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ST power-supply IC tailored for white LEDs in portable apps

Posted: 11 Oct 2005 ?? ?Print Version ?Bookmark and Share

Keywords:stmicroelectronics? st? power-supply ic? stcf02?

ST's new power-supply IC

STMicroelectronics rolled out a new power-supply IC that is specifically designed to drive the high-current 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.

The STCF02 is a high-efficiency (85 percent typical) buck-boost converter that is 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 as 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 and provides a regulated output voltage ranging from 2.5V up to 5.3V powered from a single Li-ion battery.

Three logic-input signals allow 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 200mA to 800mA; the Torch-mode current is adjustable from 25mA to 250mA; while the Medium Flash mode-current is set at a level between the Torch-mode and full Flash-mode currents.

In addition to supporting the higher flash illumination powers required by the latest high-resolution image sensors, the STCF02 chip extends battery life through its low standby current of less than 1?A 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, added ST.

To ensure reliable operation, the STCF02 integrates electrical and 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 occurs. The device includes output over-voltage 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 140C. In addition, since the STCF02 is also able to sense the white-LED temperature by means of an external negative temperature coefficient (NTC) sensor, if an excessive white-LED temperature is detected, the device is switched OFF. Alternatively, an external RC delay cell can be used in place of the NTC to limit maximum flash duration to a safe time.

The STCF02 will be priced at $1. Samples are currently available, with volume production imminent.

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