Rainbow Electronics MAX14514 - Manual (Page 13)

R
SLEW
Resistor Selection
To help reduce audible noise emission by the EL
lamps, the MAX14514 features a slew-rate control
input (SLEW) that allows the user to set the slew rate of
high-voltage outputs, V
1
, V
2
, and COM, by connecting
a resistor, R
SLEW
, from the SLEW input to GND.
Decreasing the value of R SLEW increases the slew rate
at the EL lamp output. Increasing the value of R SLEW
decreases the slew rate at the EL lamp outputs. The
output slew rate is related to R
SLEW by the following
equation:
SlewRate(V/100µs) = 12/R SLEW (M Ω)
The ideal value for a given design varies depending on
lamp size and mechanical enclosure. Typically, the
best slew rate for minimizing audible noise is between
10V/100µs and 20V/100µs. This results in R
SLEW
values
ranging from 1.2M Ω to 600kΩ. For example, if the
desired slew rate is 20(V/100µs), this leads to an R SLEW
value of 12/20(V/100µs) = 600k
Ω.
Note: Connecting R SLEW to GND does not damage the
device. However, for the device to operate correctly,
R
SLEW should be in the 100k Ω to 2.2MΩ range. R
SLEW
also affects the frequency of the boost converter (see
the
C
SW
Capacitor Selection section), the frequency of
the EL lamp (see the C EL Capacitor Selection section),
and the peak-to-peak voltage of the EL lamp.
C CAP Capacitor Selection
The MAX14514 provides a slow turn-on/-off feature that
is enabled by connecting a capacitor from CAP to
GND. For fast turn-on/-off, connect CAP to V
DD . Slow
turn-on/-off time is related by the following equation:
t ON/OFF = 0.27 x C CAP x R SLEW
R DIM Resistor Selection
The MAX14514 features dimming control inputs, DIM1
and DIM2, to control the peak-to-peak voltages on the
lamp outputs V
1 , V 2 , and COM. DIM_ is controlled by a
PWM signal, DC voltage, or by a resistor connected
from DIM_ to GND. When using a resistor, the output
voltage is related by the following equation:
V_ - V COM = 260 x R DIM /R SLEW
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
C
CS
. For a 470µH inductor and C
LAMP_TOTAL
= 20nF, a
C CS ranging from 3.3nF to 6.8nF is recommended.
C EL Capacitor Selection
The MAX14514 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. The EL lamp output fre-
quency is related to the C
EL capacitor by the following
equation:
f
EL
= 0.08175/(R
SLEW
x C
EL
)
For example, an R SLEW = 375k Ω and a C
EL capacitor
value of 872pF equals an EL lamp output frequency of
f
EL = 250Hz.
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 dri-
ving the SW input with an external clock (0 to +1.5V).
The switching frequency of the boost converter is relat-
ed to the capacitor from SW to GND by the following
equation:
f SW = 3.6/(R SLEW x C SW )
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
MAX14514 is protected by entering a thermal-shutdown
state. (See the
Thermal Short-Circuit Protection section.)
Bypass 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 MAX14514 effectively
charges capacitance ranging from 2nF to 20nF.
MAX14514
Dual Electroluminescent Lamp Driver
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