One aspect common to most available consumer ‘MP3 Players’ is that they, while very thin and easy to fit in a pocket, use a non replacable Li-Ion battery. Additionally usually they’re only rechargable via USB! The previous music player I had used AA batteries and it was very nice to be able to swap them when they got discharged. One possible way of solving this problem is by externally delivering 5V (nominal voltage for the USB bus). However as a NiMh cell is only 1.25-1.2V when discharging that would lead to using a lot of batteries! A somewhat more complex solution is to construct a voltage raising DC-DC regulator (boost, flyback, sepic etc), which would allow the use of one or two cells.
I’ve seen one switching solution on the web, called MintyBoost!, but it was limited to around 100-200mA output current which I felt was a bit on the short side, considering USB devices can draw up to 500mA. I didn’t have any data on how much current my music player drew, but it would be safe to design a regulator that could output 500mA constantly.
I’ve opted to go for a boost-type circuit as they’re relatively simple and there are a bunch of monolithic solutions requiring only an external inductor, diode, and stabilizing capacitors. The LM2621 was one (of many) circuits that offered the specifications I needed; small, high output current, monolithic, low input voltage. As it happens it uses a ‘gated oscillator’ control scheme which greatly simplifies the routing of the PCB, from an anal retentive level that typical controllers use to the easier level of super-pendant. (Using two layers would simplify routing a lot).
I pretty much followed the design shown in example 1 on page 9, with the exception of CF1 which I found tended to feed too much noise into the FB pin and caused erratic operation (most likely because of the limitations caused by a single-sided PCB). By removing it the circuit it very stable but has a larger output voltage ripple (about 100mV, an approximate value is V_OUT/1.2028*30*10^-3, this as the hysteresis on the FB pin is 30 mV and the voltage is divied from V_OUT to this pin). I managed to squeeze the entire boost part of the circuit into a box about 30×25x5mm (plus PCB thickness), and with some safety components about 40×35mm. The sustained output current is tested to around 500mA. 600mA worked for about 10 minutes before the device overheats and enters thermal shutdown. When 500mA is drawn out it gets hotter than I would like to run it, for high current applications using a 2-sided PCB with lots of cooling area on the back side is probably a good idea. 400mA is not a problem at all, it gets warm but not uncomfortably so.
The device is stable with input voltages between 2.3 and 3.5V at loaded at <=500mA. The output voltage is kept within 5.05 – 4.95V from 1mA-500mA load (there’s a power OK led that draws 1mA). When drawing small currents (approx <50mA) then it can start from only one cell (1.2-1.4V), but it never enters it’s gated control scheme, so efficiency is probably a bit lowered, though the output voltage is still kept stable.
When I tested this with my music player, to my surprise, it drew very little current, only 300-600mA on the low side (so around 100-200mA on the 5V side, with a fudged efficiency of 70%). This varied primarily on whether the backlight was on or not. This means that the 2.1Ah low self-discharge NiMh batteries I use will last long enough for it to charge completely about twice (a full charge takes about 3 hours, assuming 300mA charge current), or about 20-30 hours of playing time!
It may well be that the mintyboost is powerful enough for most players, nevertheless it feels good to know that this circuit can handle up to the maximum possible current USB is rated for.
The parts I’ve used are primarily;
Diode – http://se.farnell.com/jsp/search/productdetail.jsp?sku=8647887
Inductor – http://se.farnell.com/jsp/search/productdetail.jsp?sku=1669991
Buffering capacitor – http://se.farnell.com/jsp/search/productdetail.jsp?sku=1650938
Update: It has come to my attention that Y5v and Z5U ceramic dielectrics are really, hideously awful. Check out this PDF (look at the capacitance vs. DC bias and temperature plots), you only effectively get 5% of the rated capacitance at 65°C and >50% rated voltage! So the capacitors I thought were 22µF are only effectively 3.3µF each. In my application the output is stable, though a far better capacitor choice would be to use X5R or X7R dielectrics, which are available in nearly the same range and have far better characteristics! (see http://se.farnell.com/jsp/search/productdetail.jsp?sku=1650891 for example).
Regulator – http://se.farnell.com/jsp/search/productdetail.jsp?sku=9779248
Along with some other common odds and ends laying around (fuses, cabling, electrolytic capacitor and so on)
Eagle board and schematic can be found here, there are some deviations from the schematic on the final board. I’ve used a couple more 22µF capacitors on the output, there are 3 1W 6V2 diodes that should clamp spurious voltage spikes, and a 0.33 ohm output resistor, followed by a 220 µF electrolyte in parallel with a 22µF ceramic capacitor to filter out the worst high frequency noise.
