EDN Access-- 08.03.95 Regulator generates as many as four voltages

-August 03, 1995

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Design Ideas:August 3, 1995

Regulator generates as many as four voltages

D Hayden,
Hayden Electronics Design, San Diego, CA

The inexpensive switching regulator in Fig 1 provides as many as four voltages using a single quad comparator. The circuit can implement two positive regulated supplies and two semiregulated negative supplies that use a charge-pump technique. Fig 1's circuit, which costs $4.48 in moderate volumes, shows three of the four possible output voltages, ±9V and 150V. You can add a 150V supply using the same charge-pump techniques as the 9V supply. Output current of the +9V supply is nominally 10 mA. The 9V can typically provide 5 to 10 mA and tracks the positive supply well. The 150V supply can output around 200 µA. Four AA batteries provide a supply voltage from 4 to 6V. An RC time constant on power-up delays the comparator's supply voltage to prevent the switching transistors from turning on before the reference voltage is stable. An external clock source of 31.25 kHz swings between circuit ground and the battery level.

For the positive 9V supply in Fig 1, comparator IC1D senses the 9V level and compares it with the 1.2V reference. When the 9V output is low, the comparator output goes to ground. The 1N4148 diode holds the inverting diode input to IC1C to 0.7V, and the noninverting input is the clock signal divided by 2. (This signal swings from one-half of the battery voltage to ground.) IC1C's output, which R1 pulls up to the battery voltage, then switches at the clock frequency. Current switches through the inductor to charge up C1 until the 9V output rises enough to set the output of IC1D high. With IC1D high and equal to the battery voltage, the circuit blocks the divided clock signal from IC1C, leaving IC1C's output low. You must choose the size of the inductor so that it doesn't saturate with the maximum drive on-time and maximum battery voltage.

The waveform at Q1's drain is a square wave that swings from 9.7V to ground. To generate the 9V output, the circuit couples this signal through C2 and clamps it to ground. The waveform at the junction of the two 1N4935 diodes is a square wave swinging from +0.7 to 9.0V. C3 charges during the negative swings to about 8.3V.

The positive 150V supply works in the same way as the 9V supply but with different components to provide the higher voltage output. The circuit's measured efficiency is between 75 and 80% over the battery voltage range. You can increase the circuit's efficiency if you can tolerate higher component costs. Also, if you need only two output voltages, you can use a dual comparator, such as the LM393. (DI#1742)

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