Discussing the method of selecting white light LED flash driver

In the mobile phone market, it has become almost standard to integrate high-pixel image processors into mobile phones. As the resolution of these image processors increases, the industry also increases the demand for high-brightness flash. Xenon flash bulbs have always been the main lighting choice for digital cameras, but for the mobile phone market, the space available on the circuit board for non-phone functional components is really limited, making the larger xenon lamp solution impractical. Fortunately, mobile phone manufacturers have recently made significant technological breakthroughs in high-power white light diodes. Today, manufacturers of white light LED flash diodes have introduced products with light output exceeding 70 lumens and can handle pulse currents exceeding or equivalent to 1A. However, these technological breakthroughs also bring many questions to the designer, including how much circuit board space is available? What flash-related functions need to be added? How much power can the flash driver use? How many lumens are needed to take a good photo? If the above questions can be answered, the designer will be more comfortable when choosing a flash LED driver.

 

Solution size

 

The first question facing cell phone designers is how much board space is available for the camera flash? In the field of LED flash drivers, the two most common boost technologies are switched capacitor boost (charge pump) and inductive boost. Of the two boost topologies, the switched capacitor solution is generally smaller, and most switched capacitors consist of four ceramic capacitors and two external resistors. The recommended capacitance value for these applications is 4.7μF, and the voltage rating is 10V (helps reduce DC bias losses). These capacitors are available in 0603 form factors and are available from most capacitor manufacturers. The overall solution size of a flash driver using switched capacitors is generally about 25mm2. For example, the National Semiconductor LM2758, which uses a chip-scale package, has an overall solution size of less than 15mm2. Another advantage of the switched capacitor solution is that it is extremely thin. Generally, depending on the packaging method of the flash driver, the capacitor is usually the tallest component in the entire solution.

 

The solution size of inductive flash driver is generally larger than that of switched capacitor driver. A typical inductive flash LED driver solution occupies about 35mm2~40mm2 of board area. Inductive driver generally requires two capacitors (input and output), with an average capacitance of 10μF and a 0805 size. Inductive boost requires a rectifier section to handle the peak inductor current and output voltage. In synchronous boost topology, the flash IC usually integrates a pass FET (typically a PFET), and this integration usually makes the IC package size larger than asynchronous solution. In asynchronous topology, the pass section is implemented in the form of Schottky diode. Compared with the boost using switched capacitor, the extra space occupied by inductive boost comes mainly from the inductor itself. For applications with flash current close to 1A, the required inductor is generally 2.2μH~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 components in the entire solution, with 1.2mm being a common height.

 

Features

 

Once the topology of the flash driver is determined, the next issue to consider is the features required for the design. The first feature to consider is the type of control interface. Basic flash drivers generally have two control pins to implement 3-4 different operating modes (such as off, reminder light, flashlight and flashlight). If the designer does not need to adjust the brightness dynamically, these simple control components are sufficient. On the other hand, if the system requires a higher level of control, most flash circuits include some type of serial control interface. One of the most common serial interfaces is the Inter-Integrated Circuit interface (I2C). I2C or I2C compatible interfaces not only control basic on/off functions, but also allow users to dynamically set the brightness of the flashlight and flashlight. In addition, if the design includes features such as flash fuse timing, inductor current limiting or overvoltage protection level, these can also be configured through the I2C interface. In addition, these serial interfaces become more important when the general-purpose input/output (GPIO) lines of the microcontroller/microprocessor are not sufficient.

 

Many LED drivers, including National Semiconductor's LM3553, offer additional control pins to further assist designers in solving system-level problems. Today's typical image processors have an external strobe (strobe/flash) pin to indicate that the system is taking a picture. This strobe signal can be directly connected to multiple LED strobe drivers through the strobe enable pin. This direct connection between the image processor and the LED driver eliminates all delays that occur between the two components, which are generally caused by controller or software limitations.

On a system level, today's cell phone systems need to manage the amount of current drawn from the battery during talk/data transmission. The current drawn by the Tx/Rx power amplifiers plus the current drawn by the flash driver during talk/data transmission can often exceed the maximum current the battery can provide. Most cell phone designs can allow the load battery voltage to drop to 3.2V without entering a reset state (VBATT-LOADED = VBATT_UNLOADED (IBATT * RBATT_ESR)). To prevent a reset caused by the battery's ESR voltage drop, some newer flash LED drivers have added a transmit pin (Tx) that can help reduce the current drawn by the LED driver during talk/data transmission. By adding the Tx pin, the flash driver can force the diode current to remain low for a very short time (less than 100μs) to prevent the phone from accidentally entering a reset state during a call.

 

efficiency

 

Efficiency is an old topic in mobile phone design. The higher the efficiency of the system, the longer the talk time available to the user. Inductive boost technology enables the driver to achieve the highest efficiency over a wide range of input voltage and output current. In contrast, switched capacitor components are limited to a few fixed quantization gains (2x, 1.5x, 1x channel modes), so that the average converter efficiency that can be achieved is lower than that of inductive boost under the same input range. When evaluating an LED driver, the meaning of efficiency is a little different.

 

Converter efficiency or LED driver efficiency?

 

When it comes to flash LED drivers, certain efficiency losses must be considered before calculating the true efficiency of the solution. In order to obtain a regulated flash or flashlight/video camera LED current, the boost converter must use a current sink/source or a tightly controlled reference voltage, together with a resistor to establish the load current. However, both of these different methods have their own advantages and disadvantages, and it must be noted that the power losses caused by these two methods are not included in the efficiency calculation of the boost converter. However, the efficiency of the overall solution or LED does take these power losses into account.

