Electronic Design

Chips Help Supercaps Flash White LEDs Brighter For Higher-Res Photos

As digital-camera and phone-camera resolution increases, high-resolution image sensors require more light. Firstgeneration low-res camera phones provided barely adequate flash intensity for taking closeups of friends at parties. Even second-generation camera-phone flash is still unsuitable for image sensors with greater than 3-Mpixel resolution. And that’s just talking about still photography. Video requires in-phone camera lighting to provide movie/torch mode.

How are the phone-camera OEMs to achieve that illumination capacity without seriously impacting talk time? CAP-XX is working with a number of U.S. semiconductor companies that are designing LED-flash ICs that take advantage of supercapacitors’ deep charge-storage capabilities to drive highbrightness white LEDs, which naturally have high peak current demands.

So far, charge-pump-based white LED flash driver designs have limited current of up to about 800 mA. Alternative designs that rely on inductive-based stepup converters have been limited to 1 to 1.5 A. Higher currents would be possible, but throwing a large current demand directly onto the cell phone’s lithium battery would pull the battery voltage down because of the battery’s equivalent series resistance (ESR), possibly enough to result in a CPU reset in the phone.

Nonetheless, higher currents are needed. High-intensity LEDs require large currents. Seoul Semi, Philips Lighting, and other companies have announced high-intensity LEDs requiring more than 1 A. That implies a high forward voltage drop, because LED VF is proportional to ID. Obviously, this is a job for supercapacitors, particularly the smaller board-mounted devices with values ranging from 30 mF up to 2 F that CAP-XX specializes in.

These supercaps have breakdown voltages around 2.75 V per cell, so they must be connected in series, with appropriate cell balancing resistors. Using them requires some extra circuit complexity because their low ESR (on the order of 50 to 100 mO) requires some means to manage charge current.

According to Pierre Mars, CAP-XX’s vice president of applications engineering, Advanced Analogic Technologies was the only semiconductor company as of June to have introduced a driver chip for use with supercapacitors in white LED cell-phone flash applications. The AAT1282 combines a high-frequency boost converter with fixed input current limiting, dual-output regulated current sinks, and I2C control.

Those output current sinks each provide a regulated 1 A. The chip’s two LED current sources share the output current equally. They can be connected together to apply a full 2-A output into a single white LED, or each diode can be connected to its corresponding current source, and the driver can provide two independently controlled 1-A outputs. The battery-current limiter in the step-up converter that charges the supercapacitor prevents large bus voltage drops that might cause the phone to reset. Operating at a 2-MHz switching frequency, the boost converter needs only a 1-µH inductor and a 2.2-µF output capacitor.

The serial digital input can be used to enable, disable, and set the movie-mode (torch) current for each flash LED across 16 levels and for other control functions. More basically, a single external resistor sets the maximum flash and movie-mode current. (The ratio between flash current and movie-mode current is preset at 7.3:1, but it can be adjusted.)

A flash-enable pin initiates the flash operation. It also can be used to control a timer for shutting off the flash after a user-programmed delay. A load-disconnect function isolates the load from the power source while in the OFF or disabled state, limiting shutdown current to less than1.0 µA. All this comes in a 14-pin, 3- by 3-mm package. The device costs $1.75 in 1000-unit quantities.

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