How to choose white LED flashing light driver
Author: ◆ National Semiconductor Application Engineer / Greg Lubarsky In the mobile phone market, the integration of high-pixel image processors into mobile phones has become almost standard. As the resolution of these image processors has increased, the industry has also increased the demand for high-brightness flash lamps. Xenon flash bulbs have always been the main lighting choice for digital cameras, but for the mobile handset market, the space available on the board for placement of non-telephone functional components is limited, making the larger xenon lamp scheme impractical. . Fortunately, mobile phone manufacturers have recently made major technological breakthroughs in high-power white light diodes. Today, manufacturers of white LED flashing diodes have introduced products with a light output of more than 70 lumens and can handle pulse currents exceeding or equivalent to 1A. But these technological breakthroughs also bring a lot of problems to the designer, including how much board space is available? Is there any need to increase the functionality associated with the flash? How much power is available for the flash drive? How much lumens do you need to take beautiful photos? As long as the above questions can be answered, designers can be more comfortable when choosing a flash LED driver . Solution size The first question that mobile phone designers need to face is how much board space is available for camera flashing. In the field of LED flash drivers, the two most popular boosting technologies are switched capacitor boost (charge pump) and inductive boost. In these two boost topologies, the switching capacitor scheme is generally small, and most of the switched capacitors are composed of four ceramic capacitors and two external resistors. The recommended capacitance for these applications is 4.7μF and the voltage rating is 10V (to help reduce DC offset losses). These capacitors are available in 0603 form factor and are available from most capacitor manufacturers. The overall size of a flash driver using a switched capacitor is typically about 25 mm2. For example, the National Semiconductor LM2758 in a chip scale package has an overall solution size of less than 15mm2. In addition, the switched capacitor solution has the advantage of being extremely slim. In general, depending on the way the flash driver is packaged, the capacitor is usually the highest component in the overall solution. The size of the inductive flash driver is generally larger than that of the switched capacitor driver. A typical inductive flash LED driver solution occupies approximately 35mm2 to 40mm2 of board area. Inductor drivers typically require two capacitors (input and output) with an average capacitance of 10μF and a size of 0805. Inductive boost requires a rectified portion to handle peak inductor current and output voltage. In a synchronous boost topology, a flash integrated circuit typically incorporates a via FET (typically a PFET), and this integration typically results in a larger package size for an integrated circuit than an asynchronous solution. In the asynchronous topology, the path portion is implemented in the form of a Schottky diode. Compared with the boost using switched capacitors, the space occupied by inductive boost is mainly from the inductor itself. For applications where the flash current is close to 1A, the required inductor is typically 2.2μH to 4.7μF, and the saturation current must be greater than 1.5A. However, these inductors are typically no smaller than 3mm x 3mm and are usually the tallest component in the overall solution, with 1.2mm being the usual height. Features Once the topology of the flash drive is determined, another problem to be faced is the functional features required for the design. The first feature to consider is the type of control interface. Basic flash drivers typically have two control pins to perform three to four different modes of operation (eg, shutdown, cue, flashlight, and flash). If the designer does not need to adjust the brightness in a dynamic form, these simple control components are sufficient. Conversely, if the system requires a higher degree of control, most of the flash circuits contain some type of serial control interface. One of the most common serial interfaces is the Inter-Integrated Circuit Interface (I2C). The I2C or I2C compatible interface not only controls the basic on/off function, but also allows the user to dynamically set the brightness of the flashlight and flash. In addition, if the design includes flash lamp timing, inductor current limit or overvoltage protection level, it can also be configured through the I2C interface. In addition, these serial interfaces become even more important when the microcontroller/microprocessor's general purpose input/output (GPIO) lines are not sufficient. Many LED drivers, including National's LM3553, offer additional control pins to further assist designers in solving system-level problems. Today's typical image processors have an external strobe/flash pin to indicate that the system is taking a photo. This flash signal can be directly connected to multiple LED flash drivers via a flash enable pin. This direct connection between the image processor and the LED driver eliminates