A typical digital camera lens system can be extremely clever and effective. Yet it's also mechanically complex, requiring intricate gears and motors. Size, cost, and power consumption are key considerations, especially in camera phones. A new system, though, may remove those limitations.
Rogers Corp. and Varioptic have collaborated to create a miniature high-resolution auto-focus system that uses a liquid lens (Fig. 1). It doesn't have any moving parts. Instead, it's based on the not-so-well-known phenomenon called electrowetting. The lens uses two isodensity liquids—one is an insulator, and the other is a conductor.
By applying a high dc voltage to the liquids, the curvature of the liquid-to-liquid interface changes, changing the focal length of the lens. The lens can focus with a range of 5 cm to infinity. Compared to a mechanical auto-focus, this system is smaller and saves both cost and power. And, the lens will never wear out.
To make the lens work, Rogers Corp. created a special driver IC with all the needed functions. Known as the Durel LL3 driver, this circuit takes its input from a battery with a 2.8- to 5.5-V range and boosts it up to a higher dc voltage that then is converted to ac (Fig. 2). The output is a 60-VRMS ac signal that can drive into a capacitive load of up to 500 pF.
The control input can come from an I2C serial interface or a pulse-width modulation (PWM) interface. A comparator continuously monitors the high voltage generated across the capacitor through a resistive divider and compares it to the I2C or PWM input. The comparator output drives some control logic, which excites an inductor to develop the desired high-voltage dc.
The dc is fed to an H-bridge that converts the dc into ac by way of the bridge-connected lens load. The system typically draws 40 mW while focusing at 5 cm and 20 mW when focusing at infinity. The standby current is a low 1 µA, and the system can be shut down completely to conserve battery power. The driver comes in a 4- by 4-mm, 16-pin leadless flat package.