MAX1582 Datasheet, PDF (7/10 Pages) Maxim Integrated Products – High-Efficiency, 26V Step-Up Converter for White LED Main and Subdisplay Backlighting

High-Efficiency, 26V Step-Up Converter
for White LED Main and Subdisplay Backlighting
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.
Soft-Start
The MAX1582 attains soft-start by charging C_COMP
gradually with a current source. When V_COMP rises
above 1.25V, the internal MOSFET begins switching,
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. The protec-
tion 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.
Design Procedure
Adjusting LED Current
Adjusting the MAX1582’s 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):
I_LED
=
V_CTRL
10 × R_SENSE
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:
R_SENSE
=
1.62
10 × I_LED(MAX)
Power dissipation in R_SENSE is typically less than 5mW;
therefore, a standard chip resistor is sufficient.
PWM Dimming Control
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.
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:
t_SS
=
C_COMP
×
 1.25V 
 5µA 
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:
I_PEAK ≅
V_OUT1(MAX) × I_LED(MAX)
0.8 × V_IN(MIN)
+
V_IN(MIN) × 0.8µs
2× L
Schottky Diode Selection
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_OUT1. The RMS diode
current can be calculated from:
I_DIODE(RMS) ≅ I_OUT1 × I_PEAK
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.
_______________________________________________________________________________________ 7