all occurrences between the two components. Delays, which are generally caused by controller or software limitations. In terms of system level issues, today's mobile phone systems need to manage the amount of current drawn from the battery during a call/data transfer. The current drawn by the Tx/Rx power amplifier during the call/data transfer plus the current drawn by the flash driver can often exceed the maximum amount of current the battery can provide. Most handset designs allow the load cell voltage to drop to 3.2V without going into reset (VBATT-LOADED = VBATT_UNLOADED (IBATT * RBATT_ESR)). To prevent resetting caused by the ESR voltage drop of the battery, some newer flash LED drivers have a transfer pin (Tx) that helps reduce the current drawn by the LED driver during a call/data transfer. . By adding a Tx pin, the flash driver can force the diode current to remain at a low level for a short period of time (less than 100μs) to prevent the phone from being mistakenly reset during a call. effectiveness Efficiency is already an old issue in mobile phone design. As long as the system is more efficient, the longer talk time available to users. Inductive boost technology enables drivers to achieve maximum efficiency over a wide range of input voltages and output currents. Conversely, switched capacitor components are limited to a few fixed quantization gains (2x, 1.5x, 1x path modes), so that the average converter efficiency that can be achieved over the same input range is better than that of inductive boost. low. When evaluating an LED driver, the meaning of efficiency is a bit different. Converter efficiency or LED drive efficiency? Equation 2 The first thing to note is that even two different converters can have absolutely the same converter efficiency, and their LED drive efficiency is only 5% to 10% difference. In other words, if the efficiency of the two converters is equal, as long as the loss caused by the current adjustment element is lower, it is a more efficient converter. LED drive efficiency or luminous efficiency? However, the single LED drive efficiency does not fully reflect the overall performance. For example, suppose there are two different flash LED drivers and two different flash LEDs in front of them. The first driver has a converter efficiency of 85%, and the LED voltage at 4A is 4V, while the other converter efficiency and LED voltage (also at 1A) are 80% and 3V, respectively. At a given current, the two LEDs produce the same amount of light output and have a 350mV feedback voltage. Using Equation 1 and citing the worst-case input voltage or 3.2V, the first driver draws 1.6A from the battery, while the second driver draws only 1.3A from the battery. Leaving the first flash driver is more efficient, it requires more than 300mA to produce the same amount of light as the second flash driver. This example highlights the impact of LED luminous efficiency. The LED in Example 2 was 33% higher than that in Example 1 in terms of light efficiency. When the flash LED driver is operating in continuous film shooting or flashlight illumination mode, the efficiency of the converter is very important due to the long operation time, but in normal flash conditions, the operation time is only an instant. Therefore, the importance of converter efficiency is reduced. Conversely, the efficiency of interest here is whether a given output power can be given to the flash driver at a given input power to produce a brighter flash. High-efficiency flash LEDs with efficient flash drivers minimize the flash current from the battery, giving handset designers more flexibility in power management for the rest of the system. Optimization of light output The LED output driver's light output optimization involves two main factors (if the cost is three): the illuminance required by the mobile phone image processor, and how much power can be used for illumination? A given input power budget There are three ways to increase the brightness of the flash to help the designer achieve the desired lighting requirements. This is LED choice, LED current drive and LED configuration, which play a pivotal role in flash LED driver optimization. Select LED The first optimization element mentioned above is to choose an LED with high luminous efficiency. A LED with higher efficacy can emit a larger luminous flux (lumen) at a given power. At a given current, the luminous efficiency is equal to the LED luminous flux divided by the product of the LED driving current and the forward voltage. The luminous flux curve can be found in the specification sheet provided by most LED manufacturers. Increase drive current After selecting the LED, the second way to increase the light output is to increase the actual drive current. Using the luminous flux curve in Figure 2, it can be seen that when the diode current is increased from 500 mA to 1 A, there will be an increase in light output of 30 lumens. However, increasing the diode current can also have some adverse effects. If the diode current is doubled, the increased diode current and forward voltage will cause the LED's power to increase more than double, and this LED power boost will increase the system's input power requirements. Figure 3 shows the effect of forward current on the forward voltage of the