Bright Idea For The Latest Images

Power-supply IC will drive the high-current white LEDs used for flash illumination in the latest generation of mobile imaging equipment.

Geneva: STMicroelectronics introduced a power-supply IC, the STCF02, that's designed to drive high-current white LEDs. Specifically, it's targeted at white LEDs used for flash illumination in the latest generation of mobile imaging equipment such as cameras for mobile phones, digital still cameras, and PDAs.

In addition to supporting the higher flash-illumination powers required by the latest high-resolution image sensors, the device extends battery life thanks to the low standby current of less than 1µA that it draws during shutdown mode. The STCF02 also incorporates comprehensive protection circuitry, including the ability to automatically sense and respond to excessive LED temperature.

Today's approach to the camera-flash LED is based on two topologies: multiple white LEDs connected either in series or in parallel. Both topologies are considered adequate for cameras up to 1.3Mpixels, even though the maximum distance between the subject and the camera flash range from 0.5m to 1m to obtain a good image in low-light conditions. However, for the latest generation of mobile phones with higher megapixel resolution, higher-power flash is necessary. Thus, the preferred solution for these cameras is to use a single, high-power white LED.

The STCF02 is a high-efficiency (85% typical) buck-boost converter that's able to deliver a controlled current to a single high-power flash white LED. Accurate current control over the range of battery voltages is essential since the white LED's light output is proportional to the current flowing through it. The STCF02 controls the current by automatically changing the operating mode between buck (step down) and boost (step up) to accommodate the forward voltage of the white LED according to the battery voltage level. The device uses a PWM (pulse-width modulation) control scheme with a switching frequency of 1.8MHz. It provides a regulated output voltage ranging from 2.5V up to 5.3V powered from a single lithiumion-battery. Three logic-input signals enable the selection of four different operating modes: flash, medium flash, torch, and shutdown. Flash-, medium-flash-, and torch-mode currents are set by external resistors. In shutdown mode, the LED is disconnected from the battery, cutting off any leakage current drawn from the battery.

The flash-mode current ranges from 200 to 800mA, while the torch-mode current is adjustable from 25 to 250mA. The medium flash mode-current is set at a level between the torchmode and full flash-mode currents.

To ensure reliable operation, the STCF02 integrates electricaland thermal-protection circuits. Should the inductor current exceed an internally set current limit (2.3A), the PWM controller turns the internal power switch OFF until the next PWM oscillation. The device includes output overvoltage protection (OVP) that stops the operation of the PWM controller when the OV is detected. Normal operation is restored automatically when the output voltage falls below an internal fixed value. An internal thermal-shutdown protection circuit switches the device off if the junction temperature exceeds 140°C. In addition, since the STCF02 is also able to sense the white-LED temperature by means of an external negative temperature coefficient (NTC) sensor, the device will switch OFF if an excessive white-LED temperature is detected. Alternatively, an external RC delay cell can be used in place of the NTC to limit maximum flash duration to a safe time.

Thanks to its buck-boost smart dual-control mode and the high-output-power capability, as well as its outstanding conversion efficiency, the STCF02 is the optimum fully integrated solution to drive the high-power white LED used in the new generation of high-resolution camera sensors. The STCF02 offers the maximum allowable W-LED power to be extended to 3W, while the maximum distance between the subject and the camera is increased to 1.5m.

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