MIC4826 datasheet(6/12 Pages) MICREL

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MIC4826

Micrel

MIC4826

August 2004

Functional Diagram

Oscillator

GND

EL LAMP

COUT

220

µH

RWS

RSW

CIN

REL

Switch

Oscillator

VDD

VREF

VIN

Figure 1. MIC4826 Block Diagram

Functional Description

See “Application Information” for component selection and

pre-designed circuits.

Overview

The MIC4826 is a high voltage EL driver with an AC output

voltage of 160V peak-to-peak capable of driving EL lamps up

to 6 in2 . Input supply current for the MIC4826 is typically 21

µA

with a typical shutdown current of 10nA. The high voltage EL

driver has two internal oscillators to control the switching

MOSFET and the H-bridge driver. Both of the internal oscil-

lators’ frequencies can be individually programmed through

the external resistors to maximize the efficiency and the

brightness of the lamps.

Regulation

Referring to Figure 1, initially power is applied to V

DD. The

internal feedback voltage is less than the reference voltage

causing the internal comparator to go low which enables the

switching MOSFET’s oscillator. When the switching MOSFET

turns on, current flows through the inductor and into the

switch. The switching MOSFET will typically turn on for 90%

of the switching frequency. During the on-time, energy is

stored in the inductor. When the switching MOSFET turns off,

current flowing into the inductor forces the voltage across the

inductor to reverse polarity. The voltage across the inductor

rises until the external diode conducts and clamps the voltage

at V

OUT+VD1. The energy in the inductor is then discharged

into the C

OUT capacitor. The internal comparator continues to

turn the switching MOSFET on and off until the internal

feedback voltage is above the reference voltage. Once the

internal feedback voltage is above the reference voltage, the

internal comparator turns off the switching MOSFET’s oscil-

lator.

When the EL oscillator is enabled, V

A and VB switch in

opposite states to achieve a 160V peak-to-peak AC output

signal. The external resistor that connects to the REL pin

determines the EL frequency.

TIME (2ms/div)

IN = 3.0V

L = 220

µH

OUT = 0.01µF

Lamp = 2in2

SW = 332k

EL = 3.32M

Figure 2. 108Hz Typical Output Waveform

Switching Frequency

The switching frequency of the converter is controlled via an

external resistor between RSW pin and VDD pin of the

device. The switching frequency increases as the resistor

value decreases. For resistor value selections, see “Typical

Characteristics: Switching Frequency vs. Switch Resistor”

or use the equation below. The switching frequency range is

8kHz to 200kHz, with an accuracy of

±20%.

fkHz

()

Ω

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