MC33441 Datasheet, PDF (4/12 Pages) ON Semiconductor – Electroluminescent Lamp Driver IC

MC33441
VDD = 2.65V
Lamp Freq. = 365Hz
EL Lamp = 2.2nF
X = 1ms/div, Y = 50V/div
Figure 3. Output Waveform vs. Time
+ ǒ ^{OPERATING DESCRIPTION}
General
The MC33441 is a DC–AC inverter integrated circuit for
FOSC
6
driving EL lamps. It can boost the supply voltage to the level
1
REXT
Ǔ + _CINTHz
1.667 10¹⁰
REXT
Hz
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 1.8V to 3.5V and is capable to
supply a typical 140Vpp AC output voltage. The standby
B FCOIL = FOSC 4
B FEL = FOSC 384
where CINT is about 10pF.
current of the device is typically 10nA which is ideal for low ^{Coil Driver}
power portable products. Externally, one inductor and one
The coil driver is basically a simplified boost converter. It
resistor are needed to generate the desirable voltage charge and ^{takes a higher frequency clock signal from the frequency divider}
to fine tune the oscillator’s frequency. This device is offered in ^{to turn on/off the main switch alternatively. When the main}
8–Pin TSSOP packages. The operating temperature is –20°C to ^{switch is on, current will flow through the coil to ground. Once}
70°C.
the switch is being turned off, the energy stored in the coil will
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 1.27V. This 1.27V 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
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 70mA & max. 150mA) 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
following equation:
http://onsemi.com
4