Power Electronics

LCD Bias and White-LED Backlight Supplies

Maxim Integrated Products’ new MAX1578/MAX1579 is the first complete LCD bias and white-LED power supplies with only one inductor and no charge-pump diode. Four regulated outputs supply all the voltages required for small active-matrix, thin-film transistor (TFT) LCDs in handheld devices where minimum external components and high efficiency are important. Three integrated, advanced charge pumps provide fixed +5 V, +15 V and –10 V for the LCD bias circuit, while a 34 V step-up dc-dc converter drives up to eight white LEDs in series.

The charge pumps require only ceramic capacitors and no external diodes. All outputs are sequenced during startup and shutdown. During shutdown, the outputs are disconnected and actively discharged to zero. The efficiency is 83% for the bias power (+5 V at 25 mA and +15 V/-10 V at 100 mA).

The step-up dc-dc converter in each power supply provides uniform illumination because the white LEDs are connected in a serial configuration and driven by the same current. The converters also feature PWM or analog dimming control, soft-start to limit inrush current and overvoltage protection. Fast, 1-MHz PWM operation allows for small components. Backlighting efficiency is 84% (six LEDs at 20 µA). Additionally, the MAX1579 features a temperature-derating function that allows fewer LEDs to generate the same light output.

The input-voltage range is 2.7 V to 5.5 V. Both devices also feature independent enable inputs for LED and bias power, thermal shutdown protection and a low 1-µA quiescent current during shutdown mode.

For more information, visit www.maxim-ic.com.

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