Standalone regulator controls high-brightness power LEDs

The constant-current MAX16819/MAAX16820 drivers control HBLEDs (high brightness power light emitting diodes). MAX16820 circuit yields a stand-alone brightness regulator, which requires no microprocessor or microcontroller. It increases LED brightness when the ambient-to-LED ratio rises, and reduces brightness when that ratio declines. The circuit operates with a supply voltage of 6 V (four 1.5-V AA cells in series), and consists of three main sections, including ambient light sensor Q1, LED driver, Analog-to-PWM signal converter that converts the measured analog signal to a PWM signal for use in regulating the LED driver. Because the constant-current driver (MAX16820) includes a step-down converter, the efficiency is high. Its operating voltage range is 4.5 to 28 V, and it delivers up to 25 W of output power. The Analog/PWM unit includes a dual ultra-low-power comparator (MAX972), and a dual op amp with rail-to-rail input/output characteristics (MAX4092).