SP4480 Datasheet, PDF (5/13 Pages) Sipex Corporation – Dual Electroluminscent Lamp Driver

VBATT
C1
0.1µF
ELEN1
ELEN2
ROSC
402kΩ
1nF
1
2
VDD
14
SP4480
8
COIL
ROSC
3
9
CAP
7
VSS
12 13
11 10
EL1 EL2 EL3 EL4
* optional device
L1
470µH
D1
1N4148
CINT
1800pF
EL Lamp
EL Lamp
Figure 2: Typical Application Circuit of the SP4480
lighting situations. These features make EL
ideal for attractive, battery powered products.
THEORY OF OPERATION
Coil Switch
The SP4480 has an inductor-based boost con-
verter to generate the high voltage used to drive
the EL lamp. Energy is stored in the inductor
according to the equation E_L = 1/2 (LI_pk2) where
I_pk = (t_ON) (V_BATT - V_CEsat) /L. An internal oscil-
lator controls the coil switch. During the time
the coil switch is on, the coil is connected
between V_DD and the saturation voltage of the
coil switch and a magnetic field develops in the
coil. When the coil switch turns off, the switch
opens, the magnetic field collapses and the volt-
age across the coil rises. The internal diode
forward biases when the coil voltage rises above
the H-Bridge voltage and the energy enters the
EL lamp. Each pulse increases the voltage
across the lamp in discrete steps.
As the voltage approaches its maximum, the
steps become smaller. (see figure 4).
The brightness of the EL lamp output is directly
related to energy recovery in the boost con-
verter. There are many variations among coils
such as magnetic core differences, winding dif-
ferences and parasitic capacitances. For sug-
gested coil suppliers refer to page 10.
Oscillator
The internal oscillator generates a high fre-
quency clock used by the boost converter and H-
Bridge. An external resistor from VDD to
ROSC sets the oscillator frequency. Typically
a 402kΩ resistor sets the frequency to 57kHz.
The high frequency clock directly controls the
coil switch. This high frequency clock is di-
vided by 128 to generate a low frequency clock
which controls the EL H-Bridge and sets the EL
lamp frequency. The oscillator has low sensi-
tivity to temperature and supply voltage varia-
tions, increasing the performance of the EL
SP4480DS/06
SP4480 Dual Electroluminescent Lamp Driver
© Copyright 2000 Sipex Corporation
5