MAX1561/MAX1599
High-Efficiency, 26V Step-Up Converters
for Two to Six White LEDs
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Detailed Description
The MAX1561/MAX1599s’ high efficiency and small size
make them ideally suited to drive up to six series-con-
nected LEDs. The device operates as a boost DC-DC
converter that regulates output current rather than volt-
age. The MAX1561/MAX1599 provide even illumination
by sourcing the same output current through each LED,
eliminating the need for expensive factory calibration.
The fast 1MHz (500kHz for the MAX1599) internal oscil-
lator allows for a small inductor and small input and out-
put capacitors while minimizing input and output ripple.
The single analog control input allows easy adjustment
of LED brightness and on/off control. This allows either
simple logic-level on/off control, analog voltage control,
or PWM duty-cycle control of both brightness and shut-
down. In shutdown, supply current is reduced to a low
0.3µA. A soft-start gradually illuminates the LEDs, elimi-
nating the inrush current during startup.
Soft-Start
The MAX1561/MAX1599 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 at a reduced duty cycle. When V
COMP
rises above
2.25V, the duty cycle is at its maximum. See the Typical
Operating Characteristics for an example of soft-start
operation.
Shutdown
The MAX1561/MAX1599 enter shutdown when V
CTRL
is
less than 100mV for more than 8.2ms (16.4ms for the
MAX1599). In shutdown, supply current is reduced to
0.3µA by powering down the entire IC except for the
CTRL voltage-detection circuitry. C
COMP
is discharged
during shutdown, allowing the device to reinitiate soft-
start when it is enabled. Although the internal N-chan-
nel MOSFET does not switch in shutdown, there is still
a DC current path between the input and the LEDs
through the inductor and Schottky diode. The minimum
forward voltage of the LED array must exceed the max-
imum input voltage to ensure that the LEDs remain off
in shutdown. However, with two or more LEDs, the for-
ward voltage is large enough to keep leakage current
low, less than 1µA (typ). Typical shutdown timing char-
acteristics are shown in the Typical Operating
Characteristics.
Overvoltage Protection
Overvoltage lockout (OVLO) occurs when V
OUT
is
above 27V. The protection circuitry stops the internal
MOSFET from switching and causes V
COMP
to decay
to 0V. The device comes out of OVLO and into soft-
start when V
OUT
falls below 25V.
FUNCTION
1 OUT
Overvoltage Sense. When V
OUT
is greater than 27V, the internal N-channel MOSFET turns off until V
OUT
drops below 25V, then the IC reenters soft-start. Connect a 0.1µF capacitor from OUT to PGND.
2INInput Voltage Supply. Input voltage range is 2.6V to 5.5V. Connect a 2.2µF capacitor from IN to PGND.
3 CTRL
Brightness Control Input. LED brightness is controlled by the voltage applied to CTRL. Varying the voltage
from 0.24V to 1.62V adjusts the brightness from dim to bright, respectively. Any voltage above 1.62V does
not increase brightness. If CTRL is held below 100mV for more than 8.2ms (16.4ms for the MAX1599), the
IC shuts down.
4CS
Current-Sense Feedback Input. Connect a resistor from CS to GND to set the LED bias current. The
voltage at CS regulates to V
CTRL
/ 10 or 0.162V, whichever is lower.
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