben was ich oben geschrieben habe, dass ich die Bauteile für ein Uni Projekt benötige.
Für diesen Service vielen Dank an Linear Technology!
Nach dieser Erfahrung werde ich häufiger LT Bauteile einsetzen.

Nun noch ein paar Bilder vom Auspacken:

I just recently found a defect Panasonic RF-B45 All Band Receiver that had been lying in a box waiting to be repaired. The Panasonic RF-B45 is a great little radio that can be run of 6V DC or 4 Alkaline Batteries. It was actually still working, but could not be turned on via the ON/OFF switch, as it was broken. I disassembled the device and found that the keyboard uses cheap radial tactile switches that could be easily replaced. I ordered 30 new “ALPS” switches from China, reassembled the device and waited.
Finally the switches arrived and I could start replacing them. In the following I will document the necessary steps.

Step 1: Disassembling the receiver

The Panasonic RF-B45 is easily cracked open. Just remove the antenna and then six screws. In the above image I have marked the position of all screws. After the back is taken off the mainboard can be removed.

The switches are on an extra PCB that is connected via a ribbon cable. Remove the connector carefully and then the two screws holding back the PCB. Again, the screws and the connector are marked in the image.

Step 2: Unsoldering the switches

Having the PCB removed one can start the tedious task of unsoldering all switches. You can of course just replace the broken switches, but to be on the safe side I just replaced all of them. One pad lifted from the PCB while I unsoldered the switches, but all in all the PCB endured this procedure quite well. As it is neither plated through nor of FR4 material you have to be careful not to destroy the delicate pads and traces. The metal shield on the back of the LC display covers three switches and therefore has to be removed, too. This is time consuming work but not difficult at all.

Step 3: Soldering the new switches

After all switches are gone clean the PCB. Then start soldering the new switches. This again takes some time. I soldered one leg at first, then reflowing the joint and pressing the switch flush to the PCB. I then soldered the second leg. After finishing all joints cut the legs, drown them in flux and reflow them again. Do a short inspection under magnification and make sure that all joints are nice and shiny. Then clean the PCB again.

Step 4: Assembling the device and testing

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Now you can put the receiver back together again. Fasten the two screws fixing the PCB and then carefully connect the ribbon cable connector. Put the mainboard back in the front part of the case and put the back on. After all screws are fastened and the antenna is mounted the receiver should be working as normal.
Congratulations, you have rescued one more device from going to the landfill.