Many small microcontrollers require
so little power that often they can draw
what they need through their loads. This
can simplify a system, reduce its cost,
increase its reliability, and provide unexpected
benefits.
One example is an automotive check
system that monitors a car's brake lights
and indicates any faults through an incandescent
bulb in the instrument cluster,
LMP1 (Fig. 1). Microcontroller U1 is
"fused" to run off its internal, 128-kHz
watchdog oscillator. Since U1 idles when it
has nothing to do, its power consumption
is less than 1 mW. With a few simple parts,
it can draw this power through LMP1.
U1 pulse-width modulates LMP1 to
light it and still draw power. LMP1 is an
extra-bright, 2-W bulb with one side tied
to 12 V. To light the bulb, U1 grounds the
other side through automotive low-side
driver U2, but only 75% of the time. This
duty cycle dims the bulb to match the
instrument cluster's other 1.2-W bulbs.
The other 25% of the time, while the
bulb is off, U1 draws power through the
simple, inexpensive shunt regulator,
through VCC and GND. RN1 pulls down
and idles U2 during power-up and reset.
C1 delivers power while LMP1 is on
and during brief power outages that
might otherwise reset U1. U1's brownout
level is fused at 1.8 V to tolerate outages
lasting hundreds of milliseconds.
Using such an arrangement eliminates
the cost, bulk, and weight of a separate
supply wire and increases reliability.
As long as the instrument cluster has
power and the bulb is intact, the microcontroller
should continue to function
and report faults.
The shunt regulator and PWM scheme
provide a couple more, perhaps less
obvious benefits. Zener diode D2 fixes
U1's supply at about 5.6 V and fully protects
it against the gamut of automotive
transients that can enter through U1's
supply-or through its inputs and ESD
diodes. Also, U1 counts 128-kHz oscillator
cycles to scan its inputs and pulse
LMP1 at a nominal 90 Hz.
The oscillator is fairly stable but is
uncalibrated. However, its frequency can
be determined to fix the number of cycles
for 90 Hz as U1 and U2 pulse LMP1. The
top trace of Figure 2 shows U2's activelow
output after determining and adjusting
the 90-Hz cycle count. The bottom
trace shows the supply ripple across C1.
D2's somewhat soft Zener "knee" causes
most of the decay while U2 is on.
Replacing the shunt regulator with a
low-power series regulator and, where
needed, adding a shunt resistor across
the load will allow U1 to consume more
power or draw power from a more sensitive
load, such as an LED. A less expenexpensive
MOSFET can replace R2 and U2 in a
less severe environment.
JOHN FIRESTONE, system
simulation engineer, holds
an AB geophysics (AB
applied mathematics) from
the University of Chicago,
Ill., and a PhD in geophysics
from the University of Washington,
Seattle.