Adjusting OLED Bias Output Voltage
Adjusting the MAX8608’s output voltage changes the
OLED bias headroom. An analog input (CTRL) and a
resistor voltage-divider set the output voltage. The reg-
ulation voltage at FB_ is given by:
V
FB_
= V
CTRL
/ 5
The V
CTRL
voltage range for adjusting output voltage is 0
to 1.65V. Applying V
CTRL
voltage above 1.65V does not
increase the output voltage any further. To set the maxi-
mum output voltage, choose a value for R2 (Figure 1)
between 1kΩ and 10kΩ and then calculate R1 when
V
CTRL
is at its maximum as follows:
R1 = R2 ((V
OUT
/ 0.33) - 1)
For loop stability and good transient response, place a
feed-forward capacitor (1000pF typ) in parallel with R1.
The feed-forward capacitor value is not critical.
Calculate the approximate value as:
PWM Dimming Control
When both OUT1/FB1 and OUT2/FB2 are configured
for current regulation for WLED loads, CTRL can also
be used as a digital input, allowing LED brightness
control with a logic level (greater than 1.65V) PWM sig-
nal applied directly to CTRL. Use a 200Hz to 200kHz
frequency range. A 0% duty cycle corresponds to full
current. The error amplifier and compensation network
form a lowpass filter such that PWM dimming results in
DC current to the LEDs without the need for any addi-
tional RC filters. For this to work correctly, change the
compensation network to a 0.1µF capacitor from COMP
to GND (with R
COMP
= 0Ω).
Capacitor Selection
The exact values of input and output capacitors are not
critical. The typical value for the input capacitor is 2.2µF,
and the typical value for the output capacitor is 1µF.
Larger-value capacitors can be used to reduce input
and output ripple, but at the expense of size and higher
cost. C
COMP
stabilized the converter and controls soft-
start. Connect a 10kΩ resistor and 0.01µF capacitor in
series from COMP to GND. For applications with both
outputs configured for WLED current regulation, change
the compensation network to a 0.1µF capacitor from
COMP to GND (with R
COMP
= 0Ω).
Inductor Selection
Inductor values range from 10µH to 47µH. When using
1MHz operation, a 22µH inductor optimizes the efficien-
cy for most applications. When using 500kHz opera-
tion, a 47µH inductor optimizes the efficiency for most
applications. With input voltages near 5V, a larger value
of inductance may be more efficient. To prevent core
saturation, ensure that the inductor saturation current
rating exceeds the peak inductor current for the appli-
cation. Calculate the peak inductor current for 1MHz
switching with the following formula:
Schottky Diode Selection
The MAX8608’s high switching frequency demands a
high-speed rectification diode (D1) for optimum effi-
ciency. A Schottky diode is recommended due to its
fast recovery time and low forward-voltage drop.
Ensure that the diode’s average and peak current rat-
ing exceed the average output current and peak induc-
tor current. In addition, the diode’s reverse breakdown
voltage must exceed V
OVP
. The RMS diode current can
be calculated from:
Applications Information
PC Board Layout
Due to fast-switching waveforms and high current
paths, careful PC board layout is required. An evalua-
tion kit (MAX8608YEVKIT) is available to aid design.
When laying out a board, minimize trace lengths
between the IC and R
SENSE
(and/or feedback resis-
tors), the inductor, the diode, the input capacitor, and
the output capacitor. Keep traces short, direct, and
wide. Keep noisy traces, such as the LX node trace,
away from FBA and FBB. The IN bypass capacitor
(C
IN
) should be placed as close to the IC as possible.
PGND and GND should be connected directly to the
exposed paddle underneath the IC. The ground con-
nections of C
IN
and C
OVP
should be as close together
as possible. The traces from IN to the inductor and
from the Schottky diode to the LEDs may be longer.
.
.
