AN11470 Datasheet, PDF(3/25 Page) NXP Semiconductors – Leadless Schottky diodes in a DC-to-DC step-up converter

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AN11470 Datasheet, PDF (3/25 Pages) NXP Semiconductors – Leadless Schottky diodes in a DC-to-DC step-up converter

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NXP Semiconductors

AN11470

Leadless Schottky diodes in a DC-to-DC step-up converter

1. Introduction

Most of the mobile communication and computing devices like mobile phones, tablet PCs,

navigation systems and notebooks use LCDs. An LCD display needs backlight that is

placed behind the LCD shutter system. For each pixel of the display LCD crystals can

block light transmission when a black pixel is displayed or they let light pass to generate

e.g. a white pixel. The degree of transmissibility is controlled for each pixel to allow gray

level in between. In modern LCD devices white LEDs are chosen as an energy efficient

light source. They combine advantages like a long life time, a constant white temperature

over time and low heat dissipation. The light produced by LEDs is distributed with plastic

light spreaders to achieve a uniform brightness for all areas of the display. White LEDs

have a forward voltage VF of 3.2 V to 3.6 V. The LEDs are clustered into strings, where a

defined number of LEDs is placed in series to build a string. The forward voltage of a

string is the multiple of VF of a single LED.

The supply voltage of mobile devices is usually lower than the voltage required to drive

LED strings for the LCD backlight. In smartphones normally a single lithium-ion cell

provides roughly 3.8 V. Therefore a voltage up conversion or a boosting of the battery

voltage towards a suitable voltage of the backlight is required. The voltage conversion has

to work like the LEDs themselves with a high energy efficiency. Furthermore small and low

weight components have to be chosen for the circuits.

The voltage up conversion is usually implemented in a switch mode topology. The

conversion block requires an electronic switch, a diode, an inductor and a capacitor as

discrete key components.

This application note focuses on the Schottky diodes that can be found in backlight

step-up converters. New package technologies and enhanced Schottky processes allow

more compact designs in combination with an excellent energy efficiency.

AN11470

Application note

All information provided in this document is subject to legal disclaimers.

Rev. 1 â 22 April 2014

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