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Anelok: boost converter (2/3)

Werner Almesberger werner at almesberger.net
Mon Apr 10 14:51:10 UTC 2017

For measuring the performance of the boost converter, I set up a lab
power supply as "battery" and connected some instruments to it. The
parameters are:

- Vsrc: nominal voltage set at the power supply
- Vbat: actual voltage measured on the PCB (*)
- Ibat: measured battery current
- 3V3: voltage measured on the 3V3 rail (*)
- EN: voltage measured on the enable signal of the boost converter.
  This is generated by a power-on-reset chip that is also new, and
  that should solve the problem with handovers between battery and
  USB. (*)
- FB: measured feedback voltage. This is the output voltage of the
  boost converter, divided such that the result is exactly 0.5 V
  when the output voltage is at the desired level. (*)
- FBnom: nominal feedback voltage given the measured 3V3 voltage,
  using nominal values for the voltage divider. I.e., this tells us
  what we should see when measuring FB.

(*) Measured with an UT60G. My much more accurate Fluke 8845A was
    used to measure the current.

The first round of measurements was with the board #2, which has no
RF chip yet:

Vsrc	Vbat	Ibat	3V3	EN	FB	FBnom
(V)	(V)	(mA)	(V)	(V)	(V)	(V)
-------	-------	-------	-------	-------	-------	-------
1.550	1.51	22.04	3.23	1.40	0.64	0.50
1.500	1.46	22.92	3.24	1.34	0.61	0.50
1.400	1.34	25.00	3.23	1.24	-	0.50
1.300	1.24	27.47	3.23	1.14	-	0.50	
1.200	1.13	30.82	3.23	1.04	-	0.50
1.100	1.00	37.9	3.26	0.92	-	0.50
1.000	0.89	40.8	3.26	0.93 ?	-	0.50	system unstable

Since the MCUs are not yet programmed, they just run in reset loops.
This way, they consume a few mA, which is enough for this first round
of tests.

With the components used in this prototype, the "3V3" voltage should
be 3.233 V. We can see that the regulator does its job well, and the
system becomes unstable only when the real battery voltage reaches
0.9 V.

The regulator is specified for a minimum battery voltage of 0.9 V, so
I would have expected a little bit of margin. However, given that the
power supply is not a real battery and connected through about 1 m of
cables, there may very well be regulation problems that would not
occur with a real battery. So, no worry there this far.

But what is going on with FB ? We should see exactly 0.5 V there, not
something 28% higher. More confusingly, the 3V3 voltage is exactly
what we should get if FB is indeed 0.5 V. So what's going on ?

The answer is simple: if your circuit works but your measurements
don't make sense, suspect the measurements. When looking at FB with
an oscilloscope, I saw a fair amount of noise. It turns out that the
UT60G seems to favour the peaks of that noise, thus always reading
too high.

I then proceeded with board #1. I first got completely nonsensical
results, like this:

Vsrc	Vbat	Ibat	3V3	EN	FB	FBnom
(V)	(V)	(mA)	(V)	(V)	(V)	(V)
-------	-------	-------	-------	-------	-------	-------
1.550	1.52	15.92	1.64	1.33	0.52	0.25	voltage unstable

... but that was caused by the resistors in the voltage divider for
FB having incorrect values. I guess I must have picked the wrong

After fixing this, plus a number of small soldering issues, I got:

Vsrc	Vbat	Ibat	3V3	EN	FB	FBnom
(V)	(V)	(mA)	(V)	(V)	(V)	(V)
-------	-------	-------	-------	-------	-------	-------
1.50	1.45	21.39	3.10	1.38	0.53	0.48
1.40	1.34	23.4	3.12	1.29	0.53	0.48
1.30	1.24	25.9	3,13	1.20	0.53	0.48
1.20	1.12	29.1	3.14	1.10	0.53	0.49
1.10	1.00	34.4	3.16	1.00	0.53	0.49
1.00	0.89	39.5	3.19	0.91 ?	-	0.49	system unstable

This is a lot more reasonable. 3V3 is a bit on the low side, but
it's still well within the tolerance of the resistors I used. The now
a bit more correct-looking FB voltage also suggests that this board
has less noise.

The voltage at EN has to be at least 80% of the battery voltage in
order to turn on the boost converter. This is always the case, with
plenty of margin.

These are good results so far. The boost converter works as expected,
at least at moderate loads. The noise needs more looking into, since
it wouldn't be so great to have 3V3 bounce around by +/- 10%.

Coming next: yet another fixture variant.

- Werner

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