I compared the Anelok board that had the boost converter rework
with the one that didn't. I used a fairly elaborate setup for this,
with lab power supply, meter, scope, another meter, and all this
lovingly interwoven by a ragtag collection of all sorts of cables.
We'll get to the problems this setup caused later. First the results:
Voltage Board behaviour
(V) original reworked
------- --------------- ---------------
3.0 ok ok
...
2.6 ok ok
2.5 display errors ok
2.4 turned off ok
...
1.9 - ok
1.8 - unstable
The operation I tried was to have Anelok sit at the setup menu, and
then I scrolled up and down.
"Unstable" here means that the display brightness oscillated and
that the board reset / hung when scrolling.
What's good is that the rework greatly improved the minimum voltage
at which the board would work, i.e., 1.9 V instead of 2.6 V.
However, 1.9 V is still pretty high and when looking at my voltage
measurements, I saw that what really arrived at the end of the
instrument chain was at least 100 mV below what the power supply
provided. This means that I had a fairly large resistance in the
path.
I then simplified the setup and only proceeded with the modified
board. This time I extended the test cycle to power-on ("start")
and entering the setup menu ("setup").
Voltage (V) "Battery" current (mA) Behaviour
Start Setup
--------------- --------- --------- ----------------
3.0 6.8-17.5 10.0-31.5 ok
2.9 6.9-17.9 10.2-32.6 ok
...
2.5 9.2-21.5 12.6-39.0 ok
...
2.1 11.8-26.4 15.5-50.2 ok
2.0 didn't start
The MCU should run with as little as 1.71 V and will reset itself
at 1.6 +/- 0.06 V. So 2.0 V should be sufficient. However, the
whole start process also involves starting the boost converter
and this produces a brief current surge that trips the system.
But let's worry about this later. First, I wanted to determine
how low I can go. The boost regulator should be able to turn on
with as little as 1.0 V and, once on, keep on running even with
0.65 V.
So I raised the voltage, brought the device up again, and reduced
the cycle to going between "standby" (OLED dark) and the setup
menu.
Voltage (V) "Battery" current (mA) Behaviour
Setup
--------------- ---------------------- ----------------
2.0 16.9-51.2 runs fine
1.9 18.2-57.0 ok
...
1.7 21.3-68.7 ok
1.6 23.1-80.0 faded and turned off when scrolling
I gave it a little rest and tried again.
1.6 22.5-69.1 runs fine
1.5 25.1-78.6 ok
1.4 58.8-104.0 display dims and flickers
1.3 turns off
So this is a bit better than the first run, with stable operation
all the way down to 1.7 V and maybe even as low as 1.5 V.
Now, the battery current is quite high. This is in part caused by
the firmware not making much of an effort to be power-efficient,
but it also means that any situation where power demands are
high could be a problem, especially when also adding memory card
and RF.
My plan is now to make the firmware pay more attention to not
burning a lot of power then idling, and especially to try not to
have the MCU run when turning on anything especially power-hungry.
After that, the boost converter and surrounding circuit deserve
a closer look. Those stability problems are happening at much higher
voltages than I expected. Perhaps some capacitors need a bit of
adjusting.
- Werner
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