HV857L
External Component Description
External
Component
Diode
C
S
Capacitor
Description
Fast reverse recovery diode, BAS21 diode or equivalent.
0.003µF to 0.1µF, 100V capacitor to GND is used to store the energy transferred from the inductor.
The EL lamp frequency is controlled via an external R
EL
resistor connected between REL-Osc and VDD
of the device. The lamp frequency increases as R
EL
decreases. As the EL lamp frequency increases,
the amount of current drawn from the battery will increase and the output voltage V
CS
will decrease. The
color of the EL lamp is dependent upon its frequency.
R
EL
Resistor
A 2MΩ resistor would provide lamp frequency of 205Hz. Decreasing the R
EL
resistor by a factor of 2 will
increase the lamp frequency by a factor of 2.
f
EL
=
(2MΩ)(205Hz)
R
EL
R
SW
Resistor
The switching frequency of the converter is controlled via an external resistor, R
SW
, between RSW-Osc
and VDD of the device. The switching frequency increases as R
SW
decreases. With a given inductor, as
the switching frequency increases, the amount of current drawn from the battery will decrease and the
output voltage, V
CS
, will also decrease.
(560kΩ)(77kHz)
R
SW
f
SW
=
L
X
Inductor
The inductor L
X
is used to boost the low input voltage by inductive flyback. When the internal switch is
on, the inductor is being charged. When the internal switch is off, the charge stored in the inductor will
be transferred to the high voltage capacitor C
S
. The energy stored in the capacitor is connected to the
internal H-bridge, and therefore to the EL lamp. In general, smaller value inductors, which can handle
more current, are more suitable to drive larger size lamps. As the inductor value decreases, the switch-
ing frequency of the inductor (controlled by R
SW
) should be increased to avoid saturation.
A 220µH Murata (LQH32CN221) inductor with 8.4Ω series DC resistance is typically recommended.
For inductors with the same inductance value, but with lower series DC resistance, a lower R
SW
resistor
value is needed to prevent high current draw and inductor saturation.
As the EL lamp size increases, more current will be drawn from the battery to maintain high voltage
across the EL lamp. The input power, (V
IN
x I
IN
), will also increase. If the input power is greater than
the power dissipation of the package, an external resistor in series with one side of the lamp is recom-
mended to help reduce the package power dissipation.
Lamp
Split Supply Configuration
The HV857L can also be used for handheld devices operat-
ing from a battery where a regulated voltage is available.
This is shown in Figure 2. The regulated voltage can be used
to run the internal logic of the HV857L. The amount of cur-
rent necessary to run the internal logic is 150µA maximum at
a V
DD
of 5.0V. Therefore, the regulated voltage could easily
provide the current without being loaded down.
Enable/Disable Configuration
The HV857L can be easily enabled and disabled via a logic
control signal on the R
SW
and R
EL
resistors as shown in Fig-
ure 2 below. The control signal, which can be from a micro-
processor, has to track the V
DD
supply. R
SW
and R
EL
are typi-
cally very high values. Therefore, only 10’s of microamperes
will be drawn from the logic signal when it is at a logic high
(enable) state. When the microprocessor signal is high, the
device is enabled, and when the signal is low, it is disabled.
5
SUPERTEX [ Supertex, Inc ]
