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issue: May 2006 APPLIANCE Magazine

Electronics Report
Efficient LED Driver


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Ideal for the hand-held electronics market, a new step-up converter can drive nine white LEDs in a series at 20 mA from a single-cell lithium ion battery.

The MP1528 is a nine-lamp, 36-V white LED driver that uses current-limited, variable frequency architecture to regulate the LED current while maintaining high efficiency. A BIAS pin measures the output voltage and turns off the converter if an over-voltage condition is present to prevent damage due to an open circuit condition.

The MP1528 from Monolithic Power Systems is said to offer design engineers an efficient power management solution small enough for today’s battery-powered consumer electronics. Available in tiny QFN6 (3-mm by 3-mm) and MSOP8 (3-mm by 5-mm) packages, the drivers can be used in personal digital assistants (PDAs), cell phones, digital and video cameras, and small LCD displays.
The devices feature an input voltage range of 2.7 V to 36 V and have an overall operating efficiency greater than 80 percent. According to Dr. Wei Chen, vice president of Marketing and Applications, the wide input voltage range was achieved by integrating a high-voltage (HV) power field effect transistor (FET) and drive mechanism. “These power devices have very low operational power loss due to low ON-State resistance and fast switching characteristics,” Chen explains. “The key is to optimize the power FET’s structure in terms of gate threshold voltage and to reduce the ‘on’ resistance per unit silicon area. Also important is to optimize the switching characteristics to achieve three to four times improvement versus the regular external power MOSFET.”
The integrated 0.63 ohm power MOSFET ensures a maximum drive current of at least 20 mA, which Chen says is a common output power rating for white LEDs used for LCD backlight. “By guaranteeing at least 20 mA output current, customers can take full advantage of the maximum brightness levels of the white LEDs,” he says. “It should be noted that a higher input voltage and/or fewer LEDs can result in even higher output current greater than 20 mA.”
The drivers regulate LED current via a fixed off-time switch control loop determined by a low 0.4-V feedback threshold. LED brightness is adjusted by applying a DC voltage or a pulse width modulation (PWM) signal to the driver. Additional features include integrated soft-start, over-current, thermal, and output over-voltage protection.
Chen says that with a such a highly integrated device, having a thermal shutdown feature is important. “If the junction temperature reaches 160°C the device will shut itself down until the junction temperature drops to 130°C [and] then the device will start again,” he says. “This feature adds insurance for the design engineers to prevent some abnormal operation caused by thermal runaway.”

 

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