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LNK604PG-TL Datasheet(PDF) 3 Page - Power Integrations, Inc. |
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LNK604PG-TL Datasheet(HTML) 3 Page - Power Integrations, Inc. |
3 / 18 page Rev. F 01/10 3 LNK603-606/613-616 www.powerint.com LinkSwitch-II Functional Description The LinkSwitch-II combines a high voltage power MOSFET switch with a power supply controller in one device. Similar to the LinkSwitch-LP and TinySwitch-III it uses ON/OFF control to regulate the output voltage. In addition, the switching frequency is modulated to regulate the output current to provide a constant current characteristic. The LinkSwitch-II controller consists of an oscillator, feedback (sense and logic) circuit, 6 V regulator, over-temperature protection, frequency jittering, current limit circuit, leading-edge blanking, inductance correction circuitry, frequency control for constant current regulation and ON/OFF state machine for CV control. Inductance Correction Circuitry If the primary magnetizing inductance is either too high or low the converter will automatically compensate for this by adjusting the oscillator frequency. Since this controller is designed to operate in discontinuous-conduction mode the output power is directly proportional to the set primary inductance and its tolerance can be completely compensated with adjustments to the switching frequency. Constant Current (CC) Operation As the output voltage and therefore the flyback voltage across the bias winding increases, the FEEDBACK pin voltage increases. The switching frequency is adjusted as the FEEDBACK pin voltage increases to provide a constant output current regulation. The constant current circuit and the inductance correction circuit are designed to operate concurrently in the CC region. Constant Voltage (CV) Operation As the FEEDBACK pin approaches V FBth from the constant current regulation mode, the power supply transitions into CV operation. The switching frequency at this point is at its maximum value, corresponding to the peak power point of the CCCV characteristic. The controller regulates the feedback pin voltage to remain at V FBth using an ON/OFF state-machine. The FEEDBACK pin voltage is sampled 2.5 ms after the turn-off of the high voltage switch. At light loads the current limit is also reduced to decrease the transformer flux density. Output Cable Compensation This compensation provides a constant output voltage at the end of the cable over the entire load range in CV mode. As the converter load increases from no-load to the peak power point (transition point between CV and CC) the voltage drop introduced across the output cable is compensated by increasing the FEEDBACK pin reference voltage. The controller determines the output load and therefore the correct degree of compensation based on the output of the state machine. Cable drop compensation for a 24 AWG (0.3 W) cable is selected with C BP = 1 mF and for a 26 AWG (0.49 W) cable with CPB = 10 mF. Auto-Restart and Open-Loop Protection In the event of a fault condition such as an output short or an open loop condition the LinkSwitch-II enters into an appropriate protection mode as described below. In the event the FEEDBACK pin voltage during the flyback period falls below 0.7 V before the FEEDBACK pin sampling delay (~2.5 ms) for a duration in excess of ~450 ms (auto-restart on-time (t AR-ON) the converter enters into auto-restart, wherein the power MOSFET is disabled for 2 seconds (~18% auto- restart duty cycle). The auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is removed. In addition to the conditions for auto-restart described above, if the sensed FEEDBACK pin current during the forward period of the conduction cycle (switch “on” time) falls below 120 mA, the converter annunciates this as an open-loop condition (top resistor in potential divider is open or missing) and reduces the auto-restart time from 450 msec to approximately 6 clock cycles (90 ms), whilst keeping the disable period of 2 seconds. Over-Temperature Protection The thermal shutdown circuitry senses the die temperature. The threshold is set at 142 °C typical with a 60 °C hysteresis. When the die temperature rises above this threshold (142 °C) the power MOSFET is disabled and remains disabled until the die temperature falls by 60 °C, at which point the MOSFET is re-enabled. Current Limit The current limit circuit senses the current in the power MOSFET. When this current exceeds the internal threshold (I LIMIT), the power MOSFET is turned off for the remainder of that cycle. The leading edge blanking circuit inhibits the current limit comparator for a short time (t LEB) after the power MOSFET is turned on. This leading edge blanking time has been set so that current spikes caused by capacitance and rectifier reverse recovery time will not cause premature termination of the MOSFET conduction. The LinkSwitch-II also contains a “di/dt” correction feature to minimize CC variation across the input line range. 6.0 V Regulator The 6 V regulator charges the bypass capacitor connected to the BYPASS pin to 6 V by drawing a current from the voltage on the DRAIN, whenever the MOSFET is off. The BYPASS pin is the internal supply voltage node. When the MOSFET is on, the device runs off of the energy stored in the bypass capacitor. Extremely low power consumption of the internal circuitry allows the LinkSwitch-II to operate continuously from the current drawn from the DRAIN pin. A bypass capacitor value of either 1 mF or 10 mF is sufficient for both high frequency decoupling and energy storage. |
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