LED. In order to enhance the internal optimization of the LED driver current system, an estimate of the input current must be established. Once the minimum input voltage and maximum input current are calculated, the data can be found from the LED forward voltage versus LED current (Figure 3) to further calculate the maximum allowable drive current for the flash LED driver. Configuration If you choose to have LEDs with high luminous flux and high luminous efficiency, but still do not achieve the required illuminance after the flash current optimization, then you can pull the target by adding the second or third LED in the design. near. Referring again to the curve of luminous flux versus LED current (Figure 2), it can be seen that the curve is not completely linear. The luminous flux produced by two LEDs operating with only half of the flash current will produce more light than an LED operating at full flash current. In addition, the total power of two LEDs operating at half the flash current is also lower than that of a full flash current. This allows for a larger total output current for both LEDs within a given input power budget. These two LEDs can be driven in a side-by-side or serial configuration. example The VFBs of the following three configurations are all 350Mv, and can also produce 73 lumens. When driving two LEDs, the serial configuration has more advantages than the parallel configuration. Driving the two LEDs in series ensures that the current flowing through the two flash LEDs is consistent. In a side-by-side configuration, the typical match between the two current sources and the LED current is between 1% and 3%. However, most flash drivers have only one current sink. If two LEDs are connected to a single current source/current, the mismatch of the LED forward voltage will cause serious LED power. Loss match. To solve this problem, you need to add a serial lock-flow resistor. However, adding a series resistor to the LED will simultaneously reduce the budget of the output current and reduce the amount of flash current that can be used, resulting in a weaker flash. In addition, if the LED currents in the serial and parallel configurations are the same, driving two LEDs in series can also reduce the output power dissipation caused by current control elements (resistors or current sinks) by half (PFB-Series = ILED) VFB and PFB-Parallel = ILED VFB2). to sum up Adding a white LED camera flash to a mobile phone system involves many design choices, and in determining the board space that the flash can occupy, it also indirectly determines the topology to be used. In addition, functions such as transfer and flash start pins allow other subsystems to assist with certain current management tasks and flash timing, thereby reducing battery and microcontroller/processor operating pressure. Furthermore, high optical efficiency LEDs combined with efficient boost converters can help increase the illuminance of the flash system. If a single LED is not sufficient to provide adequate illumination in a dim environment, a second LED can be added to the design to address the problem of insufficient brightness. When choosing an LED flash driver, you should consider the above problems at the beginning of the design. As long as these problems are solved early, all the troubles can be swept away. Wireless Charging Power Banks,Fast Charging Wireless Power Bank,Power Banks,Wireless Power Banks Dongguan Guancheng Precision Plastic Manufacturing Co., Ltd. , https://www.dpowergo.com
introduction
When faced with a flash LED driver, certain efficiency losses must be considered before the true efficiency of the solution can be calculated. In order to obtain a regulated flash or flashlight/short film to capture the light LED current, the boost converter must use a current sink/source or a tightly controlled reference voltage, together with a resistor to set up Load current. However, these two different methods have their own advantages and disadvantages. It must be noted that the power loss caused by these two methods will not be included in the efficiency calculation of the boost converter. However, the overall solution or the efficiency of the LED takes these power losses into account.
Equation 1
When choosing LEDs, cell phone designers should not only consider the optical performance of LEDs, but must also consider the size and cost of LEDs, as well as to increase the LED illumination to the highest lens complexity.
Equation 3
POUT CONV IIN VIN = ILED VLED + VFEEDBACK
example
VIN = 3.2V, IIN = 1.5A, CONV = 85% VFEEDBACK = 350mV
Input power = 4.8W, maximum output power = 4.08W
As can be seen from the graph, the output power of the LED at 3.6V and 1A is equal to 3.95W (PLED + PFEEDBACK), which is very close to the maximum allowable value.
1 LED @ 1A: POUT = (3.6 1A) + (1A 350mV) = 4.08W
2 LEDs @ 350mA (parallel l): POUT = (3.3V 2 350mA) + (350mA 350mV2) = 2.56W
2 LEDs @ 350mA (serial): POUT = (3.3V 2 350mA) + (350mA 350mV) = 2.43W
The VFBs of the following three configurations are all 350Mv, and their output powers are very close.
1 LED@1A: POUT = (3.6 1A) + (1A 350mV) = 4.08W 73 Lumens
2 LEDs@525mA (parallel): POUT = (3.425V 2 525mA) + (525mA 350mV2) = 3.96W 90 lumens
2 LEDs@550mA (serial): POUT = (3.45V 2 550mA) + (550mA350mV) = 3.99W 94 lumens