White LED Driver Must have Conditions

In recent years portable electronic equipment mostly switched to the forward voltage is 6V (20mA), the white LED as a backlight of the LCD, such as a mobile phone takes about 3 ~ 4 white LEDs general lithium ion battery, however, the discharge voltage is often less than 3. 6V, therefore must use in order to comply with the white LED forward voltage 3 6V demand, boost circuit in order to obtain the desired effect, means that the white LED driver must have to meet: even if the battery output voltage required for normal forward voltage UF-down, the drive can also be provided for the white LED.

Poor white LED's forward voltage drop match early white LED driver should match the current limiting resistor, to compensate for the forward voltage differences, in order to ensure white LED brightness consistency. However, the newly developed white LED driver IC supply voltage can not maintain a constant brightness. Upgrade and adjust the battery voltage driver IC can be open or closed loop controller, charge pump or inductive converter with voltage or current output.

In most applications, the white LED is in parallel or in series together. However, in individual cases, it also can be mixed in series and parallel configurations. LED connection in series or in parallel or hybrid depending on the needs of the application, each configuration has its own advantages and shortcomings.
1. The series. The white LED has a the similar diode forward voltage and current characteristics. Since the white LED brightness almost entirely by the current control, therefore, just use the same or match the current, two white LED light can be obtained the same brightness, without having to consider the difference of the forward voltage. White LED connected in series to ensure that its current is the same, therefore all white LED brightness is consistent.

2. The parallel. Most parallel white LED configurations use a constant voltage or constant current drive, and the drive is used depends on the specific application requirements. Parallel white LED driver in a constant voltage topology is very simple, the drive can be open-loop voltage output switching DC / DC regulator converter. Each white LED series with a current limiting resistor to set the current value of the match between the current white LED. Constant voltage drive with high current-limiting resistor value, you can get a good current matching, but the negative impact efficiency. Conversely, the lower the current-limiting resistor value will provide a higher efficiency, but the matching effect of the white LED current (brightness) is poor.

The first generation of constant current drive with a constant current charge pump, these IC to detect white LED current limiting resistor. They may utilize the sensed current to adjust the first white LED forward current, its Yu Baiguang LED current limiting resistor, the current value will be matched with the first white LED current. Since the forward voltage between the white LED is not identical and therefore the accuracy of the current match is very limited. Applications requiring accurate current matching color LCD display, you can use the internal drive with constant current function, these drives provide accurate current matching between the white LED, without regard to its forward voltage.

White LED forward voltage is relatively high, the latest technology trend is to white LED forward voltage is decreased to 3V or less. Its nominal voltage value of 3. 6V, white LED forward voltage is lower than 3V 2. 7 ~ 4. 2V, by a single lithium-ion battery voltage range can be lithium-ion battery directly drive a white LED circuit ( without boost), thereby reducing the number of parts and cost.

Unlike traditional white LED driver IC, the white LED driver using active matching techniques in order to achieve a constant brightness white LED with a built-in digital, analog and PWM control unit to adjust the white LED brightness. The Fairchild the latest FAN5608LED of the drive to integrate the brightness control mode (digital, analog, PWM), and supports a special configuration program. It uses two constant current source drives two white LED series branch, each SMD LED series branch has a separate brightness control, 4 white LEDs in series for each white LED series support on the road, so as to meet the driving eight white LED's requirements.