Electronic Design

Regulated LCD-bias generator requires no inductor

Astringent height limitation on pc boards employed in PDAs and palmtop computers requires using expensive, low-profile inductors in switchmode power supplies. As an alternative, however, you can replace certain switchmode circuits with one based on charge pump (see the figure). The example shown generates a regulated negative voltage suitable for biasing an LCD.

IC1 contains a regulated, inverting charge pump that produces output voltages as high as −4 VIN, in which the supply voltage VIN can range from 1.8 V to 5.5 V. The IC regulates VOUT via pulse-frequency modulation (PFM), with a maximum frequency of 450 kHz. The chip’s low quiescent current (30 mA) provides excellent light-load efficiency without sacrificing full-load capability.

Inserting an external, discrete charge pump (consisting of C3, C4, and the Schottky diodes) in the feedback path of IC1 produces an “inverter-quadrupler” circuit. Its regulated output level is set by the ratio of feedback resistors R1 and R2:

VOUT = −VIN (R1/R2)

Configured as shown, the circuit provides up to 15 mA at VOUT = −18 V, with 76% efficiency and 60 mV of output voltage ripple. Lower VOUT allows higher output currents: VOUT = −15 V yields 20 mA; while VOUT = −12 V yields 30 mA.

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