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PS1518 Datasheet(PDF) 10 Page - Power IC Ltd.

Part # PS1518
Description  High Efficiency, Fixed Frequency WLED Driver
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Manufacturer  PICSEMI [Power IC Ltd.]
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PS1518 Datasheet(HTML) 10 Page - Power IC Ltd.

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OPERATION DESCRIPTION
The PS1518 is a high efficiency, fixed frequency,
peak current mode boost regulator. It has the
architecture to regulate the voltage at the feedback
pin, so that a regulated fixed current is achieved to
drive white LEDs. The power MOS is turned ON
through the control circuitry, at the start of each
oscillator cycle and thus the charging phase is
initiated. The error amplifier, consisting of a voltage
comparator and current sense amplifier, is basically,
a PWM comparator. The voltage comparator
amplifies the difference between the reference and
feed back voltage. When the output of the current
sense amplifier, reaches the output of the voltage
comparator, the POWER MOS is turned OFF and
thus the charging phase is terminated. To prevent
sub-harmonic oscillations at duty cycles greater
than 50 percent, a stabilizing nonlinear ramp is
added to the current sense signal. In this way the
peak current level keeps the output in regulation.
The PS1518 has internal soft start mechanism, to
limit the inrush current at startup and to limit the
amount of overshoot on the output, also. An
internal blanking time is provided during start up, to
prevent the start of switching before all the circuitry
become ready for operation. The current limit is
increased by around a fourth every 15µs giving a
total soft start time of 70µs.
Setting the LED current :
PS1518
The LED current can be set, according to the
requirement, by feedback resistor, R1. The current
through the LEDs is selected by the following
equation:
ILED=100mV/R1
Current Limit :
The PS1518 includes a current limiter. It monitors
the peak current through the inductor and controls
gate of the power device.
Enable Input :
The PS1518 features an active-high Enable input
(EN) pin that allows on/off control of the regulator.
The PS1518 bias current reduces to less than
microampere when it is shutdown. The output
remains at a schottky forward voltage lower than
the SW pin, if a schottky diode is connected
between SW and OV pin, at shut. The Enable input
is TTL/CMOS compatible.
Under Voltage Lockout:
When the input supply goes too low (below 2.35V)
the PS1518 produces an internal UVLO (under
voltage lockout) signal that shuts down the chip.
This mechanism protects the chip from producing
false logic due to low input supply.
Thermal Overload Protection:
Thermal-overload protection limits total power
dissipation in the PS1518. When the junction
temperature exceeds Tj= +160°C, the thermal
sensor signals the shutdown logic and turning off
most of the internal circuitry. The thermal sensor
turns internal circuitry on again after the IC’s
junction temperature drops by 20
°C. The regulator
then starts functioning in the required mode based
on the supply voltage.
Thermal-Overload protection is designed to protect
the PS1518 in the event of a fault condition. For
continual operation, do not exceed the absolute
maximum junction temperature rating of Tj =
+150
°C.
Open Load Protection:
Open load protection in PS1518, protects the chip
from destruction, due to excessive high voltage.
When, in any case, one or more LEDs in the LED
string fails, the feedback pin is pulled down to zero.
As a result, the chip runs at maximum duty cycle,
boosting the output voltage higher and higher. The
open load protection mechanism, checks this
condition, if OV pin is tied to the top of the LED
string. If the output voltage exceeds the OV
threshold (28V), the switching stops, allowing the
output to discharge. The open load protection
mechanism includes a hysteretic comparator. The
switching is enabled again, when the output voltage
has fallen to a certain level. Because the chip
doesn’t turn OFF fully, no power recycling is
necessary, when this condition takes place. But it’s
highly recommended, not to use the chip in that
mode of operation for longer span of time.
July 2007
www.picsemi.com
© 2007 Power IC. All Rights Reserved.
10


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