MAX158 Maxim, MAX158 Datasheet - Page 7

• Manufacturer Part Number: MAX158
• Description: High-Efficiency / 26V Step-Up Converter for White LED Main and Subdisplay Backlighting
• Manufacturer: Maxim

between the input and the LEDs through the inductor
and Schottky diode. To ensure the LEDs remain off
when the MAX1582 is in shutdown, the minimum input
forward voltage of the LED array must exceed the maxi-
mum input voltage. Typically, the leakage current
through the LEDs in shutdown is less than 1µA.
The MAX1582 attains soft-start by charging C
gradually with a current source. When V
above 1.25V, the internal MOSFET begins switching,
but at a reduced duty cycle. When V
2.25V, the duty cycle is at its maximum.
OVLO occurs when V
tion circuitry stops the internal MOSFET from switching
and causes V
comes out of OVLO and into soft-start when V
below 25V.
Adjusting the MAX1582’s output current changes the
brightness of the LEDs. The LED current is set by the
voltage at CTRL (V
(R
The V
0 to +1.62V. To set the maximum current, calculate
R
Power dissipation in R
therefore, a standard chip resistor is sufficient.
CTRL is also used as a digital input, allowing LED
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.
SENSE
SENSE
for White LED Main and Subdisplay Backlighting
CTRL
when V
):
voltage range for adjusting output current is
R
SENSE
COMP
CTRL
I
LED
_______________________________________________________________________________________
CTRL
is at its maximum as follows:
OUT1
=
SENSE
to decay to GND. The device
=
) and the sense-resistor value
10
High-Efficiency, 26V Step-Up Converter
Overvoltage Protection
10
Adjusting LED Current
PWM Dimming Control
rises above 27V. The protec-
Design Procedure
×
V
is typically less than 5mW;
×
CTRL
R
1 62
I
LED MAX
SENSE
.
(
COMP
)
Soft-Start
COMP
rises above
OUT1
COMP
rises
falls
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
Connect a 0.022µF capacitor from COMP to GND. The
minimum value for C
time is found from:
Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
applications, while maintaining a low 15mV
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:
The MAX1582’s high switching frequency demands a
high-speed rectification diode (D1) for optimum
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
rating exceeds the average output current and peak
inductor current. In addition, the diode’s reverse break-
down voltage must exceed V
current can be calculated from:
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.
I
PEAK
COMP
≅
stabilizes the converter and controls soft-start.
V
OUT MAX
I
DIODE RMS
0 8
t
1
SS
(
.
Applications Information
×
(
=
)
V
COMP
C
×
IN MIN
Schottky Diode Selection
)
COMP
(
I
LED MAX
≅
is C
)
(
I
Capacitor Selection
×
OUT
Inductor Selection
OUT1
OUT



)
1 25
PC Board Layout
1
5
+
.
µ
×
/ 10. The soft-start
. The RMS diode
A
V
V
IN MIN
I
PEAK



(
P-P
2
)
×
input rip-
×
L
0 8
.
µ
s
7