As Led production costs fall, they're being used more frequently in applications ranging from handheld devices, to automotive, to architectural Lighting. Their high reliability (operational lifetimes of greater than 50,000 hours), good efficiency (175 Lumens/W), and nearly instantaneous response make them a very attractive light source. However, driving LEDs is not without its challenges.
A controlled brightness requires driving the LED with a constant current, which must be maintained regardless of input voltage. Quite often, LEDs have a dimming requirement. For instance, it may be desirable to dim a display or architectural lights. There are two ways to accomplish this: either vary the LED current, or use pulse width modulation (PWM). The least effective way is varying the current because the light output isn't completely linear with current and the LED color spectrum tends to shift at currents below full rating.
It's important to remember that human perception of brightness is exponential, requiring a large percentage change in current for full dimming. This has a profound impact on the circuit design as a 3% regulation error at full current can become a 30% or more error at 10% load due to circuit tolerances. Dimming current waveform through PWM is more accurate, although the response speed becomes an issue. In lighting and displays, it's desirable to PWM above 100 Hz so the human eye doesn't perceive flicker.
Figure 1 shows a very simple, low-cost, buck regulator driving a single LED that implements a very fast dimming feature. This is based on a MC33063 which has an internal switch, a current-limit comparator, an oscillator, and an internal reference. A disable function is provided by the pin that's usually used for voltage regulation. In this scenario, a voltage greater than 1.25 V disables the supply while a lower voltage enables it. With the circuit enabled, the controller operates in a current limit/hysteretic mode, because voltage feedback has been eliminated.
1. This MC33063 forms the heart of a low-cost led driver.
The oscillator generates a start pulse that causes the power switch to be turned on. This puts the input voltage across the current sense resistor, LED, and inductor. The current-limit comparator senses when the current reaches about 350 mA and turns off the power switch. The inductor voltage reverses and flies above the input voltage, causing the freewheeling diode to conduct. The inductor and LED current continue to circulate until the switch turns on at the next switching cycle. This circuit is very adaptable for a wide range of applications. Using a switching regulator with a voltage rating of 40 V and a current rating of 1.5 A is useful in handheld portable devices, white goods, and automotive applications where simplicity and low cost are desirable. The basic topology could be adapted for a wider range of applications although it may be a challenge to implement the hysteretic control and enable functions so simply.
2. As shown, the hysteretic current control provides rapid PWM response. In the images, the green represents the output current, while the blue represents the PWM signal. The top image shows 500 us/div, while the bottom image shows 10 us/div.