“It pays to take a system-level approach to powering LED backlights in small LCDs. Light-emitting diode (LED) technology is widely used to illuminate the pixels in small-sized liquid crystal displays (LCDs) in battery-powered applications. The white light emitted by the LED is transmitted through a polarizer to the LCD, where it can be blocked or attenuated, and sent to an RGB color filter to produce colored light.
It pays to take a system-level approach to powering LED backlights in small LCDs.
Light-emitting diode (LED) technology is widely used to illuminate the pixels in small-sized liquid crystal displays (LCDs) in battery-powered applications. The white light emitted by the LED is transmitted through a polarizer to the LCD, where it can be blocked or attenuated, and sent to an RGB color filter to produce colored light.
Figure 1: Backlight LED drive system. Figure 1 shows a system-level view of a backlight LED driver consisting of a DC/DC converter and one or more regulated current sources. Furthermore, RGB-LED-based backlights require temperature-based feedback control, which translates to higher costs than white LED-based backlights. How much PCB area can be used? What functionality is required? How much power does the system consume? Answering these questions can guide designers in choosing the right backlight LED driver.
DC/DC converter for LED backlight
In portable applications with a single-cell Li-Ion source, the sum of the voltage drop of the white, green or blue LED and current source can be lower or higher than the battery voltage. This means that while red LEDs can be powered directly from a single-cell Li-Ion battery, white, blue or green LEDs require battery voltage to sometimes be boosted.
The first aspect to consider when choosing an LED driver for battery-powered applications is the total area occupied by the IC driver and external components (Figure 2).
Figure 2: Example of a typical PCB layout: charge pump (left), Inductor boost (right).
Two boost techniques are widely used: boost DC/DC converters, also known as inductive boost, and switched capacitor converters, also known as charge pumps. The charge pump implementation requires only four ceramic capacitors and one low-power resistor, which often results in a smaller solution size. The recommended capacitor value for these applications is 0.47µF to 1µF, rated at 10V (to help with DC bias losses). These capacitors can be found in 0402 or 0603 case sizes from many capacitor manufacturers. Total solution sizes of less than 21 mm 2 are quite common and also have the advantage of being very thin, less than 1 mm. Depending on the LED driver package, the capacitor can be the tallest component in the solution. Compared to switched capacitor drivers, inductor boost based LED drivers tend to have a larger solution size. A typical solution size for an inductor boost based LED driver is close to 30mm 2 of board area. Inductive drivers typically require two capacitors, one at the input and one at the output, with values from 1µF to 2.2µF, and are available in 0603 and 0805 case sizes. Inductor boost requires a rectifier element that can handle the peak inductor current and output voltage. In synchronous boost, the pass-through PFET can be integrated into the IC. However, this integration often results in IC packages that exceed the size of asynchronous solutions. The power conversion efficiency is also reduced by about 10% in the presence of an integrated high voltage PFET or Schottky diode. In an asynchronous topology, the pass element consists of Schottky diodes. The main area increase for an inductive boost compared to a switched capacitor boost is the inductor itself. 15mA to 20mA applications with 6-8 LEDs typically require a 10µF to 22µH inductor with saturation current between 0.4A and 0.5A. These inductors can be found in a footprint smaller than 3.0mm x 3.0mm. The inductor is also the tallest component in the solution, with heights ranging from 0.8mm to 1.2mm.
The easiest way to boost the battery voltage is to use a step-up DC/DC converter (Figure 3). The advantage of this approach is very high efficiency at all load and input voltage conditions, as the input voltage can be boosted to the sum of the LED forward voltage and the current source headroom voltage. As mentioned earlier, this significantly optimizes cost and PCB area efficiency.
Figure 3 shows the working principle of the magnetic boost regulator. When the NFET switch is closed (solid arrow), the inductor current iL
At t = t1, the NFET switch is turned off and the energy stored in the inductor L is now transferred to the output capacitor and the load through the Schottky diode (dashed arrow). Therefore, the inductor current drops to the previous Ia value during time t2. The output voltage must be greater than the input: if this voltage relationship is incorrect, the inductor will not discharge into the output network. In other words, when the NFET is turned off, the voltage across the inductor reverses because the current discharge does not happen immediately. The input voltage increased by the reverse magnetic voltage causes the output voltage to be higher than the input voltage. When driving 10 LEDs in series, the required supply voltage can be as high as 35V. Another advantage of the boost topology is the simplified PCB routing: only two connections are required between the driver and the LED string. The second way to boost the battery voltage is to use a charge pump (a simple implementation of which is shown in Figure 4), which takes advantage of the following properties of capacitors: capacitor charge accumulation does not occur instantaneously, which means that the initial voltage change across the capacitor is equal to zero .
