Increasing the voltage on the DIM input increases the
peak-to-peak voltage, and decreasing the voltage on
the input decreases the peak-to-peak voltage. The EL
lamp peak-to-peak voltage is also adjusted by applying
a PWM signal to the DIM input. The duty cycle of the
PWM determines the EL lamp output peak-to-peak volt-
age. As the duty cycle is increased, the peak-to-peak
output voltage is increased, and as the duty cycle is
decreased, the peak-to-peak voltage is decreased. The
MAX4990 also features a slow turn-on and slow turn-off
time feature that is enabled by connecting a resistor and
capacitor from DIM to GND (see the
Typical Application
Circuits
and
the R
DIM
Resistor and C
DIM
Capacitor
Selection
section). This slow turn-on/-off feature causes
the peak-to-peak voltage of the EL outputs to slowly rise
from zero to the maximum set value when the device is
enabled. This feature also causes the peak-to-peak volt-
age of the EL outputs to fall from the maximum set value
to zero when the device is placed into shutdown. The
slow rise and fall of the peak-to-peak EL output voltage
creates a soft fade-on and fade-off of the EL lamp,
rather than an abrupt change in brightness.
Boost Converter
The MAX4990 boost converter consists of an external
inductor from VDD to the LX input, an internal DMOS
switch, an external diode from LX to the CS output, an
external capacitor from the CS output to GND, and the
EL lamp, CLAMP, connected to the EL lamp outputs.
When the DMOS switch is turned on, LX is connected
to GND, and the inductor is charged. When the DMOS
switch is turned off, the energy stored in the inductor is
transferred to the capacitor CCS and the EL lamp.
Note: Keeping SW high shorts LX to GND, causing the
internal die temperature to increase. The MAX4990 is
protected by entering a thermal-shutdown state (See
the
Thermal Short-Circuit Protection
section.)
The MAX4990 boost converter frequency uses an inter-
nal switch oscillator to set the desired frequency of the
boost converter. The boost converter frequency is
adjusted by either 1) the combination of a resistor from
SLEW to GND and an external capacitor from SW to
GND, or 2) by driving a PWM signal directly into the SW
input. When SW is driven with an external PWM signal
at a suggested 90% duty cycle, the boost converter fre-
quency is changed to the frequency of the external
PWM signal. (See the
C
SW
Capacitor Selection
section
for choosing the CSW capacitor value.)
Dimming Control
The MAX4990 features a dimming control input, DIM,
that controls the peak-to-peak voltage on the lamp out-
puts VAand VB. DIM is controlled by a resistor con-
nected from the DIM input to GND, a PWM signal
applied to the DIM input, or a DC voltage applied to the
DIM input. (See the
R
DIM
Resistor and C
DIM
Capacitor
Selection
section.)
The duty cycle of a PWM signal to the DIM input is
internally translated into a DC voltage with the 0 to
+1.22V range. The DIM input accepts the frequency
range of 200kHz to 1MHz. As the duty cycle increases,
the peak-to-peak voltage of the output increases, and
as the duty cycle decreases, the peak-to-peak voltage
of the output decreases.
The peak-to-peak voltage is adjusted by applying a DC
voltage to the DIM input. Increasing the voltage on DIM
increases the peak-to-peak output, and decreasing the
voltage on DIM decreases the peak-to-peak output volt-
age.
The DIM input, in combination with the EN input, con-
trols the shutdown mode of the MAX4990 shutdown.
(See the
Shutdown
section.)
Slow Turn-On, Slow Turn-Off
The MAX4990 provides a slow turn-on/-off feature by
connecting a resistor in parallel with a capacitor con-
nected from the DIM input to GND (see the
R
DIM
Resistor and C
DIM
Capacitor Selection
section). When
EN is driven high, the reference current IB(set by
RSLEW) is used to charge capacitor CDIM. When EN is
driven to GND, IBis removed, and the voltage on the
capacitor CDIM and resistor decays with a time con-
stant of RDIM x CDIM. A slow turn-on effect is seen by
driving EN high. The slow rise and fall of the voltage on
DIM during transitions on the EN input modulates the
peak-to-peak voltage of the EL outputs, creating a soft
fade-on/-off effect at the EL lamp.
Shutdown
The MAX4990 features an enable logic input, EN, to
enable and disable the device. To enable the device,
apply +1.2V or greater to the EN input and +0.35V or
greater to the DIM input. To place the device in shut-
down, apply +0.2V or less to the EN input, and +0.15V
or less to the DIM input.
Undervoltage Lockout (UVL0)
The MAX4990 has a UVLO threshold of +2.1V (typ).
When VDD falls below +2.1V (typ), the device enters a
nonoperative mode.
Thermal Short-Circuit Protection
The MAX4990 enters a nonoperative mode if the inter-
nal die temperature of the device reaches or exceeds
+158°C (typ). The device turns back on when the inter-
nal die temperature cools to +150°C.
MAX4990
High-Voltage, ±15kV ESD-Protected
Electroluminescent Lamp Driver
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