MAX1553_15 Datasheet, PDF (6/10 Pages) Maxim Integrated Products – High-Efficiency, 40V Step-Up Converters for 2 to 10 White LEDs

MAX1553/MAX1554
High-Efficiency, 40V Step-Up
Converters for 2 to 10 White LEDs
Pin Description
PIN NAME
1
GND Ground
FUNCTION
2
V_CC Voltage-Supply Input. 2.7V to 5.5V. The IC is powered from V_CC.
3
EN Enable Input. Drive high or connect to V_CC to enable the IC. Drive EN low for shutdown.
Brightness-Control Input. Either an analog or PWM control signal can be used. The LED current can be
4
BRT controlled over a 10 to 1 range. The PWM signal must be between 100Hz and 10kHz, and must have an
amplitude greater than 1.72V.
5
FB
Feedback Input. Connect to the cathode of the LED string and connect a resistor from FB to GND to set the
LED current.
Soft-Start Timing-Control Input. Connect a capacitor from SS to GND to control soft-start timing. See the Soft-
6
SS Start section for information on selecting the soft-start capacitor. SS is pulled to ground with an internal 200Ω
switch when EN is low.
7
OV
Overvoltage Sense. Connect to a resistor-divider from the anode of the LED string to set the overvoltage
threshold. See Figures 1, 2, and 3.
8
LX Inductor Connection. Connect to the inductor and diode. LX is high impedance when EN is low.
—
EP Exposed Pad. Connect to GND.
Detailed Description
Control Scheme
The MAX1553/MAX1554 utilize a minimum off-time, cur-
rent-limited control scheme. If the voltage at FB drops
below the regulation threshold, the internal low-side
MOSFET turns on and the inductor current ramps up to
the current limit. Once the current-limit comparator
trips, the low-side MOSFET turns off for the minimum
off-time (250ns). After 250ns, if the voltage at FB is
above the regulation threshold, the low-side MOSFET
stays off. If the voltage at FB is below the regulation
point, the low-side MOSFET turns back on and the
cycle repeats. By using a regulation control scheme
that is not fixed frequency and that can skip pulses, the
MAX1553/MAX1554 operate with very high efficiency.
Soft-Start
Soft-start is provided on the MAX1553/MAX1554 to min-
imize inrush current. The soft-start time is set with an
external capacitor, C3 (Figures 1, 2, and 3). Use the fol-
lowing equation to solve for C3:
C3
=
2
tSS
x 10⁵
where t_SS is the soft-start time. A value of 0.1µF pro-
vides a soft-start time of 20ms.
Shutdown
The MAX1553/MAX1554 feature a low-current shut-
down feature. When EN is low, the IC turns off, reduc-
ing its supply current to approximately 0.1µA. For
normal operation, drive EN high or connect to V_CC.
Overvoltage Protection
The MAX1553/MAX1554 have an adjustable overvolt-
age- protection circuit. When the voltage at OV reaches
the overvoltage threshold (1.25V typ), the protection cir-
cuitry prevents the internal MOSFET from switching,
allowing the output voltage to decay.
The peak output voltage in an overvoltage-protection
event is set with a resistor-divider from the output con-
nected to OV (R2 and R3 in Figures 1, 2, and 3). Select
a value for R3 (10kΩ is recommended), then solve for
R2 using the following equation:
R2
=
R3
x
⎛
⎝⎜
V_OUT(PEAK)
V_OV
− 1^{⎞⎠⎟}
where V_OV is the overvoltage threshold (1.25V typ), and
V_OUT(PEAK) is the desired peak output voltage.
www.maximintegrated.com
Maxim Integrated | 6