SP4426 Datasheet, PDF (4/11 Pages) Sipex Corporation – Electroluminescent Lamp Driver with High Output Drivers

duty cycle and range from Vdd-1V to ground. An
external clock signal may be desirable in order to
synchronize any parasitic switching noise with the
system clock. The maximum external clock
frequency that can be supplied is 400kHz.
The coil is an external component connected from
V_battery to pin 9 of the SP4426. Energy is stored in
the coil according to the equation E_L=1/2LI² where
I, the the first approximation is (V_bat-V_CE)R_L. Where
V is approximately ) 0.5 volts for the internal coil
CE
driver transistor, and R is the series resistance of
L
the inductor. In order to maximize the energy
stored in the inductor and subsequently recovered
during the switching off of the coil transistor, it
is expedient to use the highest value battery,
the largest inductor with the smallest series
resistance that the system constraints will permit.
This energy recovery is directly related to the
brightness of the lamp output. There are great
variations among coils; caused by magnetic
material differences, winding differences and
parasitic capacitance. The Sipex SP4426 is final
tested using a 9mH/35Ω coil from Hitachi. For
suggested coil sources see page 8.
The supply Vdd can range from 2.2 to 6.0V. It is
not necessary that Vdd=V . V should not
battery battery
exceed max coil current specification. The
majority of the current goes through the coil and is
typically much greater than Idd.
The F signal controls a switch that connects the
coil
end of the coil at pin 5 to ground or to open circuit.
The F signal is a 94% duty cycle signal switching
coil
at the oscillator frequency. During the time when
the F signal is high, the coil is connected from
coil
V to ground and a charged magnetic field is
battery
created in the coil. During the low part of F , the
coil
ground connection is switched open, the field
collapses and the energy in the inductor is forced
to flow toward the lamp. F_coil will send 16 of these
charge pulses (see figure 2 page 8) to the lamp,
each pulse increases the voltage drop across the
lamp in discrete steps. As the voltage potential
approaches its maximum, the steps become smaller
(see figure 1 page 8).
For maximum performance a fast recovery diode
is recommended and should be connected between
pins 5 and 6 (cathode side). The low reverse
voltage capacitance of the fast recovery diode
minimizes the voltage that is charged and
discharged across it and this increases the energy
transferred to the lamp.
The H-bridge consists of two SCR structures that
act as high voltage switches. These two switches
control the polarity of how the lamp is charged.
The SCR switches are controlled by the F signal
lamp
which is the oscillator frequency divided by 64.
For a 20kHz oscillator, F =300Hz.
lamp
PDA Application
3 Volt
-
Battery ₊
Typical SP4426 Application Circuit
SP4426DS/01
Switch
IN418
1
HON C1
N/C N/C
Vss C2
CN/oCsc Vdd
Coil N/C
D1 N/C
EL1 EL2
Optional
1800pF
180pF
EL LAMP
6 SQ. IN.
SP4426 Electroluminescent Lamp Driver
4
© Copyright 1997 Sipex Corporation