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LNK458VG-TL Datasheet(PDF) 5 Page - Power Integrations, Inc. |
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LNK458VG-TL Datasheet(HTML) 5 Page - Power Integrations, Inc. |
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5 / 20 page  Rev. A 11/01/10 5 LNK454/456-458/460 www.powerint.com IC Supply and BYPASS Pin The internal 5.85 V regulator charges the bypass capacitor connected to the BYPASS pin to 5.85 V by drawing current from the voltage on the DRAIN pin whenever the power MOSFET is off. The BYPASS pin is the internal supply voltage node. When the power MOSFET is on, the device operates from the energy stored in the bypass capacitor. Extremely low power consumption of the internal circuitry allows LinkSwitch-PL to operate continuously from current it takes from the DRAIN pin. A bypass capacitor value of 1 µF is sufficient for both high frequency decoupling and energy storage. Dimming applications may require a higher bypass capacitor value. During phase angle dimming when the conduction angle is small the AC input voltage is present for only short periods of time. In that case the IC should not rely on the integrated high voltage current source, but instead external bias circuitry should be used to supply the IC from the output (D ES and RES in Figure 4). If the output voltage is less than 7 V, external bias circuitry should be implemented. This is accomplished by adding an auxiliary winding on the transformer, which is then rectified and filtered via a diode (ultrafast) and capacitor. The winding voltage (turns) should be selected such that the maximum IC consumption can be supported at the lowest operating output current. Start-up, Switching Frequency and On-time Range At start-up the controller uses an initial switching frequency f MIN and minimum on-time t ON(MIN). The charging of the output capacitor together with the energy delivery to the output LEDs determines a step-by-step increase of the power MOSFET switching frequency and on-time updated every half-cycle of the AC input voltage. The steady state switching frequency and on-time are determined by the line voltage, voltage drop across the LEDs and converter efficiency. At light load when the device reaches the minimum frequency f MIN and on-time tON(MIN), the controller regulates by skipping cycles. In this mode of operation the input current is not power factor corrected and the average output current is not guaranteed to fall within the normal range. The FEEDBACK pin cycle skipping threshold is reduced from approximately twice the normal regulation level down to just above the level required to limit output power delivery under these conditions. A properly designed supply will not operate in this mode under normal load conditions. A power supply designed correctly will operate within the switching frequency range [f MIN … fMAX], with an on-time falling between t ON(MIN) and tON(MAX) when connected to a normal load. Overload Protection In case of overload, the system will increase the operating frequency and on-time each AC half-cycle until the maximum frequency and maximum on-time are reached. When this state is reached, the controller enters auto-restart protection, thus inhibiting the gate of the power MOSFET for approximately 1.28 s if the main line frequency is 50 Hz, 1.02 s if it is 60 Hz. After this auto-restart off-time expires, the power MOSFET is re-enabled and a normal start-up is initiated, i.e. at f MIN and t ON(MIN), stepping up until regulation is achieved again. In case of a persistent overload condition, the auto-restart duty cycle DC AR is ~33%. Overload protection is inhibited during phase dimming when the TRIAC conduction duty cycle is less than 60%. Output Overvoltage Protection If a no-load condition is present on the output of the supply, the output overvoltage Zener (DZ OV in Figure 4) will conduct once its threshold is reached. A voltage V OV in excess of VFB(AR) = 2 V will appear across the FEEDBACK pin and the IC will enter auto- restart. Output Short-Circuit If the output of the supply (i.e. the LED load) is short-circuited, then a large amount of energy will be delivered to the sense resistor, generating a high voltage at the FEEDBACK pin. If this condition develops more than 2 V on the FEEDBACK pin, then the IC will interpret this event as an output short-circuit and will enter auto-restart. Safe Operating Area (SOA) Protection If 3 consecutive cycles of the power MOSFET are prematurely terminated due to the power MOSFET current exceeding the current limit after the leading edge blanking time, SOA protection mode is triggered and the IC will enter auto-restart. Hysteretic Thermal Shutdown The thermal shutdown circuitry senses the die junction temperature. The thermal shutdown threshold is set to 142 °C typical with a 75 °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 75 °C, at which point the power MOSFET is re-enabled. |
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