As mobile phones, PDAs and other handheld devices continue to upgrade the performance of built-in cameras, many of the companies building these devices look for ways to make better use of white LEDs (WLEDs) for illumination. These solid-state light sources are desirable because they can operate in either a flash mode for taking still photographs or a torch mode for recording video. That dual-mode operation is something a Xenon flash lamp cannot do. LEDs have other advantages over Xenon bulbs: they're more rugged and can be powered off of a smaller power circuit.
In the early-generation camera phones (and other camera-enabled devices), an LED flash was typically sufficient because image resolution was low (a few megapixels or less). However, as camera phones migrate to higher resolution, the light output of conventional WLEDs becomes inadequate. In these applications, the much-brighter Xenon lamps have generally been the flash of choice.
Nevertheless, there are those who would like to exploit the LED's advantages, even in the higher-resolution camera phones. For those product developers, the latest high-intensity LEDs provide a possible alternative to Xenon lamps (albeit one that still requires some tradeoff in camera performance).
The latest flash LED driver ICs have been developed to support these higher-intensity LEDs. The new chips also shrink their packaging and push switching frequencies higher to implement smaller designs, while also providing a variety of control and protection features to keep the battery, the LED and the end product running smoothly.
Supercap Helps with Peak Currents
This month, Advanced Analogic Technologies introduced a 2-A flash LED driver IC capable of delivering high-intensity light from WLEDs. Combining a high-frequency boost converter with fixed-input current limiting, dual-output regulated current sinks and I2C control, the AAT1282 uses a supercapacitor to deliver the high peak current needed to support the high-intensity flash, required for mobile phones featuring cameras with resolutions of 5 megapixels and higher, while protecting the battery.
“Several LED vendors have announced products which approach the light intensity of a conventional Xenon flash, the de facto yet bulky standard in today's high-resolution digital still cameras or smartphones,” says Phil Dewsbury, product line director for AnalogicTech.
“But, the adoption of LED flash in portable applications has until now been limited due to the high levels of battery current, forward voltage and peak current required to produce this intensity. By storing the output of a boost converter in a high-capacity supercapacitor, the AAT1282 allows designers to use WLEDs to deliver the high-intensity light needed for flash and video applications in high-megapixel cameras without draining or damaging the battery.”
Operating across a 2.7-V to 5.5-V input, the AAT1282 is designed for LED photoflash applications in a variety of single-cell Li-ion-powered products. The stepup converter features two output current sinks, each providing a regulated 1-A current.
The two LED current sources share the output current equally. They can be connected together to apply a full 2-A output current into a single WLED (Fig. 1). Alternatively, each diode can be connected to its corresponding current source and the driver can provide two independently controlled 1-A outputs.
Designed specifically to charge a supercapacitor, the stepup converter features a fixed-input current limiter to limit the battery current and to avoid large voltage drops that can cause the phone to reset. The current limit is set at 800 mA for the AAT1282, but other values can be requested. A 2-MHz switching frequency allows the use of a compact 1-µH inductor and 2.2-µF output capacitor.
The WLED driver uses an I2C input to enable, disable and set the movie-mode (torch) current for each flash LED across 16 levels. The I2C interface also is used to set the flash-/movie-mode current ratio, output channel control and safety timer. Maximum flash- and movie-mode current is set by a single external resistor. The ratio between flash- and movie-mode current is preset at 7.3:1, but can be adjusted to a wide range of other values.
An independent flash enable is provided to initiate the flash operation and to control a default timer, which can be used to terminate a flash event at the end of a user-programmed delay or as a safety feature to prevent the LED from overdissipating in the event of a software failure.
The AAT1282 also features a true-load disconnect function that isolates the load from the power source while in the off or disabled state. By guaranteeing a shutdown current of less than 1 µA, this feature essentially eliminates leakage current. To protect the device in the event of an output short-circuit condition, this driver IC also features an integrated thermal management system. Built-in circuitry prevents excessive inrush current during startup.
Available in a 14-pin, 3-mm × 3-mm TDFN package, the AAT1282T sells for $1.75 each in quantities of 1000. For more information, see www.analogictech.com.
Adjustable Overvoltage Protection
In March, National Semiconductor introduced a high-current LED driver that enables dual LED operation for the camera-flash function in portable multimedia devices. A member of National's PowerWise energy-efficient product family, the LM3553 drives one or two high-current LEDs in series.
The LM3553 flash LED driver is a fixed-frequency, stepup converter with two regulated current sinks, driving loads up to 1.2 A from a single-cell Li-ion battery. Using the driver's adjustable overvoltage protection (OVP) circuitry allows designers to drive two high-current LEDs in a series configuration, which maximizes the illumination-to-power ratio. The LM3553 can drive the camera in a high-power flash mode for still photography or a low-power torch mode for video recording (Fig. 2).
To configure the driver to fit their application, designers can use the adjustable 128-flash current levels and 16-flash timer durations through an I2C-compatible interface. The LM3553 also features built-in timeout protection to protect the flash LEDs in case of an error condition.
The flash LED driver maximizes the light output for low-power systems with peak efficiency equal to or greater than 90%. The LM3553 features an adjustable switch current limit for the use of small inductors with lower saturation currents. The voltage mode offers a 5-V rail for backlight LEDs and audio amplifiers. One or more high-current LEDs can be driven either in a high-power flash mode or a lower-power torch mode controlled by either an internal register or the FEN and TX pins.
The TX input forces the flash pulse into a low-current torch mode, allowing for synchronization to RF power amplifier events or other high-current conditions. A GPIO pin adds a hardware resource to the system. The hardware RESET pin offers full control over the device in case of system failures. Additional features include soft-start and a 1.2-MHz switching frequency.
The LM3553 comes in a 12-pin LLP. For more information, see www.national.com/pf/LM/LM3553.html. Available now, the LM3553 is priced at $1.20 each in 1000-unit quantities.
This month, Texas Instruments is introducing its TPS61054, a high-power white LED driver that operates in torch and flash modes, delivering up to 75 mA and 700 mA, respectively, to illuminate a single WLED. The driver consists of a 2-MHz, fixed-frequency synchronous boost converter with standard logic interface. The high switching frequency allows use of low-profile 2.2-µH inductors — enabling the total design to fit in less than 25 mm2 of pc-board area.
Despite the fast switching, the boost converter achieves up to 96% efficiency. Input current is reduced to 0.3 µA typical in shutdown.
The TPS6105x also functions as a voltage boost regulator. This allows the chip to power other system functions such as an audio power amplifier. The LED current or desired output voltage can be programmed through two logic signals. For flash synchronization with the camera, the chip includes a trigger pin for fast LED turn-on time.
Other features include an LED turn-on safety timer, a zero-latency transmit-masking input, an integrated low-light dimming mode, protection against an open or shorted LED and overtemperature protection. Along with the TPS61054, the company offers a lower-current version (TPS61055) rated for 500 mA of flash current. Both devices are available in 12-pin NanoFree chipscale packaging and in 10-pin QFNs. In quantities of 1000, unit pricing in the QFN package driver is $1.40 for the TPS61054 and $1.35 for the TPS61055.