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MC33441 Datasheet PDF : 12 Pages
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MC33441
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 1.8V to 3.5V and is capable to supply a
typical 140Vpp AC output voltage. The standby current of the
device is typically 10nA 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 1.27V. 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
+ ǒ Ǔ + circuits. The oscillator frequency is governed by the following
equation:
FOSC
6
1
REXT
CINT Hz
1.667 1010 Hz
REXT
B FCOIL = FOSC 4
B FEL = FOSC 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 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 will be turned off and the
energy will release to the EL lamp. And, those switches will
remain off until the next clock cycle.
H–Bridge Network
To achieve the 140V peak–to–peak voltage, H–bridge network
is used to charge and discharge the EL lamp. The switching
frequency of the bridge network is controlled by a clock signal
from the divider with its frequency much lower than the one to the
coil–driver. Moreover, to reduce the current consumption, the
biasing current to the two low–side switches of the H–bridge is
not activated until the coil–driver circuit needed to release the
energy to the EL lamp. Then, the biasing circuit will be on and be
ready before the main switch in the coil–driver really starts to turn
off.
External Components
System designer will base on the application to decide the
size and the type of the EL lamp to be used. The external
resistance (REXT) at RT1 pin determines the excitation frequency
(FEL) for the lamp. The relationship between REXT and the
frequency is:
B FEL = FOSC 384
By substitute the equation of FOSC from Oscillator &
Frequency Divider.
+ FEL
4.341 107
REXT
Hz
so
+ W REXT
4.341 107
FEL
Moreover, if a low TC resistor is used, the oscillator
frequency’s TC can be kept low. The filter capacitor is to provide
a smooth and more stable output waveform for the EL lamp. The
value of this capacitor depends on the input voltage and the coil’s
inductance value. Equations below can be used to estimate filter
capacitor’s value at different input voltage.
Best Case Approximation for the Filter Capacitor:
+ * ń CFILTER 0.026 (Vin VSW)2 (L FOSC 2)
Worst Case Approximation for the Filter Capacitor:
+ * ń CFILTER 0.085 (Vin VSW)2 (L FOSC 2)
where VIN is the input voltage, VSW is voltage across the
switch when it is on, L is the coil’s value and FOSC is the clock
frequency.
Measurement below is recorded with the condition: coil =
1mH, EL lamp = 2.2nF and at room temperature.
Table 1: Reference for CFILTER
VDD
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 1.8V
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 2.0V
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 2.5V
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 3.0V
REXT
W 100K–130K
W 100K–130K
W 100K–130K
W 100K–130K
CFILTER
5n–10nF
10n–22nF
10n–22nF
22nF–33nF
4
MOTOROLA ANALOG IC DEVICE DATA
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