Low-voltage flash solution for high-brightness LEDs

Flashlight is an auxiliary tool with a wide range of uses. The emergence of low-voltage flashlights with high-brightness LED as the flashlight source has brought a revolutionary change to traditional flashlights. Low-voltage flashlights do not require oscillation circuits, step-up transformers, and large energy storage capacitors. Flash LEDs only need a DC voltage of 3.5-4.5V and a current of 100mA to emit 2000mcd-3000mcd of high-brightness light to illuminate the subject that needs auxiliary light. LED low-voltage flashlight circuits are simple, efficient, power-saving, low-cost, and occupy a very small PCB area. They are particularly suitable for mobile phones, digital cameras, and handheld devices, and will therefore win the favor of the entire handheld imaging product market.

High voltage flash lamp circuit

Currently, most commonly used flashlight circuits are high-voltage flashlight circuits, which are composed of an oscillation circuit, a step-up transformer, a large energy storage capacitor, a high-voltage coil, and an inert gas flashlight. The typical circuit diagram is shown in Figure 1.

Low voltage flash lamp solution

The circuit of low-voltage flash lamp is very economical, mainly composed of boost and voltage-stabilizing charge pump, Flash LED (LED flash lamp), and flash control switch, as shown in Figure 2.

Advantages of low voltage flash lamps

• The low voltage flash circuit is simple, efficient, power-saving, low-cost, and occupies very small PCB area;

• It has multiple flash modes, namely automatic flash, pulse flash, continuous single flash, red-eye reduction flash, and can also be used as DVC lighting;

• The whole circuit mainly consists of charge pump and Flsah LED;

• No high voltage is required, eliminating the need for a step-up transformer and large energy storage capacitors;

• High brightness SMD Flsah LED can emit light up to 2000-5000mcd;

• SMD Flsah LED has low power consumption, no heat and long life;

• Capacitive charge pumps have high conversion efficiency and fewer peripheral devices;

• The charge pump has an SHDN input terminal that can accept control signals from the host and operate automatically;

• All devices are in ultra-small packages, occupying small space and low installation cost;

• Suitable for mobile phones, DSC, DVC;

• Suitable for mobile phones with camera function to protect the privacy of the person being photographed.

Typical circuit of mobile phone camera flash

The resolution of mobile phone cameras is 200,000 to 1 million pixels. Since mobile phones are relatively small, the space available for installing flash is very limited. The geometric size of Flash LED is 5X6X2.5mm, and there will be smaller packages of 3X3X2mm in the future. The charge pump is SOT-23 package, all of which are Mini type, which just meets the requirements of mobile phone design.

Figure 3 is a circuit diagram of a mobile phone camera flash with a peak current of 100-200mA. It uses an AAT3110IGU-4.5V capacitor charge pump to boost and stabilize the voltage of the mobile phone lithium battery to 4.5V, and provides a 4.5V operating voltage and 100-200mA peak current to an EL-61-25UWC Flash LED. The FDG335N MOSFET is used as a flash switch, and the peak current forms a loop through it. The input filter capacitor of the charge pump is 10uF, the output filter capacitor is 4.7uF, and the energy storage capacitor is 1uF. All of them are X7R and X5R ceramic capacitors with small equivalent series resistance (ESR). RB is the Flash LED balancing resistor, and RF is the peak current adjustment resistor. Changing this resistor can set the peak current size, as shown in Table 1 AAT3110IGU-4.5V peak Flash LED current test table. Figure 3 is a current change diagram of the Flash LED before and after the flash.

Typical circuit of digital camera flash

Increasing the number of Flash LEDs can meet the different requirements of digital cameras for flash brightness and flash distance. Figure 4 is a typical circuit of a digital camera flash that can provide 300-400mA current. One AAT3110IGU-4.5V charge pump can only provide 200mA peak current (maximum IF=250mA/100ms), so two AAT3110IGU-4.5V charge pumps are required to be connected in parallel to output 400mA peak current; the IFP of a Flash LED is 100mA, so 400mA peak current can drive four Flash LEDs. Two AAT3110s share CIN and COUT, which can save PCB space and cost. The second AAT3110 (B) and the flash gate control share the same signal. During the flash stop period, the diode forms the RC discharge delay circuit of the flash gate.

LED flash

LED flash lights currently all use the structural principle of installing five blue LED tube cores and a yellow phosphor color filter in one LED flash light to emit white light. This is also the most economical and practical solution. Using an RGB (red, green, and blue) three-color tube core LED can also synthesize white light, but an RGB LED is more expensive than an LED flash light. The brightness of the red, green, and blue colors must be adjusted separately to synthesize white light, and the cost of use is relatively high. It is only used in advanced solutions that require adjustable flash color temperature and colorfulness.

LED flashes are equipped with reflectors and lenses, so they provide uniform light at a distance of several meters.

The appearance and circuit diagram of Flash LED are shown in Figure 5. The main technical parameters of Flash LED are brightness (IV), viewing angle, forward voltage (VF), forward current (IF), power consumption (Pd), peak current IF (Peak), and size. Table 2 is a table of commonly used Flash LED performance.

Capacitor Charge Pump

In order for a low-voltage flash LED to emit dazzling white light, it must be supplied with a constant voltage and a large current. In order to reduce the impact of the working frequency of the boost device on RF, a capacitive charge pump with a capacitor as the intermediate for energy transfer is generally selected; the efficiency of the capacitive charge pump is divided into two types according to its boost method: frequency doubling and fractional frequency doubling. The efficiency of the former is about 90%, and the efficiency of the latter is about 93-95%. The calculation of the efficiency is related to the input and output voltages; the higher the switching operating frequency of the capacitive charge pump, the smaller the capacity of the filter capacitor required, and the smaller the interference to RF; the capacitive charge pump is divided into constant voltage output and constant current output according to its output; the parallel constant voltage and high current instantaneous drive solution is generally adopted for Flash LED driving.