Convertors drive white-LED backlights

The XC9116 series of fixed-frequency constant-current step-up DC/DC convertors is ideal for driving the white LEDs used in LCD backlighting applications in a wide range of handheld designs.

The XC9116 series of fixed-frequency constant-current step-up DC/DC convertors is ideal for driving the white LEDs used in LCD backlighting applications in a wide range of handheld designs, such as PDAs, cellular phones and digital cameras.

Backlighting with white LEDs is by far the simplest way of producing a "brighter" LCD in handheld devices.

The new XC9116 has an input voltage range of 2.5 to 6.0V and can deliver output voltages up to 17.5V.

Thus from a 2.5V input, four white LEDs can be driven in series.

Alternatively, using a 3.2V input, the new DC/DC convertor is able to drive a network of two parallel banks of three LEDs.

The luminance of the LEDs is controlled by varying the duty cycle of a 10kHz PWM signal applied to the device's CE pin.

The new XC9116 is a highly integrated device combining reference voltage, ramp wave circuit, error amplifier, PWM comparator, phase compensation circuit, overvoltage limit, current limit and N-channel MOS driver transistor (2ohm on resistance) in a single package.

All that is required to achieve an ultracompact low-profile solution for driving white LEDs is a chip coil and a tiny output capacitor of 0.22uF.

A switching frequency of 1.0MHz and the device's low feedback reference voltage, which ensures that RLED losses are minimal, combine to make the XC9116 a highly efficient solution.

For example, when driving three LEDs in series efficiency is 88%.

A "B" version of the XC9116 has an additional fault detection circuit so that if the LEDs are disconnected or damaged in such a way that feedback is prevented, then excess or runaway voltage is prevented by stopping the drive to the internal MOSFET.

The XC9116 is available in SOT-25 and ultracompact USP-6C packages.

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