Datasheet

MAX4990
lamp outputs. A slow turn-on effect is produced by dri-
ving EN high. Slow turn-on/-off time is related by the fol-
lowing equation:
t
ON
= 2.6 x R
DIM
x C
DIM
t
OFF
= 1.2 x R
DIM
x C
DIM
For this equation to be valid, R
DIM
/R
SLEW
must be
≤ 1.3.
C
CS
Capacitor Selection
C
CS
is the output of the boost converter and provides
the high-voltage source for the EL lamp. Connect a
3.3nF capacitor from CS to GND and place as close to
the CS input as possible. When using an inductor value
larger than 220µH, it may be necessary to increase the
C
CS
. For a L
X
= 470µH and C
LAMP
= 20nF, a C
CS
ranging from 3.3nF to 6.8nF is recommended.
C
EL
Capacitor Selection
The MAX4990 EL lamp output frequency is set by con-
necting a capacitor from the EL input to GND together
with a resistor from SLEW to GND or by driving the EL
input with an external clock (0 to +1.5V). The EL lamp
output frequency is related to the C
EL
capacitor by the
following equation:
For example, an R
SLEW
= 375kΩ and a C
EL
capacitor
value of 1000pF equals an EL lamp output frequency of
F
EL
= 217Hz.
C
SW
Capacitor Selection
The boost converter switching frequency is set by con-
necting a capacitor from the SW input to GND, together
with the resistance from the SLEW input to GND, or driving
the SW input with an external clock (0 to +1.5V). The
switching frequency of the boost converter is related to the
capacitor from SW to GND by the following equation:
Connect the SW input to GND to turn the switch oscilla-
tor of the boost converter off. Although the optimal f
SW
depends on the inductor value, the suggested f
SW
range is 20kHz to 150kHz.
Note: Driving SW with a logic-high causes LX to be dri-
ven to GND. Keeping SW high shorts LX to GND, caus-
ing the internal die temperature to increase. The
MAX4990 is protected by entering a thermal-shutdown
state. (See the
Thermal Short-Circuit Protection section.)
C
B
Capacitor Selection
Bypass V
DD
with a 0.1µF ceramic capacitor as close to
the IC as possible and a 4.7µF ceramic capacitor as
close to the inductor as possible
Diode Selection
Connect a diode, D
1
, from the LX node to CS to rectify
the boost voltage on CS. The diode should be a fast-
recovery diode that is tolerant to +150V.
EL Lamp Selection
EL lamps have a capacitance of approximately 2.5nF to
3.5nF per square inch. The MAX4990 effectively
charges capacitance ranging from 2nF to 20nF.
Applications Information
PCB Layout
Keep PCB traces as short as possible. Ensure that
bypass capacitors are as close to the device as possi-
ble. Use large ground planes where possible.
Chip Information
PROCESS: BiCMOS-DMOS
f
RC
SW
SLEW SW
=
×
361.
f
RC
EL
SLEW EL
=
×
0 0817.
High-Voltage, ±15kV ESD-Protected
Electroluminescent Lamp Driver
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