Electronic Design

Simple Circuit Additions Power A Microcontroller Through Its Load

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.

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