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High-Efficiency Step-Up Converters
for White LED Main and Subdisplay Backlighting
t SS COMP × ?
= C
? 5 μ A ?
V OUT 1 MAX ) × I LED ( MAX ) V IN ( MIN ) × 0 . 8 μ s
0 . 8 × V IN ( MIN )
I PEAK ? +
path between the input and the LEDs through the
inductor and Schottky diode. To ensure the LEDs
remain off when the MAX1582/MAX1582Y are in shut-
down, the minimum input forward voltage of the LED
array must exceed the maximum input voltage.
Typically, the leakage current through the LEDs in shut-
down is less than 1μA.
Soft-Start
The MAX1582/MAX1582Y attain soft-start by charging
C COMP gradually with a current source. When V COMP
rises above 1.25V, the internal MOSFET begins switch-
ing, but at a reduced duty cycle. When V COMP rises
above 2.25V, the duty cycle is at its maximum.
Overvoltage Protection
OVLO occurs when V OUT1 rises above 27V (18V for the
MAX1582Y). The protection circuitry stops the internal
MOSFET from switching and causes V COMP to decay to
GND. The device comes out of OVLO and into soft-start
when V OUT1 falls below 25V (16.7V for the MAX1582Y).
Design Procedure
Adjusting LED Current
Adjusting the MAX1582/MAX1582Ys’ output current
changes the brightness of the LEDs. The LED current is
set by the voltage at CTRL (V CTRL ) and the sense-
resistor value (R SENSE ):
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
0.1μF. Larger value capacitors can be used to reduce
input and output ripple, but at the expense of size and
higher cost.
C COMP stabilizes the converter and controls soft-start.
Connect a 0.022μF capacitor from COMP to GND. The
minimum value for C COMP is C OUT / 10. The soft-start
time is found from:
? 1 . 25 V ?
?
Inductor Selection
Recommended inductor values range from 10μH to
47μH. A 22μH inductor optimizes the efficiency for most
applications, while maintaining a low 15mV P-P input rip-
ple. With input voltages near 5V, a larger value of induc-
tance may be more efficient. To prevent core saturation,
ensure that the inductor saturation current rating exceeds
the peak inductor current for the application. Calculate
the peak inductor current with the following formula:
2 × L
I LED =
V CTRL
10 × R SENSE
Schottky Diode Selection
The MAX1582/MAX1582Ys’ high switching frequency
The V CTRL voltage range for adjusting output current is
0 to +1.62V. To set the maximum current, calculate
R SENSE when V CTRL is at its maximum as follows:
demands a high-speed rectification diode (D1) for opti-
mum efficiency. 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
R SENSE =
1 . 62
10 × I LED ( MAX )
rating exceeds the average output current and peak
inductor current. In addition, the diode’s reverse break-
down voltage must exceed V OUT1 . The RMS diode
Power dissipation in R SENSE is typically less than 5mW;
current can be calculated from:
therefore, a standard chip resistor is sufficient.
PWM Dimming Control
CTRL is also used as a digital input, allowing LED
I DIODE ( RMS ) ?
I OUT 1 × I PEAK
brightness control with a logic-level PWM signal
applied directly to CTRL. The frequency range is from
200Hz to 200kHz, while 0% duty cycle corresponds to
zero current and 100% duty cycle corresponds to full
current. The error amplifier and compensation capaci-
tor form a lowpass filter, so PWM dimming results in DC
current to the LEDs without any additional RC filters
required.
Applications Information
PC Board Layout
Due to fast-switching waveforms and high-current
paths, careful PC board layout is required. An evalua-
tion kit (MAX1582EVKIT) is available to speed design.
_______________________________________________________________________________________
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