Abstract: Typical integrated circuit (IC) specifications are more important than minimum and maximum specs when optimizing battery life.
Many engineers wouldn't think of using "typical" IC specs
in power supply designs. But typical specs can actually be very useful,
sometimes critical, when trying to optimize battery life. By keeping a
clear view of design goals, and considering how data sheet limits are
set, typical specs offer guidance that is sometimes missing from minimum
and maximum limits.
IC production methods continually evolve. Those who've
been designing analog systems for a while have probably noticed that today,
fewer analog parts have A, B, and C "grades." Most power devices
have just one. Silicon has gotten cheaper as wafer diameters grow and
transistor dimensions shrink, but testing has remained relatively costly.
Relentless price pressure forces more focused testing, and less critical
parameters are given "by design" guarantees. The good news is
that this has not reduced product quality. Quite the contrary, today's
IC manufacturing flows generate far fewer "out of spec" parts
than in previous times when "binning" was often a crutch for loose
processes. Today there are larger safety margins between "on the bench"
typical performance and min/max specifications. In most cases it's hard
to argue that this margin is not a benefit, but when designing for battery
life, conservative min/max IC limits can be misleading.
Battery life has the most value as a "typical" parameter
(except in medical or other critical uses). Admittedly, operating times
quoted for consumer products can be described as "optimistic";
but a guaranteed limit serves no purpose if the quoted operating time
is only a fraction of the real performance. Adding up IC maximum operating
currents generates mostly useless battery life numbers. Typical parameters
provide more valid results, particularly data from typical operating curves
and graphs, as well as typical specs from the spec table.
The last important step is to understand the system's
power profile in each operating state (i.e., off, sleep, run, etc.), then
determine how much time is spent in each state. This is important because
it tells you where most of the energy is going, and where to spend power
supply money and design effort. If your product spends most of its time
off, but still biases low power circuitry in the off state, then money
spent on high power efficiency might be wasted. Instead concentrate on
low-load efficiency and low quiescent operating current to optimize the
performance where most battery energy is used. Conversely a product with
a real hardware ON/OFF switch, that disconnects the battery, should be
designed for peak efficiency at the typical operating load current, not
the peak. Of course you still need to consider the maximum loads to ensure
that the power supply can supply peak load when needed, but the typical
numbers will be the best guide to help place the efficiency "sweet
spot" where it is needed most.
A similar version of this article appeared in the April
15, 2002 issue of Planet Analog magazine.
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