LM3500

Synchronous Step-up DC/DC Converter for White LED

Applications

General Description

The LM3500 is a fixed-frequency step-up DC/DC converter

that is ideal for driving white LEDs for display backlighting

and other lighting functions. With fully intergrated synchro-

nous switching (no external schottky diode required) and a

low feedback voltage (500mV), power efficiency of the

LM3500 circuit has been optimized for lighting applications

in wireless phones and other portable products (single cell

Li-Ion or 3-cell NiMH battery supplies). The LM3500 oper-

ates with a fixed 1MHz switching frequency. When used with

ceramic input and output capacitors, the LM3500 provides a

small, low-noise, low-cost solution.

Two LM3500 options are available with different output volt-

age capabilities. The LM3500-21 has a maximum output

voltage of 21V and is typically suited for driving 4 or 5 white

LEDs in series. The LM3500-16 has a maximum output

voltage of 16V and is typically suited for driving 3 or 4 white

LEDs in series (maximum number of series LEDs dependent

on LED forward voltage). If the primary white LED network

should be disconnected, the LM3500 uses internal protec-

tion circuitry on the output to prevent a destructive over-

voltage event.

A single external resistor is used to set the maximum LED

current in LED-drive applications. The LED current can eas-

ily be adjusted using a pulse width modulated (PWM) signal

on the shutdown pin. In shutdown, the LM3500 completely

disconnects the input from output, creating total isolation and

preventing any leakage currents from trickling into the LEDs.

Features

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Synchronous rectification, high efficiency and no

external schottky diode required

n

Uses small surface mount components

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Can drive 2-5 white LEDs in series

(may function with more low-V

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2.7V to 7V input range

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Internal output over-voltage protection (OVP) circuitry,

with no external zener diode required

LM3500-16: 15.5V OVP; LM3500-21: 20.5V OVP.

n

True shutdown isolation

n

Input undervoltage lockout

n

Requires only small ceramic capacitors at the input and

output

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Thermal Shutdown

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0.1µA shutdown current

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Small 8-bump thin micro SMD package

Applications

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LCD Bias Supplies

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White LED Backlighting

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Handheld Devices

n

Digital Cameras

n

Portable Applications

Typical Application Circuit

20065701

February 2005

© 2005 National Semiconductor Corporation

DS200657

www.national.com