Small size, high light output and flash or continuous video recording lighting output power LED lights have been developed specifically for mobile phone camera, light source, but have been overwhelmed by make battery a heavier burden. This paper discusses the current mobile phone camera light applications to optimize the efficiency, accuracy and control LED current power IC products and application technology.
A function of more and more popular in the phone built-in digital camera that can shoot high-resolution still and video images. With the wide bandwidth of 3G networks roll out, the demand for higher resolution photos as well as new applications (such as video conferencing) to enhance the performance of the integrated camera. High-power white light source in the camera when used indoors or in dim light, the camera performance enhancements also put forward higher requirements. The white LED lamps are already widely used in the color display backlight, but also become the main light source for camera phones. The white LED has a combination of stylish mobile handset designers the required performance: small size, high light output and flash or continuous video capture lighting. The output power LEDs have been developed specifically for the integrated camera light source. Although these dedicated camera LED is ideal for landscape lighting, but they are also the consumption of the battery is another important factor.
From the system designer’s point of view, there are three main problems with driving high current white LED: 1. To provide efficient power; 2 to adjust the LED current; camera light source is turned off to ensure that the LED power supply is completely cut off. Like an ordinary diode, the electrical characteristics of high current white LED forward voltage is 3.4V or a typical 350mA forward current (see Table 1).
Increase the LED current (“flash” mode, 500mA-700mA or more) will cause a higher forward voltage. The low current (“torch” or “video capture” mode, 100mA-350mA) to produce a low forward voltage. The forward voltage will decrease as the temperature increases, will have hundreds of millivolts of drift in the temperature range of the phone work as a diode forward voltage. Therefore, effective to provide LED power supply is very challenging, LED forward voltage due to different working conditions may be higher or lower than the voltage of the lithium.
LED driver should be applied to the buck and boost DC / DC conversion, with minimal pressure drop in the current control circuit to adjust the LED current.
If two or more parallel LEDs use a camera light, due to the great changes between any two LED’s forward voltage drop, therefore need some additional devices to ensure current matching. The most obvious way is the LED in series, the boost converter works well. If, however, need to be connected in parallel, the solution must be able to provide independent current control of each LED. In any way, the current control circuit in the current-sensing resistor should be at the lowest possible pressure drop. Another complex points in the camera light applications from as low as a microamp forward current-current LED visible light. No inherent LED disconnect, needs to be a separate switch in series on the LED to ensure the shutdown, the LED will not have a current flows through.
High power LEDs produce visible light is very simple, but in the existing design to produce high-performance power supply and current control solution is very difficult. In these applications, there is no dedicated power supply IC to solve the above problem, the battery life will still suffer. Linear Technology’s LTC3453 is specifically designed to optimize the efficiency, accuracy and control LED current in the current camera light applications.
LTC3453 use of a synchronous buck – boost power supply and programmable low-dropout current source to adjust the LED bulbs current. Devices based on VIN and LED forward voltage synchronous buck, synchronous boost, as well as four switch buck – boost mode automatically converted. Each current source has its own independent control loop can be adjusted in a very low LED pin voltage currents to reduce power consumption. Truly cut off the LED current source can be turned off. 4 LED current source (LED1 to LED4) LEDs can be connected in parallel or together to jointly drive a single high output LED. By regulating VOUT to ensure that all current source is the minimum voltage of the regulator state, the overall efficiency can be optimized. Within the scope of the entire lithium batteries and LED forward voltage synchronous buck – boost topology, the efficiency of the “flashlight” mode is greater than 90%, increased by 20% to 30% than ordinary step-up solution.