MC33441 데이터 시트보기 (PDF) - ON Semiconductor

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MC33441

Electroluminescent Lamp Driver IC

ON Semiconductor 

MC33441

VDD = 2.65 V

Lamp Freq. = 365 Hz

EL Lamp = 2.2 nF

X = 1 ms/div, Y = 50 V/div

Figure 3. Output Waveform versus Time

OPERATING DESCRIPTION

General

The MC33441 is a DC−AC inverter integrated circuit for

driving EL lamps. It can boost the supply voltage to the level

required by EL lamps and also provide high voltage AC lamp

excitation. It consists of an oscillator, a frequency divider, a coil

driving circuit and a switched H−bridge network. The input

supply voltage range is from 1.8 V to 3.5 V and is capable to

supply a typical 140 Vpp AC output voltage. The standby

current of the device is typically 10 nA which is ideal for low

power portable products. Externally, one inductor and one

resistor are needed to generate the desirable voltage charge and

to fine tune the oscillator’s frequency. This device is offered in

8−Pin TSSOP packages. The operating temperature is −20°C to

70°C.

Oscillator and Frequency Divider

Two circuits are put together to form the oscillator. They are

Vref and Ibias. The functionality of Vref block is to generate a

zero temperature coefficient (TC) voltage reference which is

about 1.27 V. This 1.27 V will then be used in Ibias circuit to

provide current biasing to all of the internal circuits with the

value equal to Vref divided by an internal resistor. Besides of

that, an external resistor is also connected to this circuit block for

setting the oscillator’s frequency. The temperature coefficient is

dominated by the value of that resistor. Therefore, if a low TC

resistor is used, the oscillator frequency’s TC can be kept low.

The current mirrors with the induced current equal to the Vref

divided by an external resistor are used to charge and discharge

an internal capacitor to provide a 50% duty cycle clock signal.

This original clock pulse will then be fed into the frequency

divider which will generate two additional clock signals with

different frequency and duty cycle to the coil−driver and the

H−bridge circuits. The oscillator frequency is governed by the

following equation:

ǒ Ǔ FOSC + 6

1

REXT

CINT Hz

+

1.667 1010

REXT

Hz

FCOIL = FOSC B 4

FEL = FOSC B 384

where CINT is about 10pF.

Coil Driver

The coil driver is basically a simplified boost converter. It

takes a higher frequency clock signal from the frequency divider

to turn on/off the main switch alternatively. When the main

switch is on, current will flow through the coil to ground. Once

the switch is being turned off, the energy stored in the coil will

be released to the external capacitor (EL lamp) through an

internal diode. According to the frequency of the clock signals

between the coil driver and the H−bridge, the external capacitor

(EL lamp) will be charging to the desirable level.

Current limit circuit (typical 70 mA & max. 150 mA) is

implemented in this device. Since the current through the coil

will increase corresponding to the input voltage, if the input

voltage is high and the inductance of the coil is small, the coil

can be saturated. The current limit feature is used to avoid this

happen. The main switch is parallel to a much smaller switch

which has their collector and their base connected together.

However, the emitter of the smaller switch is tied to a sensing

resistor while the emitter of the main switch is connected to

ground. The coil current will split into two according to the

sizing ratio between the main and the smaller switch. The

current through the smaller switch will also flow through the

sensing resistor and generates a voltage. If the voltage across this

sensing resistor is above the pre−set value, then both switches

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