Charge-pump-based Drivers Power up to Six LEDs in Parallel for Uniform Display Backlighting
These white LED drivers feature a dual-gain architecture that enables the regulators to automatically select between one of two gains from input voltage to output voltage, maximizing the efficiency of the regulator. The selection is based on the input voltage of the regulator and the forward voltage of the LED.
"The dual-gain architecture of these new products provides excellent efficiency and extends the battery life of portable devices," said Mark Davidson, marketing director for National Semiconductor's Power Management Division. "The exceptional current matching is ideal for uniform lighting in thin handheld devices."
For uniform brightness across the display, the white led drivers offer current matching at just one percent between each LED. The LM27951, LM27952 and LM27964 are adaptive 1.5X/1X switched-capacitor current drivers that include pulse-width modulation (PWM) brightness control. The LM27964 also includes an I2C compatible interface to easily control the brightness of the LEDs, for a total output current of 180 mA.
Key Technical Specifications of National's LM27951 and LM27952 White LED Drivers
The LM27951 and LM27952 charge-pump-based drivers are capable of driving up to four LEDs with 30 mA through each LED. Four tightly regulated current sources ensure excellent matching of LED drive current and LED brightness. The LED drive current value is programmed by an external resistor. The drivers operate over an input voltage range of 3.0V to 5.5V and require just four low-cost ceramic capacitors.
The LM27951 and LM27952 provide high efficiency without the use of an inductor by operating the charge pump with dual-gain architecture. Maximum efficiency is achieved over the input voltage range by actively selecting between a gain of 3/2 and a gain of one, based on the LED forward voltage requirements.
The LM27951 and LM27952 use constant frequency pre-regulation to minimize conducted noise on the input. Each has a fixed, 750 kHz switching frequency optimized for portable applications. To save energy, the drivers consume less than 1 uA of supply current when shut down.
Key Technical Specifications of National's LM27964 White LED Driver
The LM27964 charge-pump-based driver drives up to six LEDs in parallel along with multiple keypad LEDs, for a total output current up to 180 mA. Regulated internal current sources deliver excellent current matching to all LEDs.
The LED driver current sources of the LM27964 are split into two independently controlled groups. The primary group of four LEDs can be used to backlight the main phone display, and the second group of two LEDs can be used to backlight a secondary display. A single keypad LED driver can power up to 16 keypad LEDs with a current of 5 mA each. The LM27964 has an I2C compatible interface that allows the user to independently control the brightness on each bank of LEDs.
The LM27964 operates from an extended Li-Ion input voltage range of 2.7V to 5.5V. The device provides excellent efficiency without the use of an inductor by operating the charge pump in a gain of 3/2, or in pass-mode. The proper gain for maintaining current regulation is chosen based on LED forward voltage, so that efficiency is maximized over the input voltage range.
About National's Power Management Products
National Semiconductor is the number one supplier of power management ICs with 14.1 percent market share, according to iSuppli's 2005 voltage regulator/reference survey. National's power management revenue grew 37.3 percent in 2004 over the previous year, compared to 24 percent growth for the industry. National's innovative power management products include best-in- class linear regulators; power monitoring, control and reference ICs; and switching converters, such as high-voltage ICs for DC-DC conversion, switched capacitor converters, lighting management ICs and inductive switching converters. National also offers online tools that enable power designers to select the best power management chips for their applications. National's Webench(R) design tool allows designers to choose a product, create and analyze a design, and then build custom prototypes that can be delivered within 24 hours. More information about National's power management products is available at http://power.national.com .
Pricing and Availability
The LM27951, LM27952 and LM27964 charge-pump-based white LED drivers are available now. The LM27951 and LM27952 are packaged in thermally enhanced 14-lead LLP(R) packages and are priced at $1.50 each in 1,000-unit quantities. The LM27964 is available in a thermally enhanced 24-lead LLP(R) package and priced at $1.86 each in 1,000-unit quantities. Lead-free package options are also available.
For more information about the LM27951 or to order samples, visit http://www.national.com/pf/LM/LM27951.html . For more information about the LM27952 or to order samples, visit http://www.national.com/pf/LM/LM27952.html . For more information about the LM27964 or to order samples, visit http://www.national.com/pf/LM/LM27964.html . For more information on National's white LED driver products, visit power.national.com. A large selection of white LED drivers is supported by National's WEBENCH online design tool. To explore these solutions, visit http://webench.national.com . To learn more about designing with white LED drivers, visit National's library of online seminars at http://www.national.com/onlineseminar/ .
About National Semiconductor
National Semiconductor, the industry's premier analog company, creates high-value analog devices and subsystems. National's leading-edge products include power management circuits, display drivers, audio and operational amplifiers, communication interface products and data conversion solutions. National's key analog markets include wireless handsets, displays and a variety of broad electronics markets, including medical, automotive, industrial, and test and measurement applications. Headquartered in Santa Clara, California, National reported sales of $1.91 billion for fiscal 2005, which ended May 29, 2005. Additional company and product information is available at www.national.com.
NOTE: National Semiconductor, WEBENCH and LLP are registered trademarks of National Semiconductor Corporation. All other brand or product names are trademarks or registered trademarks of their respective holders.
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