Design Ideas: February 15, 1996

The photovoltaic power-supply circuit in Figure 1 delivers 3.34V (±1.67V) at 5 to 20 mA to a low-power data-acquisition circuit. The total conversion efficiency is 40%; you can thus use a low-cost laser as the power source. The LM385Z shunt-regulates the photovoltaic converter at 1.67V output. This voltage is close to the optimum power-out value for the converter, which specs a 2V open-circuit output. The MAX665 voltage-inverter circuit provides about 95% efficiency vs the 83% you'd obtain from a boost switcher, whose high start-up currents could cause the circuit to hang up.

Moreover, the inverter doubler does not produce current spikes, and its ramp-shaped ripple is easy to filter. Low power-supply noise is important in powering a 12-bit ADC at 3.3V (1 LSB=0.8 mV). Note that pin 5 of the instrumentation amplifier connects to the 1/2V_CC point rather than to the negative rail. Otherwise, the amplifier would produce a 40-mV offset as the input voltage approaches the negative rail. The MC68HC705C8 CPU serially reads the ADC and transmits the data from its UART at 57 kbaud to the 1A229 fiber-coupled LED. The total current draw for the circuit, not including the sensor, is less than 5 mA at a 1.8-MHz CPU frequency. (DI #1823)