 

Equation 1 and Equation 2   

 

The first thing to note is that even two different converters can have absolutely the same converter efficiency, and their LED driving efficiency will only differ by 5% to 10%. In other words, if the efficiency of two converters is equal, as long as one has lower losses caused by the current regulation element, it is the more efficient converter.

 

LED driving efficiency or luminous efficiency?

 

However, LED driver efficiency alone does not tell the full story of overall performance. For example, consider two different flash LED drivers and two different flash LEDs. The first driver has an 85% converter efficiency and an LED voltage of 4V at 1A, while the other has an 80% converter efficiency and an LED voltage (also at 1A) of 3V. At a given current, both LEDs produce the same amount of light output and have a 350mV feedback voltage. Using Equation 1 and citing a worst-case input voltage or 3.2V, the first driver draws 1.6A from the battery, while the second driver only draws 1.3A from the battery. Despite the higher efficiency of the first flash driver, it would need to draw 300mA more to produce the same amount of light output as the second flash driver. This example highlights the impact of LED efficiency. The LED in Example 2 is 33% more efficient at producing light than the LED in Example 1.

 

When the flash LED driver is operating in continuous video shooting or flashlight lighting mode, the operation time may be relatively long, so the efficiency of the converter becomes very important. However, under normal flash conditions, the operation time is only a moment, so the importance of converter efficiency is reduced. On the contrary, the efficiency that is more concerned here is whether it can give the flash driver more output power at a given input power to produce a brighter flash. High-efficiency flash LEDs combined with efficient flash drivers can minimize the flash current drawn from the battery, so that mobile phone designers can more flexibly manage power for other parts of the system.

 

Optimization of light output

 

Optimizing the light output of an LED flash driver involves two main factors (three if you include cost): the light level required by the phone's image processor, and how much power is available for illumination? For a given input power budget, there are three ways to improve the brightness of the flash to help designers achieve the desired lighting requirements. These are LED selection, LED current drive, and LED configuration, which all play a vital role in optimizing the flash LED driver.

 

Selecting LEDs

 

As mentioned above, the first optimization element is to select an LED with high luminous efficacy. An LED with higher luminous efficacy can emit more luminous flux (lumens) at a given power. At a given current, the luminous efficacy is equal to the LED luminous flux divided by the product of the LED drive current and the forward voltage. Luminous flux curves can be found in the specification sheets provided by most LED manufacturers1.

When selecting LEDs, mobile phone designers should not only consider the optical performance of the LED, but also must consider the size and cost of the LED, as well as the complexity of the lens that maximizes the LED illumination.

 

Increase drive current

 

After selecting the LED, the second way to increase light output is to increase the actual drive current. Using the luminous flux curve in Figure 2, it can be found that when the diode current is increased from 500mA to 1A, the light output will increase by about 30 lumens. However, increasing the diode current will also bring some adverse effects. If the diode current is doubled, the increased diode current and forward voltage will cause the LED power to increase by more than double, and this LED power increase will increase the system's input power requirements. Figure 3 shows the effect of forward current on LED forward voltage.

 

In order to enhance the optimization of the LED driver current system, an estimated input current must be established first. Once the minimum input voltage and maximum input current are calculated, data can be found from the LED forward voltage to LED current curve (Figure 3) to further calculate the maximum allowable drive current of the flash LED driver.

 

Formula 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

 

From the curve in the figure, we can see that the output power generated by the LED at 3.6V and 1A current is equal to 3.95W (PLED + PFEEDBACK), which is very close to the maximum allowable value.

 

Configuration

 

If you already have a high luminous flux and high efficacy LED, but the required luminance is still not achieved after optimizing the flash current, then you can get closer to the goal by adding a second or third LED to the design. Looking again at the luminous flux vs. LED current curve, you can see that the curve is not completely linear. The luminous flux produced by two LEDs operating at only half the flash current will produce more than one LED operating at the full flash current. In addition, the total power of two LEDs operating at half the flash current is lower than that of one LED operating at the full flash current. This can provide a greater total output current for the two LEDs within the given input power budget. The two LEDs can be driven in parallel or series configurations.

 

example

 

The following 3 configurations all have a 350Mv VFB and can also produce 73 lumens.

 

1 LED @ 1A:  POUT = (3.6 1A) + (1A 350mV) = 4.08W 

2 LEDs @ 350mA (parallel): 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) + (550mA 350mV) = 3.99W 94 lumens

 

When driving two LEDs, the series configuration offers many advantages over the parallel configuration. Driving the two LEDs in series ensures that the current flowing through both flash LEDs is the same. In the parallel configuration, the typical matching of the two current sources to the LED current is 1% to 3%. However, most flash drivers have only one current sink. If the two LEDs are connected to a single current source/current sink, the mismatch in the LED forward voltage will cause a serious LED current mismatch. To solve this problem, a series current-locking resistor is added. However, adding a series resistor to the LED will reduce both the output current budget and the amount of available flash current, resulting in a weaker flash. In addition, if the LED current is the same in the series and parallel configurations, driving two LEDs in series can also reduce the output power dissipation caused by the current control element (resistor or current sink) by half (PFB-Series = ILED VFB and PFB-Parallel = ILED VFB2).

 

Summarize

 

Adding a white LED camera flash to a cell phone system involves many design choices, and the topology that will be chosen is indirectly determined when determining the board space that the flash can occupy. In addition, features such as transmit and flash enable pins allow other subsystems to help handle some of the current management and flash timing, thereby reducing the workload on the battery and microcontroller/processor. Furthermore, optically efficient LEDs combined with efficient boost converters can help increase the illumination of the flash system. If a single LED is not enough to provide sufficient illumination in a dim environment, a second LED can be added to the design to solve the problem of insufficient brightness. When selecting an LED flash driver, the above issues should be considered early in the design. As long as these issues are resolved early, all worries can be swept away.