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TPS61085 Evaluation Module

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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.

Rload 550 Ω
Iload 28.3 mA
Vout 15.88 V
Vripple,p-p 17.2 mV

Output ripple at high load current.

Rload ~ 156.7 Ω
Iload 101.6 mA
Vout 15.71 V
Vripple,p-p 33.6 mV

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.

Rload 360 Ω
~ 78.3 Ω
Iload 44 mA
~ 200 mA
Vin 4.99 V
Vout 15.94 V
Vtransient,p-p ~ 200 mV

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.

Rload 360 Ω
Iload 44 mA
Vin 4.99 V
Vout 15.93 V
Efficiency 82.75 %

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.

Written by Ferdinand

März 21st, 2012 at 21:51