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Maxim > Design Support > Technical Documents > Application Notes > Display Drivers > APP 1037
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 1037
Keywords: charge pump, step-up, dc dc converters, switching power supply, regulated voltage, regulated
current supply, white LED, backlight, power, boost, switched capacitor, series, parallel, charge pumps
APPLICATION NOTE 1037
Charge-Pump and Step-Up DC-DC Converter
Solutions for Powering White LEDs in Series or
Parallel Connections
Apr 23, 2002
Abstract: White LEDs are popular for portable equipment LCD backlights because they are less complex,
less expensive, and smaller than CCFL backlights. Portable equipment like PDAs, cellphones, and digital
cameras require backlights because they are converting to color LCD displays. This article discusses
charge-pump and inductor step-up converter solutions for powering white LEDs in series or parallel
configurations. Regulated-voltage or regulated-current controllers are used. Trade-offs in size, efficiency,
battery life, and LED-matching are discussed.
With the increasing popularity of color LCD displays in cell-phones, PDAs, and digital cameras, white
LEDs are becoming popular illumination sources. While monochrome displays can use colored light
sources such as electroluminescent back lights or colored LEDs, color displays require a white light
source to properly display color.
There are two main methods for providing a white light source: white LEDs and CCFLs (cold-cathode
fluorescent lamps). CCFLs have been used for years in notebook computers. However, because of their
size, complexity, and cost advantages, white LEDs are becoming the preferred light source for small
handheld devices.
White LEDs require a low DC voltage (between 3V and 4V), which means that simple inductor or
capacitor-based circuits may be used for power. CCFLs, in contrast, require a high AC voltage
(200V
RMS
to 500V
RMS
) and an expensive, bulky, transformer-based circuit for power (Figure 1).
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