MIC4834 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – Low Noise Dual 220 VPP EL Driver

Micrel
Application Information
The MIC4834 is designed to use an inductance with a
value between 100µH to 330µH. Choosing the right
inductor is always a balance of size, inductance,
efficiency, current rating and cost. A TDK (VLS4012T-
221M) 220µH inductor is recommended based on size,
efficiency and current rating.
Generally, the lower the inductance, the more current
the inductor can handle. Lowering the inductance allows
the boost regulator to draw more input current to deliver
more energy every switching cycle. As a result, a lower
inductance may be used to drive larger panels or
brighten similar sized panels. However, caution is
required as using a low inductance with a low switching
frequency may cause the voltage at the switch node and
the CS pin to exceed the absolute maximum rating. If
the application uses a low input voltage (2.3V to 3V),
then a lower value inductor, such as 100µH, may be
used in order to drive the EL panel at maximum
brightness.
Diode
The diode must have a high reverse voltage (150V),
since the output voltage at the CS pin can reach up to
130V. A fast switching diode with lower forward voltage
and higher reverse voltage (150V), such as
BAV20WS/BAS20W, can be used to enhance efficiency.
MIC4834
Output Capacitor
Low ESR capacitors should be used at the regulated
boost output (CS pin), to minimize the switching output
ripple voltage. The larger the output capacitance, the
lower the output ripple at the CS pin. The reduced
output ripple, at the CS pin, along with a low ESR
capacitor improves the efficiency of the MIC4834 circuit.
Selection of the capacitor value depends upon the peak
inductor current, inductor size, and the load. The
MIC4834 is designed for use with an output capacitance
as low as 2.2nF. For minimum audible noise, the use of
a C0G/NPO dielectric output capacitor is recommended.
TDK and AVX offer C0G/NPO dielectric capacitors in
capacitance up to 2.7nF at 200V to 250V voltage rating
in 0805 size.
EL Panel Terminals (ELA, ELB, COM)
The two EL panels are connected from ELA to COM and
ELB to COM. The ELA and ELB terminals are in phase
with each other, while the COM is out of phase with both
ELA and ELB. Since ELA and COM are out of phase,
the high voltage generated by the boost regulator is
alternated across ELA and COM by the H-Bridge. The
frequency of each cycle is internally fixed at 225Hz. The
alternating 220V peak-to-peak causes the EL panel to
emit light. Similarly, the ELB and COM are also out of
phase and allows a second EL panel to be driven at the
same time. Both EL panels may operate independently
from each other and do not have to be the same size.
For component selection, Table 2 lists recommended
values for various panel sizes up to a total of 6 in² (For
example, two 3 in² panels). Driving overly large panels
will result in a dimmer display, but will not cause damage
to the device.
October 2008
7
M9999-100908-A