LED Flash Solutions for Camera Phones

LEDs have become the standard solution for movie lighting and camera flash in mobile phones. The demand for higher picture quality and higher resolution requires brighter flash LED solutions. The challenge is how to squeeze the best light flux from the battery by achieving the most efficient solution. As a result, operation that draws high current from the battery requires many power-saving operation features and a robust system design. This article will introduce a system-level flash LED driver design and some features that ensure safe system operation and integration.

Efficient Camera Flash LED Driver

High-resolution cameras require a high-brightness flash to take pictures in minimum light conditions. Customers are demanding a flash solution as a standard feature of mobile phones. Flash-equipped mobile phones have become an attractive selling point. This feature requires high luminous flux, which poses a challenge to the design of efficient LED driver systems.

System Design

The high forward voltage and current of the flash LED in a mobile phone and a given battery voltage make the boost converter the best solution. When driving large currents, the inductor-based boost converter shows satisfactory efficiency. The LED must be current driven because the forward voltage varies not only with temperature, but also has its own variations. This variation in forward voltage comes from its production process and has a range of ±20%, see Figure 1.

A simple approach is to connect the flash LED in series with a current sense resistor and then drive it through a boost converter. Figure 1 illustrates this approach.

Figure 2: Simple LED driving method using external current sensing resistor

The output voltage of the boost regulator is controlled to match the set LED current sensed by an external resistor. Unfortunately, this deviates from the designer's goal of squeezing the highest luminous flux from the limited power provided by the battery. The external current sense resistor has high power dissipation and is sized to provide available headroom voltage even at low currents to drive continuous movie lighting. On the other hand, if the current increases, the voltage drop across the current sense resistor increases, resulting in significant power dissipation. In addition, high-precision resistors with ideal power dissipation ratings are expensive and increase the size of the solution, requiring one resistor per LED channel.

Therefore, a better solution is an active current sink or source integrated into the LED driver, as shown in Figure 3. The internal current sense resistor can be adjusted in a way that the voltage drop and the resulting power dissipation are reduced, depending on the LED current. For low LED currents, the voltage drop can be maintained high enough to obtain an accurate sense signal.

Figure 3 Improved LED driving scheme using adaptive current sink and detection

The current sink not only senses the LED current, it can also regulate it by dynamically adjusting the resistor. The resulting current sink voltage drop serves as the information needed to dynamically adjust the boost converter output voltage, aiming to keep power dissipation to an acceptable minimum at any current level.

Figure 4 Comparison of active current sensing and resistive current sensing

Figure 4 shows a comparison between using a 1Ω resistor to sense current and using an active current sensing method regulated to a 400mV drop. The active current sensing method significantly contributes to higher system efficiency due to the lower power consumption.

Squeezing light flux from the battery

In the past, the RF PA drew the highest pulse current from the mobile phone battery. With the development of multi-functional mobile phones in the past 5 years, the processor power supply and the flash LED power supply, which is the focus of this article, have drawn the highest current. For example, if you want to drive an LED current of 1.5A, the current drawn from the battery can be as high as 3A due to the voltage ratio of the boost converter. Such high currents can cause the battery voltage to drop sharply. The undervoltage threshold detection mechanism prevents the system from failing in this situation. The phone will completely shut down due to low battery voltage when the flash is turned on, which is a very bad user experience. The common solution is to let the camera software turn off the flash in the low battery voltage state, which is not too bad compared to the user experience of not using the flash. The slow battery voltage information refresh rate provided by the PMIC, battery temperature and aging effects, and more serious inaccuracies relax the safety margin.

If the flash driver itself can prevent the battery voltage from dropping too much, then a smaller safety margin can be maintained. This can be achieved by using a controlled slew rate to ramp up the LED current and continuously monitoring the battery voltage during the ramp-up period.

TI has a flash driver technology that monitors the battery voltage. To obtain a stable LED current waveform and avoid excessive battery voltage drop, the flash driver actively controls the LED current rise/fall sequence. During the rising phase (rising slope of 25mA/12µs), the input voltage is monitored. If the input voltage drops below a set threshold, the device immediately stops the LED current from rising further to the set threshold and keeps the flash current at the actual level, see Figure 5.

Figure 5 Battery voltage drop monitoring

Therefore, it is guaranteed that the safety margin is very small and the phone does not shut down. Irreversible battery voltage drops during battery cycles are avoided and the overall battery operating time is increased.

Security system integration

The spotlight is designed for safe and trouble-free operation when driving high pulse currents. Mobile phone manufacturers are pressing for a seamless system integration solution. This requires a feature set that goes beyond the standard safety operation features such as inductor current limiting, undervoltage protection, etc. The TPS61310 flash LED driver has this feature set for this demanding operation.

LED Fault Detection

LED short circuits must be detected not only during production, but also during device operation to avoid dangerous conditions. One way to detect this condition is to force a current of a few mA to flow in the forward direction. This current can test the LED in the sub-illumination range, so the end user will not notice the brightness. However, this method has some disadvantages: LED manufacturers usually do not test the sub-illumination range. Due to process differences in production, there are huge inaccuracies not only between LED types but also within a single LED type. This may result in missed detection of a short-circuited LED or false false detection. The TPS61310 is different. If one or more LEDs are in a short-circuit condition during operation, the low-side current sinks LED1, LED2, LED3 limit the maximum output current and increase their input resistance to prevent excessive current absorption according to the settings of the video illumination mode or flash mode. In addition, this process is monitored and the short-circuit LED condition is reported to the test equipment during production or to the camera engine during operation through the I2C interface. Using a similar method, the open circuit condition can also be detected.

Over temperature detection

An attractive mobile phone product design may not meet the requirements of optimal thermal design in some cases. High-power flash LEDs have limited permissible pulse handling capabilities. If the phone is exposed to high temperatures, and/or the LED temperature rises due to the preceding flash operation, the capacitive thermistor may not be able to handle the LED power loss, resulting in an irreversible drop in luminous efficacy above 85°C, shortened lifetime, or even extinction. To bridge the gap between thermal design and functional/attractive design imperatives, the TPS61310 allows one or more LED temperatures to be measured using an NTC sense resistor. If the critical temperature is exceeded, the LED on/off time pulse ratio is reduced by software, allowing the LED to cool down between flash operations. This feature can also be used as a finger burn protection feature.

The TPS61310 chip temperature is also monitored. In addition to the standard thermal shutdown function, the TPS61310 can also provide an early warning function to the camera engine to avoid the thermal shutdown function being accidentally triggered, thus providing space for the device to cool down.

TPS61310 Movie Lighting/Flash Driver

The TPS61310 is capable of driving a 1.5A single, dual or triple flash LED application. Proprietary features such as battery voltage monitoring, power-saving operation, and reliable LED short-circuit detection make it a simple integrated solution for flash driving and movie lighting. After being programmable through a high-speed I2C I/F, the device has the highest cross-platform design flexibility. As an option, dedicated logic inputs can be used for zero-delay triggering. The 2x2mm2 chip-scale package and the lack of external components to program the current or flash on-time result in an extremely small solution size.