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TNY375-380 Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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TNY375-380 Datasheet(HTML) 4 Page - Power Integrations, Inc. |
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4 / 20 page  Rev. A 05/07 TNY375-380 www.powerint.com BYPASS/MULTI-FUNCTION Pin Undervoltage The BYPASS/MULTI-FUNCTION pin undervoltage circuitry disables the power MOSFET when the BYPASS/MULTI- FUNCTION pin voltage drops below 4.9 V in steady state operation. Once the BYPASS/MULTI-FUNCTION pin voltage 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 The thermal shutdown circuitry senses the die temperature. The threshold is typically set at 142 °C with 75 °C hysteresis. When the die temperature rises above this threshold, the power MOSFET 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 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 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 current spikes caused by typical capacitance and secondary-side rectifier 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-PK 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 8192 switching cycles (or 32 ms), the power MOSFET switching is normally disabled for 1 second (except in the case of line undervoltage 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 undervoltage condition, the switching of the power MOSFET is disabled beyond its normal 1 second until the line undervoltage 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, typically 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, and after auto-restart, until the power supply output reaches regulation. Line Undervoltage Sense Circuit The DC line voltage can be monitored by connecting an external resistor from the DC line to the EN/UV pin. During power-up or when the switching of the power MOSFET is disabled in auto-restart, the current into the EN/UV pin must exceed 25 mA to initiate switching of the power MOSFET. During power-up, this is accomplished by holding the BYPASS/ MULTI-FUNCTION pin to 4.9 V while the line undervoltage condition exists. The BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to 5.85 V when the line undervoltage condition goes away. Once MOSFET switching is enabled, the DC line voltage is ignored unless the power supply enters auto-restart mode in the event of a fault condition. When the switching of the power MOSFET is disabled in auto-restart mode and a line undervoltage condition exists, the auto-restart counter is stopped. This stretches the disable time beyond its normal 1 second until the line undervoltage condition ends. The line undervoltage circuit also detects when there is no external resistor connected to the EN/UV pin (less than ~1 mA into the pin). In this case the line undervoltage function is disabled. TinySwitch-PK Operation TinySwitch-PK 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 (applicable when On-Time Extension is disabled). Since the highest current limit level and frequency of a TinySwitch-PK design are constant, the power delivered to the load is proportional to the primary inductance of the transformer and peak primary current squared. Hence, designing the supply involves calculating the primary inductance of the transformer for the maximum output power required. If the TinySwitch-PK is appropriately chosen for the power level, the current in the calculated inductance will ramp up to current limit before the DC MAX limit is reached. 0 1000 2000 Time (ms) 0 5 0 10 100 200 300 V DRAIN V DC-OUTPUT Figure 5. Auto-Restart Operation. |
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