olympic-white

A switch-mode DC/DC converter module based on a TPS65131 to provide adjustable positive and negative rails from a 2.7V-5.5V supply.

Draft: README.md will be updated after validation and debugging.

Purpose

For analog electronics, a bipolar supply is often needed, that is positive and negative rails like +15V and -15V. Unfortunately, most of time, only a single positive supply is available, in particular, USB supplies a single +5V. The object of this switch-mode DC/DC module is to propose a workaround for that.

With an input supply between 2.7-5.5V, the TPS65130 and TPS65131 DC/DC converter ICs are perfectly suited for battery operated or USB devices. For example, my Solid State Logic SSL2 USB audio interface seems to make use of a TPS65130. A TPS65131 is used here to benefit from its higher current capability.

The proposed module can be configured using 2 resistors and bridging 4 solder jumpers. It matches a DIP style footprint, hence it can be inserted in a DIP socket or used on a breadboard The previews should give you a better idea: top and bottom

Usage

The TPS65131 implements a boost converter for the positive rail and a buck-boost (inverting) converter for negative. Please refer to the Texas Instrument's website for details, including the datasheet. And then have a look at the schematics.

Each rail features an "enable" and a "power save mode" pins. Solder jumpers ENP and ENN respectively control if positive or negative outputs are enabled by external (bridge center and top pads) signals (pins ENP and ENN on J2) or always on (bridge center and bottom pads). Solder jumpers PSN and PSP control power save mode (refer to datasheet) for respectively positive and negative rails. Bridge center pad with top pad to turn-off or with bottom pad to turn-on. All solder jumpers must be appropriately bridged before use.

To program the positive output voltage, use the following formula: R2 = (R1 * V_ref) / (V_pos - V_ref). Where R1 is 470k, V_ref is 1.213V, and V_pos is the desired output voltage. For example, for V_pos = 12V, we end-up with R2 = 52851R. We can take the standard resistor value 52.3k. Here are resistor values for some common positive output voltages:

V_pos	computed R2	1% resistor value
5V	150543R	150k
9V	73213R	73.2k
10V	64881R	64.9k
12V	52851R	52.3k
15V	41351R	41.2k

Note that 5V output can be achieved only if the supply is less than 4.5V.

To program the negative output voltage, use the following formula: R4 = -(R3 * V_ref) / V_neg. Where R3 is 470k, V_ref is 1.213V, and V_neg is the desired output voltage. For example, for V_pos = -12V, we obtain R4 = 47509R, hence we can take the standard resistor value 47.5k. Here are resistor values for some common negative output voltages:

V_neg	computed R2	1% resistor value
-5V	114022R	115k
-9V	63345R	63.4k
-10V	57011R	57.6k
-12V	47509R	47.5k
-15V	38007	38.3k

For both programming resistors, use "low-noise" 1% metal film resistors. You may solder them on either (top or bottom) sides.

Fabrication

Some "ready to use" gerbers are available: gerbers/olympic-white-gerber.zip. The full Kicad project is available kicad/olympic-white.zip. JLCPCB is cheap efficient, be sure to order the stencil along. Then order the content of the BOM.

Remarks and Discussion.

This is a lower level project and this documentation is probably not enough to make the most of it.
Because the input supply is limited to 2.7-5.5V, this module can only works with battery operated or USB devices. This is unfortunate as quite a few electronic devices that could have benefit from such a module, operate at +9V (guitar effect pedals) or +12V. In retrospect, the ADP5071 with its 2.85-15V input range and its wide -39V and +39V output ranges, would have been a better choice.
For low-noise application, the outputs would probably need post-regulation (using a pair of LM2940 and LM2990 for example), or at least some carefuly designed additional filtering.
Why olympic-white ? This is the color of my Fender Jazz Bass and I figured out it would be a good project name.

Revision history and status

Version 0.8 Ready for fabrication and debugging/validation, upload on github.
- Need to send PCB to fabrication;
- Then do the assembly and debug/validate.

anotherlin/olympic-white