Many of today’s portable consumer electronic products has a display screen, such as mobile phones, PDAs, and MP3 player, the type and size of the screen is usually determined according to the application, but the design engineer must be responsible for its design of power supply and backlight circuit. The white LED backlight program to select the most commonly used by the portable consumer electronics products.
The main objective is to drive white LED lights to produce the forward current through the device, which can be a constant voltage source or constant current source to achieve. However, this method has its shortcomings, the ballast resistor will limit the current through the nonlinear VI curve of the white LED steady flow capacity of this method is very poor; In addition, as long as the applied voltage or white LED forward voltage ( VF) any changes in the white LED current will change. Rated forward voltage of 3.6 V, a 20mA current through the white LED in Figure 1, if the temperature or process changes this voltage into 4.0V (still in the normal 3V to 4V tolerance range), the forward current will drop to 14mA; In other words, the forward voltage change of 11%, the forward current is 30% of the significant changes. Extreme changes of this white LED current will affect the display brightness, which is unacceptable in many applications.
Ideal for white LED down lights drive mode using a constant current source, it can avoid the white LED forward voltage change caused by the current change constant forward current can be controlled, can provide a constant display brightness can be controlled. Constant current source is very simple controller without the power output regulator, but as shown in Figure 2, the direct voltage across the resistor for current sensing regulator, the white LED current from the power to determine the value of the reference voltage supply and current sensing resistor value. Most monitors require more than one white LED designers the flexibility to drive multiple white LEDs, they should be all white LED in series, to ensure that each white LED current is the same. To promote white LEDs in parallel, each white LED must be a ballast resistor in series to avoid differences between the current through them, but these resistors will waste power and reduce the circuit efficiency.
Inductive drive and charge pump drive
IC designed to drive white LED provides a constant current, most of which is inductive or charge pump-based solutions, both solutions have their own advantages and disadvantages. The switched capacitor charge pump solutions, or solution, the use of discrete capacitors to the power from the input sent to the output side, the whole process without using any inductors, it is a popular solution. The small size of the charge pump power supply, the design is also very simple, select the components usually just pick and choose the appropriate capacitors according to component specifications. The charge pump solutions, the main drawback is that it can only provide limited output voltage range, the vast majority of the conversion ratio of the charge pump IC can only reach up to twice the input voltage, which means that the output voltage can not be higher than twice the input voltage If you want to use a charge pump to drive more than one white LED, you must use the parallel drive way. When only the output voltage regulated charge pump to drive multiple white LED ballast resistor must be used to prevent the current uneven distribution of these resistors will reduce battery life.
Inductive solutions for small size, high efficiency, providing longer battery life for the vast majority of consumer products. This article will prove that the designer can adjust the inductance converter efficiency in order to achieve an optimal balance between size and efficiency. As most of the inductive solutions are based on the boost converte, up to drive six or seven white LEDs in series, this practice has its merits, because many of the display’s built-in white LED are used series model; even if no built-in white LED grow lights display screen, most engineers will be connected in series. The backlight driver and the white LED is usually in a different circuit board must be power from a circuit board connected to a circuit board. Drive five parallel white LED totaling six pin connector, the drive to five white LEDs connected in series requires only two pins.