Overview With the improvement of camera sensors and optical components, camera phones account for an increasing proportion of the mobile phone market. In order to get high quality photos, it is important to choose the right flash. This article refers to the address: http:// Flash types There are two main types of flash available: light-emitting diodes (LEDs) and xenon lamps. Here are the simple operations and performance they use as a flash. (I) How the xenon lamp works: The xenon lamp is a columnar tube filled with helium, and its anode and cathode are in direct contact with the gas. The trigger electrode is distributed on the outer surface of the lamp tube, and a higher voltage pulse is transmitted through the outer layer of the glass to ionize the inert gas in the lamp tube, so that the impedance is reduced to below 1 ohm, and the current of the high-brightness light source is allowed to be generated at the cathode and Flow between the anodes. As the current pulse decays, the lamp voltage drops and eventually returns to a high-impedance state, waiting for another trigger. Figure 1 is a simple block diagram of a xenon lamp. > Figure 1. Simple block diagram of a xenon lamp (II) How LEDs work: LEDs are similar to small bulbs. Unlike incandescent lamps, they have no filament and are not very hot. LEDs use a movement of electrons within the semiconductor to create a light source. What causes the LED to emit light is the inclusion of impurities in the silicon, such as gallium, arsenide, indium, and nitride, which emit photons when the current passes through the LED. Newly developed LEDs are bright enough to compete with traditional lighting technologies, so in most applications, modern LEDs have been able to replace incandescent lamps. Xenon lamp and LED driver circuit.. (I): Xenon lamp.. Figure 2 is a transformer transformer-coupled flyback converter. When the EN pin is high, the power switch is turned on, allowing the transformer to generate high voltage pulses. Filtering produces a 300 V DC output. Suitable for digital cameras and camera phones powered by 2 alkaline batteries/NiMH batteries or single-cell Li+ batteries. When the flash capacitor is fully charged, the open-drain output /DONE will generate an indication signal. The MAX8622 automatically initiates an output charge every 11 seconds to maintain the charge of the capacitor with minimal battery current. Monitoring the output voltage with an external resistor divider provides higher charging accuracy. Direct detection of the secondary side of the transformer, in addition to preventing the output capacitor from discharging through the feedback resistor, can also directly detect the output voltage to obtain the best voltage accuracy, and make it independent of the transformer turns ratio. The MAX8622 has the following key features: (1): Capable of charging any size flash capacitor, charging a 100uF capacitor to 300V in 2.8 seconds. (2): There is no inrush current. (3): The programmable input current is limited to 1.6A. (4): Automatic "Reflash" mode. (5): Input voltage monitoring can extend battery life. (6): This architecture allows the use of low cost transformers. (7): Charging completion indication. Figure 3 shows the relationship between the charging time and the input voltage of the MAX8622. >Figure 2. Typical Application Circuit for the MAX8622 with Primary Current Limit Set by Resistor >Figure 3. MAX8622 Charging Time and Input Voltage (II) LED.. There are two different ways of connecting LEDs for camera phone flashes - parallel and series. All LEDs in series mode flow through the same current for uniform brightness and keyboard backlighting for larger TFT panels. However, this solution requires a large storage capacitor in a camera phone flash application, the capacity being related to the required flash current, the maximum output current of the boost converter, the number of LEDs in series, and the flash time. When LED parallel mode is selected, we can get higher brightness (although the brightness of each LED is slightly different). It only requires a small capacitor for filtering, and Maxim offers different solutions for both approaches. (1): MAX1583: Capable of driving five series white LEDs at high current for mobile phones, PDAs and other handheld terminals. Figure 4 shows a typical application circuit for driving a series LED. Two logic inputs EN1/EN are used to select four operating modes: shutdown mode (0.5uA maximum), projection mode (current up to 100 mA), precharge mode ( Charge the storage capacitor to 24V when the LED is off and flash mode (current up to 300mA). The MAX1583 is available in three different input current-limited versions (1A..MAX1583X; 0. 5A..MAX1583Y and 0.25A..MAX1583Z) to allow the user to select the appropriate storage capacitor for the application. > Figure 4. Camera Phone Flash Drive Typical Application Circuit The MAX1583 regulates the current flowing through the LED to achieve proper control of brightness. In the show mode (EN1 is set high, EN2 is set low), the boost converter needs to be turned on and off according to the voltage to ensure that the VLED is stable at 0.6V, and the current flowing through the LED is stabilized at the MOV. The set value of the external resistor of the foot (60/RMOV). In flash mode (EN1, EN 2 are high), unless overvoltage protection is reached, the boost converter will continuously switch to provide enough current for the LED, which is stabilized at the STB pin external resistor setting Value (600/RSTB). The MAX1583's precharge mode (EN1 low, EN2 high) charges the output capacitor to 24V and quickly switches to flash mode. When Vout reaches 24V, POK becomes high impedance and the MAX1583 will stop switching until Vout is below 23.5 V and the LED goes out (high impedance) in this mode. The output storage capacitor can be calculated as: Cres = (ILED-Iboost) * Tstrobe / (24V - n * VF), ILED and Tstrobe are the required flash current and duration; Iboost is the maximum boost converter Output current; n is the number of LEDs, and VF is the forward voltage drop of each LED. (2): For LED parallel mode, the user may use multiple white LEDs in parallel or select Luxeon LED to provide higher brightness than the normal white LED to the camera phone flash. The MAX1574/MAX1576/MAX1577 charge pumps can be used to drive LEDs in parallel. For example, the MAX1576 can regulate current and drive eight white LEDs. The main LED group (LED1-LED4) can obtain a drive current of 30 mA each. The flash units (LED5-LED8) can be independently controlled, each with a drive current of 100 mA (400 mA total). With the MAX1576's automatic voltage multiplier mode and low dropout current regulator, it provides high conversion efficiency over the entire operating voltage range of a 1-cell Li+ battery. Figure 5 shows its typical application circuit. Use two external resistors to set the maximum current of the two sets of LEDs. Four control pins are used to control the brightness of the LEDs. ENM1 and ENM2 set the main LED group current to 10%, 30% or 100% of the maximum current. ENF1 and ENF2 set the flash group current to 20%, 40% or 100% of the maximum current. > Figure 5. MAX1576 Typical Application Circuit for Backlighting and Flash Drives. As discussed above, the main difference between xenon and white LEDs for camera phone flash is that xenon lamps provide higher brightness than white LEDs. The output is easier to obtain uniform brightness, but the drive circuit is more complicated, and it must generate kV voltage trigger and 300 V flash drive. The implementation has high complexity and needs to consider safety specifications. Generally, the brightness of a white LED is limited by its point output characteristics. Multiple LEDs are required in parallel or in series to increase the effective brightness. High-brightness LUXEON LEDs can also be used, but its price is slightly higher for commercial camera phones. . Motor Start Switch,Single Phase Motor Starter Switches,Single Phase Centrifugal-Switches,Electric Machine Centrifugal Switch Gear Ningbo Zhenhai Rongda Electrical Appliance Co., Ltd. , https://www.centrifugalswitch.com
