Choosing a part requires understanding LED lighting systems to make the best tradeoffs
BY BRIAN HEDAYATI
Maxim Integrated Products
Sunnyvale, CA
http://maxim-ic.com/led
As high-brightness LEDs (HBLEDs) are becoming more efficient and their lumens/wWatt increases, they become more suitable for illumination applications such as automotive exterior lighting like daytime running lights, and low beams, and high beams, as well as general lighting applications. Leading LED manufacturers have recently introduced HBLEDs with over 80 lumens/W, making it possible to replace halogen lamps in light fixtures such as the MR16 spotlights. The migration from conventional halogen light sources to HBLEDs carries the promise of significant power savings and extended product life. According to the market research firm Strategies Unlimited, growth in the high-brightness LED market in the next 3 to 5 years will be driven by lighting, display backlighting, and automotive applications, and the overall market will reach $9 billion by 2011.
LED requires constant current
The higher lumens needed for lighting applications require the use of high-power HBLEDs. The forward current for these LEDs are in the range of 350 mA to over 1 A. The forward-voltage drop varies from 2.8 to 4.5 V for the white, blue, and green HBLEDs, and 2.3 to 3.5 V for red and amber HBLEDs.
To maintain a constant color spectrum and brightness, the HBLEDs must be driven at their specified current rating. Driving the HBLEDs with a voltage source and limiting the current with a series resistor will result in unacceptable brightness and emitted spectrum variations.
Linear drivers
The best approach is to drive the HBLEDs with a constant current source. The simplest circuit to create constant current source is to have a MOSFET in series with an HBLED, measure the HBLED current, compare it to a reference voltage, and feed it back through an operational amplifier to control the gate of the MOSFET. This type of circuit behaves like an ideal current source, which keeps the current constant irrespective of forward-voltage and supply-voltage variations. Linear HBLED driver ICs such as the MAX16806 (see Fig. 1 ), with integrated MOSFET and a high-accuracy internal reference, provide brightness consistency between each light fixture.
Fig. 1 A 350-mA linear LED driverIC such as the MAX16806 eliminates the need for a microcontroller or switch-mode converter.
The benefits of using a linear driver over a switch-mode driver are that linear drivers are simpler to implement and present no EMI concerns since they have no high-frequency switching. The total solution cost is low due to minimum external component count. The MAX16806 requires an input supply that is only 1 V above the total drop across the LED string. An external sense resistor measures the LED current, thus enabling the MAX16806 to keep the current constant while the input voltage or the LED forward-voltage change.
The power dissipation in a linear driver is equal to the LED current times the voltage drop across the internal (or external) series pass device. As the LED current or input voltage increases so does the power loss, thus limiting the use of linear drivers. To limit the power dissipation in the light fixture, MAX16806 measures the input voltage, and if the voltage exceeds its preprogrammed level it will reduce the LED current resulting in reduced power dissipation. This feature can eliminate the need for a switch-mode driver for applications such as automotive dome light or day timer running light (DRL) where the light can dim down in an abnormal high-battery-voltage condition.
Switch-mode step-down, buck driver
When the input voltage is higher than the total voltage across the LED string, it is best to use a switch-mode step-down, buck converter driver (see Fig. 2), which minimizes the power dissipation and maximizes the driver efficiency in a light fixture. Unlike general-purpose buck controllers most commonly used to power HBLEDs, the MAX16820 LED driver employs hysteretic control.There is no control-loop compensation, which simplifies the design and further minimizes component count.
Fig. 2. Using a switch-mode step-down, buck converter driver minimizes the power dissipation and maximizes the driver efficiency in a light fixture.
An integrated high-voltage current-sense amplifier and up-to-2-MHz switching frequency reduce space and component-count, making it ideal for light sources such as the energy-efficient LED based MR16 as well as front and rear automotive lighting (RCL, DRL, and fog/low-beam lights) applications.
Switch-mode buck boost or boost driver
When the input supply voltage varies above or below the total voltage across the LED string, a bBuck boost driver topology is used. If the input voltage is always lower than the total voltage across the LED string, then a boost converter is necessary.
In a buck boost configuration, a floating current-sense amplifier is required to measure and regulate the LED current. Additional protection circuits are also required such as overvoltage protection in case the LEDs fail open or when the LEDs are shorted. The buck boost circuit is ideal for driving high-power LEDs in automotive front-light applications where the input voltage can change from 5.5 V for cold crank to 24 V for double-battery conditions. The driver also needs to withstand over 40-V load dump spikes.
LED dimming
Reducing the LED current will reduce the LED brightness, but its emission spectrum will shift. This approach is not recommended for some applications. To maintain the emission spectrum over different brightness levels, it is best to keep the dc current at values specified by the LED manufacturers and chop the current at certain frequency and duty cycle.
The dimming frequency should be over 100 Hz to prevent visual flicker. The dimming range is limited by the LED driver’s minimum duty cycle capability. Most of the LED drivers require a dimming signal from a microcontroller or an external timer.
Selecting the appropriate HBLED driver requires LED lighting system understanding to make the best trade-offs. One needs to start with the basic electrical specification such as input voltage, LED current, LED forward voltage and their variations. The safety, EMI, thermal and mechanical specifications as well as available board space must also be considered. Linear drivers are used for low-cost, easy-to-implement, and low-EMI applications. Switch-mode drivers are used for high-power, high-efficiency, or wide-input operating-voltage range, but they increase the parts cost and may add EMI concerns.
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