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PS1518 Datasheet(PDF) 10 Page - Power IC Ltd. |
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PS1518 Datasheet(HTML) 10 Page - Power IC Ltd. |
10 / 12 page 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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