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TNY268P Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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TNY268P Datasheet(HTML) 4 Page - Power Integrations, Inc. |
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4 / 22 page  Rev. H 02/09 4 TNY263-268 www.powerint.com Figure 5. TinySwitch-II Auto-Restart Operation. 0 1000 2000 Time (ms) 0 5 0 10 100 200 300 V DRAIN V DC-OUTPUT 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 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-II enters into auto-restart operation. An internal counter clocked by the oscillator gets reset every time the EN/UV pin is pulled low. If the EN/UV pin is not pulled low for 50 ms, the power MOSFET switching is normally disabled for 850 ms (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 850 ms time until the line undervoltage condition ends. 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 49 μA to initiate switching of the power MOSFET. During power-up, this is accomplished by holding the BYPASS pin to 4.8 V while the line undervoltage condition exists. The BYPASS pin then rises from 4.8 V to 5.8 V when the line undervoltage condition goes away. 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 850 ms 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 ~2 μA into the pin). In this case the line undervoltage function is disabled. TinySwitch-II Operation TinySwitch-II 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-II 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-II 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. Enable Function TinySwitch-II senses the EN/UV pin to determine whether or not to proceed with the next switching cycle as described earlier. The sequence of cycles is used to determine the current limit. Once a cycle is started, it always completes the cycle (even when the EN/UV pin changes state half way through the cycle). This operation results in a power supply in which the output voltage ripple is determined by the output capacitor, amount of energy per switch cycle and the delay of the feedback. The EN/UV pin signal is generated on the secondary by comparing the power supply output voltage with a reference voltage. The EN/UV pin signal is high when the power supply output voltage is less than the reference voltage. In a typical implementation, the EN/UV pin is driven by an optocoupler. The collector of the optocoupler transistor is connected to the EN/UV pin and the emitter is connected to the SOURCE pin. The optocoupler LED is connected in series with a Zener diode across the DC output voltage to be regulated. When the output voltage exceeds the target regulation voltage level (optocoupler LED voltage drop plus Zener voltage), the optocoupler LED will start to conduct, pulling the EN/UV pin low. The Zener diode can be replaced by a TL431 reference circuit for improved accuracy. ON/OFF Operation with Current Limit State Machine The internal clock of the TinySwitch-II runs all the time. At the beginning of each clock cycle, it samples the EN/UV pin to decide whether or not to implement a switch cycle, and based on the sequence of samples over multiple cycles, it determines |
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