EETimes India - September 16-30, 2008 - (Page 10) In Focus | Green Tech White LED flash drivers brighten DSCs continued from page 1 to a camera flash. In the LED flash driver arena, two boost technologies are commonly used: switched capacitor boost (charge pump) or inductive boost. Of the two boost topologies, the switched capacitor implementation is typically smaller. Most switched capacitor parts require four ceramic capacitors and two external resistors. The typical capacitor value recommended for these applications is 4.7µF with a voltage rating of 10V (to help with DC bias losses). These capacitors can be found in 0603 cases sizes from a number of capacitor manufacturers. Total solution sizes at or around 25mm2 are fairly common when using switched capacitor flash drivers. For example, the LM2758 from National Semiconductor comes in a chip-scale package and has a total solution size that is less than 15mm2. Switched capacitor solutions also have the advantage of being very thin. Once the topology of the flash driver has been decided upon, the next question to ask pertains to the feature set required for the design. The first feature to address is the type of control interface. The basic flash driver usually has two control pins, allowing for three to four modes of operation (e.g. Shutdown, Indicator, Torch and Flash). If the designer does not need to change brightness levels dynamically, these simple control parts are usually sufficient. On the other hand, if a higher level of control is desired, many flash ICs have some sort of serial control interface. One of the most popular serial interfaces used is I2C. The I2C or I2C-compatible interface not only controls the basic on/off functionality, but also allows the user to set the torch and flash levels dynamically. Other features such as the flash safety timer duration, inductor current limit level and over-voltage protection level, if present, can be configured through the I2C interface. These serial interfaces are especially to have a controlled flash or torch/movie light LED current, a boost converter must either utilise a current sink/source or a tightly controlled reference voltage along with a resistor to set the load current. While both solutions have advantages and disadvantages, it is important to remember that both methods introduce a power loss that does not get taken into account when calculating boost converter efficiency. The solution/LED efficiency takes this power loss into account. Equation 1 Equation 2 Figure 1: By asserting the Tx pin on these parts, the flash driver can force the diode current to a lower level in a very short time (less than 100m), preventing the handset from going into a reset state during a call. useful when the general-purpose input/output (GPIO) lines of the microcontroller/microprocessor are limited. Many LED flash drivers, including National’s LM3553, provide additional control pins to further aid the designer address system level concerns. Today’s imagers typically have an external strobe/ flash pin that alerts the system that a picture is being taken. This strobe/flash signal can be tied to many LED flash drivers directly via a flash enable pin. A direct connection between the imager and LED driver eliminates any delay that could occur between the two parts due to controller/software limitations. One of the largest systemlevel issues in the handset today is managing the amount of current drawn from the battery during a call/data transmission. The combination of the current drawn by the Tx/Rx power amplifier during a call/data transmission and the current drawn by the flash driver can exceed the maximum allowed battery current. Most handset designs allow operation to occur down to a loaded battery voltage of 3.2V before the handset goes into a reset state (VBATT-LOADED = V BAT T_UNLOADED – (I BAT T * R BATT_ESR)). To prevent a reset condition caused by the ESR voltage drop in the battery, some of the newer flash LED drivers have incorporated a transmission pin (Tx) to help minimize the current drawn by the LED driver during a call/data transmission. By asserting the Tx pin on these parts, the flash driver can force the diode current to a lower level in a very short time (less than 100m), preventing the handset from going into a reset state during a call. System efficiency The topic of efficiency is not new to the world of handset design. The higher the system efficiency, the longer the talk time the user can experience. Inductive boosts can yield high efficiencies over large input voltage and output current ranges. Switch capacitor parts are limited to a few fixed quantised gains (2x, 1.5x, 1x pass mode), resulting in lower average converter efficiency over the same input range. When evaluating LED flash drivers, the topic of efficiency takes on a slightly different meaning. When dealing with flash LED drivers, certain efficiency losses must be considered to obtain the solution efficiency. In order Two different converters can have the exact same converter efficiency and have LED drive efficiencies that are 5 per cent to 10 per cent different. In the end, if the converter efficiencies are the same, the converter that introduces the smallest loss due to the current regulation element should be considered the more efficient converter. Unfortunately, LED drive efficiency does not tell the whole performance story. For example, let’s assume that we have two flash LED drivers and two different flash LEDs. The first driver has a converter efficiency of 85 per cent and the LED voltage is 4V at 1A of current. The second driver has a converter efficiency of 80 per cent and a LED voltage of 3V at 1A. Both LEDs produce the same amount of light for a given current and both converters have a feedback voltage equal to 350mV. Using Equation 1, at a worst-case input voltage or 3.2V, the first driver will draw 1.6A from the battery. Using this same equation, the second driver will only draw 1.3A from the battery. Despite having a higher efficiency, flash driver #1 draws 300mA more current to produce the same amount of light as flash driver #2. This example highlights the effects of the LEDs luminous efficacy. The LED in the second continued on page 14 10 EE Times-India | September 16-30, 2008 | www.eetindia.com http://www.eetindia.com/STATIC/REDIRECT/Newsletter_080901_EETI02.htm?ClickFromNewsletter_080916
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