MIC4826YMM datasheet(9/22 Pages) MICROCHIP

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DS20006134B-page 9

MIC4826

4.0

FUNCTIONAL DESCRIPTION

See

Section 5.0, Application

Information

for

component selection and pre-designed circuits.

4.1

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 10 nA. The high voltage EL driver has two

internal oscillators to control the switching MOSFET

and the H-bridge driver. Both of the internal oscillators’

frequencies can be individually programmed through

the external resistors to maximize the efficiency and

the brightness of the lamps.

4.2

Regulation

Referring to Figure 4-1, initially power is applied to

VDD. 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 VOUT+VD1. The energy in the inductor is then

discharged into the COUT 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 oscillator.

When the EL oscillator is enabled, VA 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.

FIGURE 4-1:

108 Hz Typical Output

Waveform.

4.3

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 Figure 2-8 or use equation Equation 4-1. The

switching frequency range is 8 kHz to 200 kHz, with an

accuracy of ±20%.

EQUATION 4-1:

4.4

EL Frequency

The EL lamp frequency is controlled via an external

resistor connected between REL pin and VDD pin of

the device. As the lamp frequency increases, the

resistor value decreases. For resistor value selections,

see Figure 2-9 or use equation Equation 4-2. The EL

frequency range is 60 Hz to 1000 Hz, with an accuracy

of ±20%.

EQUATION 4-2:

TIME (2ms/div)

IN = 3.0V

L = 220mH

OUT = 0.01mF

Lamp = 2in2

SW = 332k

EL = 3.32M

fSW kHz





RSW M





-------------------------

fEL Hz





360

REL M





------------------------

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