Used in industrial, architectural, and automotive applications, as well as for warning lamps and emergency lights, high-brightness power LEDs (HBLEDs) are becoming more and more popular, due in part to their relatively high efficiency and long life. However, to achieve optimal lighting when ambient light is not constant, the driver circuit should be capable of varying the LED brightness automatically.
Fig. 1 shows a typical operating circuit for the MAX16820. A modification to this circuit yields a stand-alone brightness regulator (Fig. 2), 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:
- Ambient light sensor Q1 (Osram SFH3410) for visible-light wavelengths in the 350- to 970-nm range
- LED driver (Maxim MAX16820) with PWM dimming input
- Analog-to-PWM signal converter that converts the measured analog signal (proportional to light) 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 (typically 90%). Its operating voltage range is 4.5 to 28 V, and it delivers up to 25 W of output power. This output is proportional to the light intensity, which can be adjusted via the PWM input (DIM).
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). The first op amp (U1A) operates as a non-inverting Schmitt trigger, and the second (U1B) as an integrator.
When the first op amp asserts a high level, the integrator capacitor charges with a constant current. Because the output of the second op amp (U1B) connects to the input of the first op amp, the output of U1B decreases linearly.
That output voltage switches to low after dropping below the input-voltage threshold of the first op amp, thereby discharging the capacitor and causing the output voltage of the second op amp to increase. By constantly charging and discharging, the capacitor produces a triangular waveform at the output of U1B.
The measured voltage, VPhoto, (proportional to light) is compared with the triangular waveform in the first comparator (U2A). This causes that output to be high when VPhoto is greater than the triangular waveform, and low otherwise.
The result is a PWM signal that regulates LED brightness via the LED driver's Dim input, making LED brightness proportional to the brightness of the environment. To achieve the inverse regulation, you can insert an inverter in front of the PWM converter circuit.
|Offset adjustment||R1 and R2 = 10 kΩ|
|Hysteresis adjustment||R3 = 200 kΩ, R4 = 180 kΩ, R6 = 10 kΩ, and R7 = 120 kΩ (R3 > R4, and R7 > R6)|
|Adjustment of maximum light intensity||RSENSE = 100 mΩ|
|Adjustment of photovoltage (VPHOTO)||RPHOTO = 18 kΩ|
|Light sensor, Q1||Osram SFH3410|
|Power LED||Lumileds LXHL-MWEC|
|PWM switching frequency (f)||Proportional to R3/(3R4×R5×C)|
|Duty cycle (D)||(VPHOTO - UL)/(UH - UL)|