Voltage regulator controls scanner-lamp brightness

Chester Simpson, National Semiconductor, Santa Clara, CA -- EDN, 5/13/1999

An image-scanning device performs best when the scanner IC controls the brightness of the image reflected into the light sensors. The best way to control brightness is by adjusting the intensity, or light output, of the fluorescent lamp that shines light onto the scanned item. Traditionally, adjustable brightness has been economically unfeasible in most scanners because it is costly to implement a high-current voltage source that is also software-programmable. However, you can use a simple, low-cost circuit to implement full-range lamp-brightness control (Figure 1).

The fluorescent lamps in most low-cost scanners operate from a fixed 12V source, which operates the lamp at maximum brightness. If you reduce the source to a lower voltage, you can adjust the brightness of the lamp to virtually any level. You can accomplish the voltage change using a circuit that takes in 12V and produces a regulated dc output voltage that varies from 0 to 12V, depending on the lamp-brightness control signal coming from IC₁'s scanner IC. IC₁ sets the duty cycle of the control signal, which is a 5V pulse train.

The lamp-brightness control regulator takes in 12V and regulates this input to any voltage from 0 to 12V (Figure 2). Because the on-resistance of the NDP6020P FET is only approximately 50 mV, the maximum regulated output voltage the circuit provides is within approximately 25 mV of the input voltage at the typical lamp current of 0.5A.

The circuit generates a regulated output using error-amplifier IC_1B, the R₁/R₂ resistive divider, and an adjustable reference voltage at Pin 6 of IC_1B. The circuit produces this reference voltage by averaging the input control signal's square-wave pulse train. R₃, C₃, R₄, and C₄ filter the square waves into a dc voltage. The voltage across C₄ is the average value of the pulse train, which is directly proportional to the duty cycle:

By adjusting the duty cycle of the 5V pulse train, you can linearly vary the reference voltage at Pin 6 of IC_1B from 0 to 5V. You can program a maximum of 4095 duty-cycle values for the LM9830 scanner IC, which yields 4095 brightness levels.

IC_1B compares the voltage across C₄ with the voltage at the center of R₁ and R₂, which the circuit derives from V_OUT. The regulating action of IC_1B constantly adjusts the gate-drive voltage to Q₁, forcing V_OUT to a value that keeps equal the voltages at the inputs of IC_1B. In this way, the voltage at C₄, which is proportional to the duty cycle of the pulse train, controls the regulated voltage, V_OUT. You can calculate V_OUT using the following equation:

where V_PK is the peak amplitude of the pulse train (5V in this application) and duty cycle is the pulse on-time divided by the total period.

R₅, R₆, C₁, C₂, and C₅ are necessary for compensation and stability. For the component values in the figure, the best performance occurs when the frequency of the pulse train is 10 to 50 kHz. (DI #2356).