Computing stuff tied to the physical world

Boost revisited

In Hardware on Jun 4, 2012 at 00:01

The AS1323 boost converter mentioned a while back claims an extra-ordinarily low 1.65 µA idle current when unloaded. At the time, I wasn’s able to actually verify that, so I’ve decided to dive in again:

Screen Shot 2012 05 17 at 15 28 15

A very simple circuit, but still quite awkard to test, due to its size:

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Bottom right is incoming power, bottom left is boosted 3.3V output voltage. Input voltage is 1.65V for easy math.

The good news is that it works, and it shows me an average current draw of 4.29 µA:

SCR03

The yellow line is the output voltags, with its characteristic boost-decay cycle. For reference: the top of the graph is at 3.45V, so the output voltage is roughly between 3.30 and 3.36V (it rises a bit with rising supply voltage).

The blue line is the voltage over a resistor inserted between supply ground and booster ground. I’m using 10 Ω, 100Ω, or 1 kΩ, depending on expected current draw (to avoid a large burden voltage). So this is the input current.

The red line is the accumulated current, but it’s not so important, since the scope also calculates the mean value.

Note that there’s some 50 Hz hum in the current measurement, and hence also in its integral (red line).

Aha! – and here’s the dirty little secret: the idle current is specified in terms of the output voltage, not the input voltage! So in case of a 1.65V -> 3.3V idle setup, you need to double the current (since we’re generating it from an input half as large as the 3.3V out), and you need to account for conversion losses!

IOW, for 100% efficiency, you’d expect 1.6 µA * (3.3V / 1.65V) = 3.2 µA idle current. Since the above shows an average current draw of 4.29 µA, this is about 75% efficient.

Not bad. But not that much better than the LTC3525 used on the AA Power Board, which was ≈ 20 µA, IIRC.

More worrying is the current draw when loaded with 10 µA, which is more similar to what a sleeping JeeNode would draw, with its wireless radio and some sensors attached:

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Couple of points to note, before we jump to conclusions: the boost regulator is now cycling at a bit higher frequency of 50 Hz. Also, I’ve dropped the incoming voltage to a more realistic 1.1V, i.e. 1/3rd of the output.

With a perfect circuit, this means the input current should be around 30 µA, but it ends up being about 52 µA, i.e. 57% efficiency. I have no idea why the efficiency is so low, would have expected about 70% from the datasheet.

Further tests with 1.65V in show that 1 µA out draws 6.72 µA, 10 µA out draws 29.6 µA, 100 µA out draws 261 µA, 1 mA out draws 2.51 mA, and 10 mA out draws 30.9 mA. Not quite the 80..90% efficiency from the datasheet.

My hunch is that the construction is affecting optimal operation, and that better component choices may need to be made – I just grabbed some SMD caps and a 10 µH SMD inductor I had lying around. More testing needed…

For maximum battery life, the one thing which really matters is the current draw while the JeeNode is asleep, since this is the state it spends most of its time in. So minimal consumption with 5..10 µA out is what I’m after.

To keep things in perspective: 50 µA average current drawn from one 2000 mAh AA cell should last over 4 years. A JeeNode with Room Board & PIR (drawing 50 µA, i.e. 200 µA from the battery) should still last almost a year.

Update – when revisiting the AA Power Board, I now see that it uses 25 µA from 1.1V with no load, and 59 µA with 10 µA load (down to 44 µA @ 1.5V in). The above circuit works (but does not start) down to 0.4V, whereas the AA Power Board works down to 0.7V – low voltages are not really that useful, since they increase the current draw and die quickly thereafter. Another difference is that the above circuit will work up to 2.3V (officially only 2.0V), and the AA Power Board up to at least 6V (which is out of spec), switching into step-down mode in this case.