Nippon Precision Circuits, Inc. (NPC, Main office: 4-3 Fukuzumi 2-chome, Koto-Ku, Tokyo, President: Yoshifumi Gomi) has started sample shipment of the white LED driver with charge pump, the SM8131B that is ideal for driving the white LED used as the backlight of the color LCD panel of portable systems.

The SM8131B is an advanced-function, high-performance white LED driver IC that can drive six channels in parallel. This LED driver supports cellular telephones with two screens by extending their battery life of small and slim cellular telephones. To drive a white LED with VF^*1 of about 4V with one cell of lithium-ion battery (2.5 to 4.2V), an up converter is necessary. This IC employs a charge pump that can automatically select a voltage increment of x1 or x1.5 without a step-up coil, and is controlled, whenever possible, to maintain a step-up rate of x1 at which the power efficiency can be enhanced about 30% from that at x1.5. The SM8131B helps reduce the LED driving current variation, enhances the power efficiency, and extends battery life.

To illuminate the surface of the backlight evenly, the brightness variation of white LEDs that are driven in parallel must be reduced. To reduce the brightness variation, it is necessary to suppress the variation of the forward current flowing into each white LED. Conventionally, a ballast resistor has been connected to each white LED in series to reduce the influence of the variation in VF^*1 of the LED. This IC has six channels of LED driving current control circuit that keeps the driving current variation to within ±3% without using a ballast resistor.

The white LED driving voltage output by this IC is automatically multiplied by 1 or 1.5. The driving current of all the LEDs is monitored every 1.4 seconds so that the best multiplication factor can be selected. Even if the battery voltage temporarily fluctuates or the temperature of the white LED fluctuates, the best multiplication factor is selected about 1.4 seconds later because the bidirectional selection voltage does not have hysteresis characteristics. Selection of the multiplication factor and the LED driving current are controlled for all the LEDs connected, so that the variation of the LED driving current can be reduced.

The dual-screen supporting function of this IC can divide the six channels of the LED driving current control circuit into two groups at any ratio. This can be done by pin setting. In addition, two-wire serial input control can turn ON/OFF the LEDs in each group and adjust their brightness in 15 steps. The maximum LED driving current per channel is set by an external resistor. By changing the resistance of this resistor, the brightness can be adjusted.

When five or fewer white LEDs are used, only the unused LED driving current control pin has to be grounded. The anode and cathode of the white LED are not directly connected to an external circuit board but to pins of the IC. The on-chip ESD^*2 protection circuit of the IC protects the white LED from ESD.

NPC plans to develop more products in this series that use these core technologies.

^*1: VF: Forward voltage (of diode)
^*2: ESD: Electrostatic discharge

Product Name	Shipping of Sample	Sample Price	Mass Production	Mass Production Plans
SM8131B	March 2004	¥300/unit	April 2004	1 million units