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TNY278P Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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TNY278P Datasheet(HTML) 4 Page - Power Integrations, Inc. |
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4 / 24 page  4 E2/06 TNY274-280 Figure 5. Auto-Restart Operation. 0 2500 5000 Time (ms) 0 5 0 10 100 200 300 V DRAIN V DC-OUTPUT In addition, there is a 6.4 V shunt regulator clamping the BYPASS/MULTI-FUNCTION pin at 6.4 V when current is provided to the BYPASS/MULTI-FUNCTION pin through an external resistor. This facilitates powering of TinySwitch-III externally through a bias winding to decrease the no-load consumption to well below 50 mW. BYPASS/MULTI-FUNCTION Pin Under-Voltage TheBYPASS/MULTI-FUNCTIONpinunder-voltagecircuitry disables the power MOSFET when the BYPASS/MULTI- FUNCTION pin voltage drops below 4.9 V in steady state operation.OncetheBYPASS/MULTI-FUNCTIONpinvoltage drops below 4.9 V in steady state operation, it must rise back to 5.85 V to enable (turn-on) the power MOSFET. Over Temperature Protection Thethermalshutdowncircuitrysensesthedietemperature. The threshold is typically set at 142 °C with 75 °C hysteresis. When thedietemperaturerisesabovethisthresholdthepowerMOSFET is disabled and remains disabled until the die temperature falls by 75 °C, at which point it is re-enabled. A large hysteresis of 75 °C (typical) is provided to prevent overheating of the PC board due to a continuous fault condition. Current Limit ThecurrentlimitcircuitsensesthecurrentinthepowerMOSFET. When this current exceeds the internal threshold (I LIMIT), the powerMOSFETisturnedofffortheremainderofthatcycle.The current limit state machine reduces the current limit threshold by discrete amounts under medium and light loads. 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 currentspikescausedbycapacitanceandsecondary-siderectifier reverse recovery time will not cause premature termination of the switching pulse. Auto-Restart In the event of a fault condition such as output overload, output short circuit, or an open loop condition, TinySwitch-III enters into auto-restart operation. An internal counter clocked by the oscillator is reset every time the EN/UV pin is pulled low. If the EN/UV pin is not pulled low for 64 ms, the power MOSFET switching is normally disabled for 2.5 seconds (except in the case of line under-voltage condition, in which case it is disabled until the condition is removed). The auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is removed. Figure 5 illustrates auto-restart circuit operation in the presence of an output short circuit. In the event of a line under-voltage condition, the switching of the power MOSFET is disabled beyond its normal 2.5 seconds until the line under-voltage condition ends. Adaptive Switching Cycle On-Time Extension Adaptive switching cycle on-time extension keeps the cycle on until current limit is reached, instead of prematurely terminating after the DC MAX signal goes low. This feature reduces the minimum input voltage required to maintain regulation, extending hold-up time and minimizing the size of bulk capacitor required. The on-time extension is disabled during the startup of the power supply, until the power supply output reaches regulation. Line Under-Voltage Sense Circuit TheDClinevoltagecanbemonitoredbyconnectinganexternal resistorfromtheDClinetotheEN/UVpin. Duringpower-upor when the switching of the power MOSFET is disabled in auto- restart, the current into the EN/UV pin must exceed 25 µA to initiateswitchingofthepowerMOSFET.Duringpower-up,this is accomplished by holding the BYPASS/MULTI-FUNCTION pin to 4.9 V while the line under-voltage condition exists. The BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to 5.85Vwhenthelineunder-voltageconditiongoesaway.Whenthe switchingofthepowerMOSFETisdisabledinauto-restartmode andalineunder-voltageconditionexists,theauto-restartcounter is stopped. This stretches the disable time beyond its normal 2.5 seconds until the line under-voltage condition ends. The line under-voltage circuit also detects when there is no external resistor connected to the EN/UV pin (less than ~1 µAinto the pin). In this case the line under-voltage function is disabled. TinySwitch-III Operation TinySwitch-III devices operate in the current limit mode. When enabled, the oscillator turns the power MOSFET on at the beginning of each cycle. The MOSFET is turned off when the current ramps up to the current limit or when the DC MAX limit is reached. Since the highest current limit level and frequency of a TinySwitch-III design are constant, the power delivered to the |
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