Driven Parallel White LED

Many portable and battery-powered devices use white lcd graphic display as a backlight. Especially PDA color monitor requires a white background light, in order to restore the desired color reproduction that is very close to the original. Future 3G phone supports picture and video data, which also requires white backlight. Digital cameras, MP3 players and other video and audio devices also include the need for white backlit display. In most applications, a single white LED is not enough, need to drive several LED. Must adopt specific actions to ensure that their intensity and color consistency, even if the battery is discharged, or other conditions change.
Lithium battery is fully charged to provide output voltage of 4.2V, in a very short period of operation will drop to a nominal 3.5V. As the battery discharges, the output voltage will drop further to 3.0V. If the white LED directly from the battery shown in Figure 3, the following problems occur:
First, when the battery is fully charged, all diodes are lit, but will have different light intensity and color. When the battery voltage drops to its nominal voltage, the light intensity decreases, and becomes greater the differences in white. Therefore, designers must consider the battery voltage and diode forward voltage value, and need to calculate the resistor value. (With the battery fully discharged, part of the Low Temperature Display will be completely extinguished.) Charge pump with current control
An LED power supply is to provide a sufficiently high output voltage, and connected in parallel, the same current through the LED. Note that if all of the LED arranged in parallel with the same current, then the LED will all have the same color coordinates. Maxim offers a charge pump with current control in order to achieve this goal (MAX1912). Three in parallel as shown in Figure 8 of the LED, the charge pump having a larger range, the input voltage can be increased to 1.5 times. Early charge pumps simply the input voltage doubler, and the new technology provides better efficiency. The input voltage rises to drive the LED can work exactly level. Connected to the SET (10 pin) resistor network to ensure that all the LED current agreement. Internal circuitry maintains the SET level in 200mV, so you can calculate the current flowing through each LED ILED = 200mV/10 = 20mA. If some diodes require a lower current, can be paralleled to drive three or more LED, MAX1912 output current up to 60mA