Figure 3: LM3509, an inductive boost LED driver.
The voltage conversion is done in two stages. During the first phase, switches S1, S2 and S3 are closed, while switches S4-S8 are open. So C1 and C2 are stacked, assuming C1 equals C2, charged to half the input voltage:
The output load current is provided by the output capacitor. When the capacitor discharges and the output voltage falls below the desired output voltage, the second phase is activated to raise the output voltage above this value. During the second phase, C1 and C2 are connected in parallel between VIN and VOUT. Switches S4-S7 are closed, while switches S1-S3 and S8 are open. Since the voltage drop across the capacitor does not change instantaneously, the output voltage jumps to 1.5 times the value of the input voltage:
Figure 4: Charging a pump circuit with 1x and 1.5x gains In this way, the boost operation is accomplished. The duty cycle of the switching signal is typically 50%, as this value usually yields the best charge transfer efficiency.
Voltage conversion with a gain of 1 can be achieved by closing switch S8 and opening switches S1-S7. The benefit of the charge pump approach is that there are no inductors. Inductors are EMI noise and can affect radio performance in displays or cell phones.
Input Power and LED Efficiency in Charge Pumps
In a charge pump LED driver, the output power relationship is used for efficiency calculations, assuming that all LEDs are the same, given by:
Figure 5 shows a typical efficiency plot with steps indicating gain transitions.
However, for a given LED current, the forward voltage can vary with process and temperature. This means that the efficiency of the LED can vary and still keep the brightness constant, since the latter only depends on the current. For clarity, let’s consider an adaptive charge pump based LED driver circuit. The following specifications:
Figure 5: Charge pump LED efficiency.
It will not affect the power consumption of the battery, but it will affect the power consumption of the drive circuit. Therefore, efficiency is not sufficient to evaluate power consumption: what must be considered is the input power versus the LED brightness, i.e. the LED current. For a given LED brightness, input power is a true measure of how many electrons are drained from the battery.
Under the previous conditions, the gain is 1.5 times, and the input power is equal to 333mW regardless of VLED.
Since charge pump converters have a limited amount of voltage gain, there is always a certain amount of wasted power in the driver circuit based on application specifications. Therefore, in order to minimize the input power, it is very important to operate the charge pump with as little gain as possible.
Constant Current LED Driver
LED characteristics determine the forward voltage current required to reach the desired level, which determines the amount of light emitted. Controlling only the voltage across the LED will result in a change in light output due to changes in the LED voltage versus current characteristics. Therefore, most LED drivers use current regulation.
Figure 6: Regulated current source.
The circuit that implements current regulation is a low-dropout regulator, as shown in Figure 6. The error amplifier takes the voltage across R2, V2, compares it to the reference voltage VREF, and adjusts the LED current IDX through the series pass element NFET to the value required to drive the error signal (VERR = VREF-V2) as close to zero as possible. VREF is equal to:
This is true only if VOUT-VLED is high enough to keep the pass element from saturating. In fact, current sources need a minimum voltage across them, called the headroom voltage VHR, in order to provide the required regulation current through the LED. Headroom voltage is usually modeled with resistance:
Brightness can be controlled directly by changing the LED current (analog control) or indirectly by quickly turning off the LED to create the perception of dimming by the human eye (PWM control). In most portable applications, analog brightness control is preferred, as the backlight controller is usually remote from the LED driver. Therefore, PCB traces with PWM signals must be placed close to noise-sensitive systems (such as radio transmitters, speakers, or displays), which can cause problems. Finally, in applications that require a premium color gamut, LEDs are used for red, green, and blue. The red LED is made of InGaAlP, while the blue and green are made of InGaN. When the ambient temperature changes, the dominant wavelength of red changes significantly compared to blue and green, so some kind of temperature compensated feedback loop is required. The LP5520 (Figure 7) adjusts the RGB LED current for perfect white balance (color accuracy ΔX and ΔY
Figure 7: LP5520, backlight RGB LED driver.
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