Regulated LCD-bias generator requires no inductor

March 9, 1998
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...

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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