Better characterization of suspend current
andy at openmoko.com
Fri Feb 8 10:25:04 CET 2008
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Hi folks -
I was lying awake in bed at 3.30am musing about the suspend current.
The smart battery says on it that it is "3.7V 1250mAh", so naturally I
divided 1250 / 5 and I say to myself "wah! Does it really mean we have
a 250hr standby time now? Didn't I see that kind of number on adverts
for mobile phones? 10 days? Cool!" Then doubts and suspicion crowded
around me in the dark.
- This "5mA" figure never got tied to any specific configuration I saw,
it just got reported as a single figure. That needs specifying so the
tests are repeatable and can be understood in context. For example,
what are we measuring here? It is Vb at the battery terminal, right?
- The GSM modem seems to be "off" according to Allen's last attack on
the current monster. Is it off? What happens to suspend current when
it is left on so in can wake on incoming call? Because that will be a
very common scenario indeed.
- People avoided using batteries and use bench PSUs last I saw (hence
nobody told me about charging completely broken). What is the situation
there for this measurement? It matters because -->
- What we really fight is not the dreaded current monster but the
shapeshifting sister monster (imagine the current monster in a Japanese
schoolgirl outfit) the "Power Monster". When I see people with bench
PSUs they stuck them on 4.0V with insanely low impendence you expect
from a bench PSU, so "5mA" in that context is 20mW. But for a start the
battery voltage is lower, 3.7V nominal so we would see 5.4mA, right?
Well what about the efficiency of the various LDOs in the PMU across
input voltage? Maybe the overall power changes. We need to measure Vb
current over a few voltages that can be seen from a real battery and get
an idea if overall power changes -- maybe it reduces as the battery
tends towards 3.3V and LDO dropout goes down.
- What we really need to be able to compute is a fairly reliable
estimate of standby time with GSM modem "on for incoming call detect" to
be arrived at from these measurements, and we need to check that
lifetime on a real phone actually left in standby until it flakes out.
If we can only manage 2 days or 3 days, well, it's not great but we have
a reliable figure we can say to people and we can estimate any
improvements we manage.
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