Electronic Design

Charge-Pump VCO Increases Parts Count But Saves On Cost

This charge-pump voltagecontrolled oscillator (VCO) has a number of neat features: single positive power-supply operation, positive VCO reference voltage, fast response, high linearity, temperature compensation, and open collector output. Seldom are all of these features present simultaneously.

Though the component count is relatively high compared to commercially available devices, cost is low because all components are garden variety. Central to the design is the ubiquitous LM339 quad singlesupply comparator.

When operating, the input current reference through R1 first charges C1 to the 0.3-V threshold voltage of U1a (Fig. 1). When U1a switches, a small positive feedback through C5 results in a clean transition. When the output of U1a reaches 6 V, comparator U1b switches and quickly discharges (pumps) C2 into C1 via D1. This is the basic charge-pump circuit.

This sequence reduces the voltage across C2 to about –0.3 V (the limit of the common-mode input range of U1). When the two comparators reset, C2 recharges to the clamp voltage set by D6.

Temperature compensation is required because the forward drop of D1 and D2 are temperature-dependent. D4 and D5 provide the compensation. Two diodes are required: one to compensate for the charge (D2) and the other for the discharge (D1).

U1d prevents latchup in the event that the input node voltage ever exceeds 0.3 V and a single discharge of C2 is insufficient to reduce it below the threshold of U1a. Latchup may be demonstrated by teasing the power supply with a full reference voltage. U1d provides a convenient means of interface—either with or without a pull-up resistor.

The circuit is scaled so that 10-V input = 10-kHz output (Fig. 2). Linearity is well within 1% except for the discontinuity at the low end as shown on the graph (Fig. 3). R2 adds a little fixed bias to compensate for nonlinearity on the low end.

How about one of you IC gurus making this monolithic except for the supporting capacitors?

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