TECHNICAL

White LED Backlights

Operation and Construction

The recent commercialization of high intensity white LEDs allows them to be used for backlighting LCDs. The tiny LEDs are capable of delivering ample white light without the fragility problems, complex drive circuitry and costs associated with CCFL backlights.

The construction of a white LED backlight is very similar to the CCFL backlight. Replacing the CCFL tube as the light source is a single row of white LED chips that are assembled on a thin PCB. The white LED strip is positioned against the side of a polycarbonate waveguide into which the light is injected. The waveguide and optical films are similar to the materials used on a CCFL backlight design.

The LED chips are electrically connected on the PCB in serial, parallel or a series-parallel arrangement. This connection arrangement affects the drive method as discussed below.

Drive Method

White LEDs are semiconductors and have unique characteristics when compared to other lighting sources. The most notable characteristic is the nonlinear relationship between current and light intensity. The second most notable characteristic is the forward-voltage drop associated with an LED. Unlike an incandescent bulb, an LED is not a purely resistive load. The magnitude of the forward voltage varies with the color of the LED.

White LEDs have a relatively high forward voltage, typically 3.3 V; typical forward current is 20mA per LED. Providing a consistent voltage and current to these LEDs in a portable devices presents a challenge, as the power supply needs to adapt to the decreasing battery voltage. Otherwise the light intensity will vary with the battery voltage.

The simplest method for driving a white LED backlight is with a current-limiting resistor in series with the supply voltage. However, this method is not recommended since it is very inefficient and results in reduced backlight lifetime.

Two common topoloigies are used to maintain both a constant current and a constant voltage for white LED backlights: inductive boost regulator with the LEDs in a serial configuration and capacitive charge pump with the LEDs in a parallel configuration. There are advantages to both of these drive methods, but only one of them will provide the greatest advantage in a given application.

Boost Regulator

An inductive boost regulator uses the current storage capability of an inductor. With an inductive boost converter, the LEDs can be either serially driven or parallel driven.

The serial array assures that the current through all LEDs is identical so that the same intensity is assured. With a serial array, the output voltage of the driver must equal or exceed the summation of the forward voltages of all the LEDs. In some applications that can be as much as 24 V. That higher voltage requires the use of a Si process with a breakdown voltage in excess of 24 V, which typically impacts the cost of the device. In addition, the efficiency of a boost converter suffers as the output voltage increases.

Although the converter doesn't need to boost the voltage very high (for example, to 3.3 V) to drive a parallel array, a parallel topology requires current regulation for each LED. Since the intensity of an LED varies with current, the current in all of the LEDs needs to be matched in order to have consistent intensity from each LED. This adds complexity and cost to the system, but the advantage of the parallel topology is the efficiency.

Charge Pump 

A capacitive charge pump uses capacitors to store energy and boost the input voltage. Using an array of switches and a clock, the capacitors are alternatively charged in parallel and discharged serially to produce a boost in the output voltage. The maximum output voltage of this regulator depends on the number of capacitors and the time allotted for charging and discharging.

Charge pumps are primarily restricted to driving a parallel array because the output voltage is dependent on the number of charge capacitors used. There are some benefits associated with charge pumps. Typically they require less board space because the capacitors can be as small as a 1 x 0.8-mm package size. This can be a compelling feature, especially when the end product is a portable.

The added benefit of charge pumps is the lower EMI generated. Even with a shielded inductor, the inductive boost regulators generate more EMI noise than the typical charge pump.

Dimming

White LED backlights can be analog dimmed, but this scheme does not provide the high dimming ratio required by the more demanding applications. LED backlights are best dimmed using pulse width modulation (PWM) techniques. In PWM dimming, the LED backlight is pulsed on and off at a fiexd frequency and the duty cycle is modulated to provide variable brightness.

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