On the market each year must be hundreds of new mobile phones, these phones are the same basic functions, that is communication. Around the phone’s design is to increase the phone’s additional features, the main way to increase mobile phone selling point as well as a new profit point. Different phone difference between live mainly in the peripheral functions, such as appearance, screen colors, brightness, multimedia features, Bluetooth, a camera function. Phone integrated camera feature is the inevitable trend of mobile phone design cell phone camera pixels are increasingly high. To get a better image effect, the mobile phone camera flash function is also becoming increasingly important.
The current mobile phone camera with LED flash. The flash of the LED only requires 3.5 to 4.5V DC voltage 120 to 250mA current can be issued 2000 ~ 7500mcd high brightness light. LED low-voltage flash circuit is simple, efficient, energy-saving, low cost, PCB area, especially for mobile phones, digital cameras and handheld devices, and popular with the favor of hand-held imaging products market.
LED flash drivers to control the forward current program
According to the output characteristics of the drive circuit, the LED flash driver circuit can be divided into the constant voltage type and the constant current type; circuit works can be divided into a step-up circuit and charge pump circuit.
LED is a current-driven devices, and their brightness is proportional to the current relationship. In the constant voltage drive circuit, often a resistor in series with the LED is used to determine the cause of the forward current is expected to the required voltage to the LED lights. In this way there is a drawback, any change of the LED forward voltage will lead to changes in the LED current, which can not be guaranteed to flow through the LED current is equal to the default, will not be able to ensure that the LED’s constant brightness.
, By detecting the series resistance of the voltage on the LED constant current drive circuit to ensure constant flow through the LED current. This way you can eliminate the forward voltage changes caused by current changes, it can be fixed LED brightness.
Since the scope of work of the cell phone battery voltage is typically 3.6V ~ 4.2V, LED forward voltage is 3V to 4V, so keep low voltage input, the output of high voltage, you must boost circuit voltage will be increased to drive LED. Flash-driven commonly used are two ways to boost, a boost circuit inductor for energy storage component, and the other is the charge pump capacitors for energy storage component.
The boost circuit using inductors as energy storage elements, the advantage of relatively high efficiency.