Basic knowledge and drivers of standard and white LEDs (2)

Many portable or battery-operated devices use white LEDs as backlights. In particular, PDA color displays require white background light to restore the desired color, and the restored color must be very close to the original. Future 3G mobile phones will support picture and video data, which will also require white backlighting. Digital cameras, MP3 players and other video and audio equipment also include displays that require white backlighting. In most applications, a single white LED is not enough and several LEDs need to be driven simultaneously. Specific operations must be employed to ensure that their intensity and color are consistent, even when the battery discharges or other conditions change. Figure 7 shows the current-voltage curves of a randomly selected set of white LEDs. Loading these LEDs with a 3.3V voltage (top dotted line) will produce a forward current ranging from 2mA to 5mA, resulting in white light of varying brightness. In this area (as shown in Figure 5), the Y coordinate changes drastically, which will cause the display color to be unrealistic. Likewise, LEDs also have different light intensities, which can produce uneven brightness. Another issue is the minimum supply voltage required. LEDs require a voltage higher than 3V to drive. If it is lower than this voltage, several LEDs may completely dim. Figure 7: The curve shows considerable differences between the current-voltage characteristics of different white LEDs, even randomly selected LEDs from the same product batch, so driving such several LEDs in parallel with a constant 3.3V will result in White light of varying brightness (upper dashed line). Lithium batteries can provide an output voltage of 4.2V when fully charged, which will drop to the nominal 3.5V within a short period of operating time. As the battery discharges, its output voltage will further drop to 3.0V. If the white LED is driven directly by the battery, as shown in Figure 3, the following problems will occur: First, when the battery is fully charged, all diodes will be lit, but with different light intensities and colors. As the cell voltage drops to its nominal voltage, the light intensity decreases and the difference between white lights becomes greater. Therefore, the designer must consider the values ​​of the battery voltage and diode forward voltage and need to calculate the value of the series resistor. (As the battery is completely discharged, some LEDs will go out completely.) The goal of a charge-pump LED power supply with current control is to provide a high enough output voltage and load the same current on the LEDs connected in parallel. Note (as shown in Figure 5) that if all LEDs in a parallel configuration have consistent current, then all LEDs will have the same color coordinates. Maxim offers a charge pump with current control to achieve this goal (MAX1912). For the three LEDs connected in parallel shown in Figure 8, the charge pump has a larger range and can increase the input voltage to 1.5 times. Early charge pumps could simply double the input voltage, while newer technologies offer better efficiency. Increase the input voltage to a level that is just right for driving the LED. The resistor network connected to SET (pin 10) ensures consistent current flow to all LEDs. The internal circuit keeps the SET level at 200mV, so that the current flowing through each LED can be calculated ILED=200mV/10=20mA. If some diodes require lower current, more than 3 LEDs can be driven in parallel at the same time, and the output current of MAX1912 can reach 60mA. Further applications and diagrams can be found in the MAX1912 data sheet. Figure 8: The IC includes a charge pump and current control internally. The charge pump provides sufficient driving voltage for the white LED, and the current control ensures uniform white light by loading the same current to each LED. Simple Current Control White LEDs can be easily driven if the system provides a level higher than the forward voltage of the diode. For example, digital cameras often include a +5V power supply. In that case, there would be no need for a boost function since the supply voltage is sufficient to drive the LED. For the circuit shown in Figure 8, a matching current source should be selected. For example, MAX1916 can drive three parallel LEDs at the same time (as shown in Figure 9). Figure 9: A single external resistor (RSET) sets the current value flowing through each LED. Simple brightness control (dimming function) can be achieved by loading a pulse width modulation signal on the IC's enable pin (EN). The operation is simple: the resistor RSET sets the current loaded into the connected LED. This method takes up very little PCB space. Apart from the IC (a small 6-pin SOT23 package) and a few bypass capacitors, only one external resistor is required. The IC has excellent current matching, with a difference of 0.3% between different LEDs. This structure provides the same color area, so each LED has consistent white light brightness. Dimming changes light intensity Some portable devices adjust the brightness of their light output based on ambient light conditions, and some devices reduce their light intensity through software after a short period of idle time. This requires the LED to have adjustable light intensity, and such adjustment should affect each forward current in the same way to avoid possible color coordinate shifts. Uniform brightness can be obtained by using a small digital-to-analog converter to control the current flowing through the RSET resistor. A 6-bit resolution converter, such as the MAX5362 with I2C interface or the MAX5365 with SPI interface, can provide 32 levels of brightness adjustment (as shown in Figure 10). Since the forward current affects the color coordinates, the LED white light will change as the light intensity changes. But this is not a problem because the same forward current will cause each diode in the group to emit the same light.