Computing stuff tied to the physical world

Another AA booster

In Hardware on Jan 16, 2012 at 00:01

Out of curiosity, I’ve been doodling around with the AS1323 boost regulator chip, as suggested by someone in the comments. It’s not super efficient, but it does claim an extraordinarily low quiescent current of 1.6 µA.

Hooking up such a tiny chip is quite a challenge:

DSC 2856

The trouble is that this stuff can’t be spread out too much – the specs say that both input and output caps have to be within 5 mm of the chip. So I tied it to a pcb – mostly for stability – and then wired everything up around it.

Without any load, this thing operates in quite an efficient mode:


The yellow trace is the voltage drop across a 10 Ω low-side resistor, the blue trace is the output, but AC coupled, i.e. showing only the ripple voltage.

The red line is the math function to integrate total power use in Coulombs – more on that later.

If you look at the top overview graphs, you can see the current blips which are over 160 ms apart, i.e. this thing is activating only 6 times a second. And each current pulse lasts only about 150 µs. The peak current on the input side, i.e. drawn from the battery, is about 12.5 mA.

Let’s put a 1 kΩ load on the 3.3V output line:


I’ve adjusted the scale a bit. The switcher is now operating at about 11 KHz. The peak current drawn is almost 18 mA, but note also that the current never drops to 0 anymore – the baseline of that yellow trace is 2 divisions down from the top, so that’s about 6 mA (as expected, since the load is always drawing current).

Now let’s push it and change the output load to 100 Ω, i.e. about 33 mA @ 3.3V. To make that work, I had to change the input sense resistor to 1 Ω:


The baseline for the yellow trace is now halfway, same as for the blue line.

I also added a third probe, the green line monitors output voltage, which is indeed steady at 3.3V (both red and green lines are based at the bottom of the screen). Note the huge peak current draw on the battery: over 290 mA!

Let’s try to understand what’s going on in this last case. First of all, with a 1 Ω sense resistor, a 190..290 mA current draw creates a voltage drop of around 0.25V – which means the battery voltage isn’t really reaching the switching regulator. The battery was measured to be at 1.37V, so the switcher is getting only about 1.1V on average. The datasheet says that it will only be 70..75% efficient on such an input voltage, when generating the 3.3V output.

The 100 Ω output load draws about 33 mA. That’s at 3.3V, so a perfect step-up regulator would need to draw 3x as much when running off 1.1V, i.e. 100 mA. A 70% efficient switcher would draw about 150 mA (0.70 x 150 ≈ 100). What I’m seeing here is more like a 40% efficient switching result (0.40 x 250 = 100) – hm, not sure why…

The other way to determine average current consumption, is to do some Coulomb counting…

In the first screenshot, each blip uses about 900 nanocoulombs (the red line rises 4.5 divisions over the entire width of of the screen). With 6 blips per second, we use 5.4 µC each second, i.e. 5.4 µA average current draw (not quite the 1.6 µA I expected, but still very impressive for an unloaded step-up regulator).

The second graph is trickier. We need to figure out the Coulombs increase per repetitive cycle. My guess would be around 820 nC. Multiply by the switching frequency of 11.25 KHz, and you get 9.2 mC per second, i.e. 9.2 mA average battery current to deliver about 3.3 mA @ 3.3V.

Gotta be a bit careful here. It turns out that the battery (which is a bit old), still delivers 1.44V at this lower power level. Also, since I’m using a 10 Ω current sense resistor in this case, there’s 92 mV wasted in that resistor. That leaves about 1.35V going in. A perfect switcher would draw 3.3V / 1.35V * 3.3mA = 8.07 mA. We’re pulling 9.2 mA, which is about 87% efficiency. That seems a bit optimistic, since the AS1323 doesn’t really go much further than 80%. Oh well, there are probably several measurement errors in my quick and dirty test setup.

For the third case with the 100 Ω load, I end up with a figure of 215 mC/s, i.e. an average current draw of 215 mA. Better than before but still under 50% efficiency.

One more datapoint: with a 100 kΩ load, the switching frequency goes to 120 Hz, while still using about 800 nC per cycle, i.e. ≈ 100 µC per second, or 100 µA. Again, pretty good for what is essentially a 33 µA load @ 3.3V – even if all these estimates are off by perhaps 25%.

So this chip might work quite well for bursty ultra-low power contexts such as a mostly-sleeping JeeNode!

  1. Clever chip – the synchronous rectification is keeping the efficiency good (and saving an external component). The heavy load current waveforms don’t look quite right – what is the rated saturation current for the inductor?

    (Minor typo last para – 120Khz)

  2. How does the chip behave if the input voltage is higher than 2V (datasheet specifies 0.75V – 2V) or even higher than the nominal output voltage? I cannot find this info in the DS.

  3. JBecker, since the chip is designed specifically for a single-cell input, it is a boost only topology so is unlikely to handle Vin>Vout. The Absolute Maximum Ratings (-0.3v +5v) are there to indicate do not exceed without damage numbers.

    Interestingly, it may well tolerate Vin closer to Vout. There are no diode drops involved – the energy transfer is almost all the time via resistive drops in the FET’s. A couple of Figures on the spec sheet show operating up to Vin = 2v. Perhaps AustriaMicrosystems has a technical helpline?

    • Martin, I like to always know the limits of the chips I use. The absolute maximum ratings are one thing to ‘absolutely’ stick to. If you cross these lines, you have to take damage into account. But sometimes even more interesting is the behaviour at other limits (or even over the whole allowed ranges). I have seen too much ‘strange’ behaviour of components not documented in datasheets.

      And if JC has the hardware still lying around, I thought he could perhaps do the tests :-))

  4. Oops, I stand corrected twice. The unit is kHz, (unlike MHz or GHz) but as JCW points out, is actually 120Hz for that observation.

    Hmm – note to self, read the screen scrapes more closely….

  5. @jcw, Did you stick to the recommended values for the inductor and caps ? They look very tiny for 10µF.

    • Yes, 10 µF and 10 µH. I’ll probably redo this measurement at some point, with different components.

  6. @jbecker, a valid question – perhaps it could handle the ~2v from a lead acid cell; still often the chemistry of choice for energy capture/deep discharge.

    • Lead acid cells can have up to 2.7V during charge (even 3V in certain cases), this was what I had in mind. I am at the moment working on a universal battery (cell) monitoring circuit for lead acid, NiXX and all sorts of lithium cells (although especially LiFePO4). But I do not need this low quiescent current there, natural choice would be a cheaper alternative than the AS1323. Input range should go as low as 0.8V during operation (for a drained Ni-cell). But this is going off-topic…

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