APPLICATION NOTE 239

Regulated LCD-Bias Generator Requires No Inductor


Abstract: This design idea shows how a regulated LCD bias generator requires no inductor. A charge-pump design based on the MAX868 regulated, inverting charge pump replaces a switch-mode design to generate a negative voltage for biasing an LCD. A few components in the feedback path of a negative, inverting charge pump produce an inverter-quadrupler circuit.

A similar idea appeared in the March 9, 1998 issue of Electronic Design.

A stringent height limitation on the PC boards for personal digital assistants (PDAs) and palmtop computers compels the use of expensive, low-profile inductors in switch-mode power supplies. As an alternative, however, certain switch-mode circuits can be replaced with one based on a charge pump (Figure 1). This example generates a regulated negative voltage suitable for biasing an LCD.

Figure 1. Adding a few inexpensive components in the feedback path of IC1 enables the generation of regulated output voltages nearly as high as -4VIN.
Figure 1. Adding a few inexpensive components in the feedback path of IC1 enables the generation of regulated output voltages nearly as high as -4VIN.

IC1 contains a regulated, inverting charge pump that produces output voltages as high as -2VIN, in which the supply voltage (VIN) can range from +1.8V to +5.5V. The IC regulates VOUT through pulse-frequency modulation (PFM), with a maximum frequency of 450kHz. The IC's low quiescent current (30µA) 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 whose 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 15mA at VOUT = -18V, with 76% efficiency and 60mV of output voltage ripple. Lower VOUT allows higher output currents: VOUT = -15V yields 20mA, and VOUT = -12V yields 30mA.