Yak-2 Voltage Regulator

Introduction

This documents the custom voltage regulator board constructed for the Yak-2. Heavily based on work by Steve Mann and Charmed Technology, the board provides power to the SBC and its subsystems.

Specifications

Input voltage 6.5-17.0V
Output voltage 5.0V +/- 3.0%
Current out 3A (max)
Efficiency 94-99% (7V), 92-95% (14V)
Connectors V1 input: 2.64mm mini banana plugs (slot directly to Sony infoLithium)
V2 input: 2.64mm mini banana jacks
Vout output: Standard PC power connector
Physical dimensions 50 x 40 x 21 mm (width/depth identical to Sony NP-F550)
Other features Each input can take either 1 or 2 Sony 7.2V infoLithium batteries in series
Batteries and/or DC power can be hot-swapped (as long as at least one is plugged in!)
Inputs reverse-polarity protected (30V max) and fused for short circuits
Manual switch for turning on and off

Usage notes

Schematic

Schematic of regulator circuit

Board layout

Component layout on 0.1" breadboard with horizontal traces, to scale
Neither up to date nor complete

Components

V1+/- 0.108" miniature banana plugs
VOUT Floppy-type power jack (AMP 171822-4 or equiv)
F1, F2 250V 5mm fuses in holders
RECT1 IRF 32CTQ030 30A Schottky rectifier
REG1 Datel UNS-5/3-D12 non-isolated 5V/3A DC/DC converter
under construction...

Engineering trivia

The other DC regulator often seen in wearable use is the Powertrends PT6302. However, the PT6302 is slightly less efficient (~90%) and it requires a minimum input voltage of 9V, ie. at least two batteries in series. On the plus side, its profile is less wide and it can take up to 4 batteries (28V) in series, although then efficiency goes down to 80%. Yak-2 doesn't have all that much space for batteries, and hot-swapping with 2 sets of 2 would have been impossible, so I went with the Datel. The circuit above should function identically with the PT6303A if you change C1 to a 1µF ceramic (or remove it entirely) and, optionally, reduce C2 to 100 µF.

We can't simply place V3 parallel to V1/V2 before REG1, because the regulator requires a minimum input voltage of 5.5V.

Diode D1 is not simply in series with input V3, because this would drop VOUT to 4.5V. Banana plugs are not the smallest of solutions, and they have the distinct disadvantages of requiring two cables for positive and negative and not restricting polarity. (The upside is being easy to insert and remove while being difficult to disconnect by accident.) I was sorely tempted by some of Hypertronics' offerings (check the D-series), but banana plugs are standardized and available anywhere, bizarro custom plugs aren't...

Currently switching on and off is handled simply by mechanically cutting the connection, but the Datel (and the Powertrends) feature an On/Off pin for shutting off regulator output. It would not be terribly hard to add a relay controlled by (say) the serial port for controlling the system. The Datel version takes a TTL signal (2.4-5V high, 0-0.5V low, 100 µA max), which according to a quick calculation could be generated by adding a line from the switch output to ground with 100k and 50k resistors in series, taking the TTL signal from between them:

Vin 7V: VCTL 2.45V, ICTL 46 µA
Vin 14V: VCTL 4.90V, ICTL 93 µA
Reference level is common ground and obviously VCTL can never be larger than Vin, so this should be safe. I think. If somebody actually implements this, let me know!