Design of a portable camera flashlight driver circuit using TPS65552A

　　High-end equipment for film or digital photography requires xenon flash tubes to shoot in low light conditions. Xenon flash tubes provide a momentary, high-intensity light source, which is essential when shooting distant, fast-moving or low-light subjects. The spectrum produced by the xenon discharge tube is very close to that of the sun, providing very accurate color reproduction.

　　Once a trigger signal is applied, a Xenon flash requires a high voltage across its electrodes (usually around 300V) to fire. All the energy needed to fire the flash is stored in a large capacitor called the "flash capacitor". Once the flash is triggered, all this energy stored in the flash capacitor is released through the flash tube to produce light. This energy stored in the flash capacitor is provided by a specialized step-up converter that charges the flash capacitor from a very low battery input voltage to a voltage of up to 300V. In the past, this converter consisted of bulky discrete components that were difficult to integrate into small devices such as cameras.

　　TI's TPS65552A greatly simplifies the design and reduces the size of camera flash charger circuits. Figure 1 shows a flash capacitor charger based on this device. The TPS65552A provides all the necessary charging control, output feedback, charge completion status, insulated gate bipolar transistor (IGBT) drivers, and some circuit protection necessary to implement a small, efficient flash charger.

　　Figure 1 TPS65552A Camera Flash Capacitor Charger

　　The TPS65552A is based on a flyback topology. During the time when the internal switch is off, it senses the output voltage. During this time, the output voltage is reflected back to the input through the transformer. This eliminates the need for a bulky, high-voltage feedback network at the output, while also providing electrical isolation from the input to the output. Once this output voltage reaches its target value, the TPS65552A automatically stops charging and the open collector output goes low, signaling a "flash ready" state. This output can drive a status LED or an input to a microcontroller.

　　The I_PEAK pin of the TPS65552A controls the peak current flowing through the primary winding of the flyback transformer T1 during all switching cycles. To adjust the capacitor charging time, the main current can be dynamically adjusted between 0.9A and 1.8A by changing the voltage applied to the I_PEAK pin. This feature allows the microcontroller to dynamically control the charger's outflow current for power management. For example, in a digital camera, the microcontroller can reduce the charger current when the high-current zoom motor is operating so that the zoom motor and charger can function simultaneously without exceeding the maximum current capability of the camera battery (see Figure 2). This feature can also be used to extend battery life. Reducing the peak current during charging also reduces the average current consumption, allowing a low battery to charge the flash capacitor.

　　Figure 2: Complete camera flash module with power management and flash management functions

　　In the past, the flash was triggered by a push button switch or silicon controlled rectifier (SCR). However, newer flash modes (such as red-eye reduction) use multiple Xenon lamp exposures. The flash is triggered for a short flash, which does not completely discharge the camera flash capacitor. Then, after a short delay, the flash is retriggered for the main flash. Push buttons and SCRs cannot reliably start and stop the flash mid-flash. IGBTs are capable of handling currents of typically 150A during a flash. However, like MOSFETs, the IGBT gate requires a large current pulse to turn on quickly; therefore, a high current driver is required. The TPS65552A has an integrated high current buffer to drive the IGBT gate used in the trigger circuit. During the flash, the IGBT gate can be driven on and off to support flash modes such as red-eye reduction, or to evaluate the IGBT gate through the lens (E-TTL).