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TNY254 Datasheet(PDF) 3 Page - Power Integrations, Inc. |
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TNY254 Datasheet(HTML) 3 Page - Power Integrations, Inc. |
3 / 20 page C 7/01 TNY253/254/255 3 circuit is sampled at the rising edge of the oscillator Clock signal (at the beginning of each cycle). If it is high, then the power MOSFET is turned on (enabled) for that cycle, otherwise the power MOSFET remains in the off state (cycle skipped). Since the sampling is done only once at the beginning of each cycle, any subsequent changes at the ENABLE pin during the cycle are ignored. 5.8 V Regulator The 5.8 V regulator charges the bypass capacitor connected to the BYPASS pin to 5.8V by drawing a current from the voltage on the DRAIN, whenever the MOSFET is off. The BYPASS pin is the internal supply voltage node for the TinySwitch. When the MOSFET is on, the TinySwitch runs off of the energy stored in the bypass capacitor. Extremely low power consumption of the internal circuitry allows the TinySwitch to operate continuously from the current drawn from the DRAIN pin. A bypass capacitor value of 0.1 µF is sufficient for both high frequency de-coupling and energy storage. Under Voltage The under-voltage circuitry disables the power MOSFET when the BYPASS pin voltage drops below 5.1V. Once the BYPASS pin voltage drops below 5.1 V, it has to rise back to 5.8V to enable (turn-on) the power MOSFET. Hysteretic Over Temperature Protection The thermal shutdown circuitry senses the die junction temperature. The threshold is set at 135 °Cwith70 °Chysteresis. When the junction temperature rises above this threshold (135 °C) the power MOSFET is disabled and remains disabled until the die junction temperature falls by 70 °C, at which point it 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 primary-side capacitance and secondary-side rectifier reverse recovery time will not cause premature termination of the switching pulse. TinySwitch Operation TinySwitch is intended to 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. The maximum on- time of the MOSFET is limited to DC MAX by the oscillator. Since the current limit and frequency of a given TinySwitch device are constant, the power delivered is proportional to the primary inductance of the transformer and is relatively independent of the input voltage. Therefore, the design of the power supply involves calculating the primary inductance of the transformer for the maximum power required. As long as the TinySwitch device chosen is rated for the power level at the lowest input voltage, the calculated inductance will ramp up the current to the current limit before the DC MAX limit is reached. Enable Function The TinySwitch senses the ENABLE pin to determine whether or not to proceed with the next switch cycle as described earlier. Once a cycle is started TinySwitch always completes the cycle (even when the ENABLE pin changes state half way through the cycle). This operation results in a power supply whose output voltage ripple is determined by the output capacitor, amount of energy per switch cycle and the delay of the ENABLE feedback. The ENABLE signal is generated on the secondary by comparing the power supply output voltage with a reference voltage. The ENABLE signal is high when the power supply output voltage is less than the reference voltage. In a typical implementation, the ENABLE pin is driven by an optocoupler. The collector of the optocoupler transistor is connected to the ENABLE pin and the emitter is connected to the SOURCE pin. The optocoupler LED is connected in series with a Zener across the DC output voltage to be regulated. When the output voltage exceeds the target regulation voltage level (optocoupler diode voltage drop plus Zener voltage), the optocoupler diode will start to conduct, pulling the ENABLE pin low. The Zener could be replaced by a TL431 device for improved accuracy. The ENABLE pin pull-down current threshold is nominally 50 µA, but is set to 40 µA the instant the threshold is exceeded. This is reset to 50 µA when the ENABLE pull-down current drops below the current threshold of 40 µA. ON/OFF Control The internal clock of the TinySwitch runs all the time. At the beginning of each clock cycle the TinySwitch samples the ENABLE pin to decide whether or not to implement a switch cycle. If the ENABLE pin is high (< 40 µA), then a switching cycle takes place. If the ENABLE pin is low (greater than 50 µA) then no switching cycle occurs, and the ENABLE pin status is sampled again at the start of the subsequent clock cycle. At full load TinySwitch will conduct during the majority of its clock cycles (Figure 4). At loads less than full load, the TinySwitch will “skip” more cycles in order to maintain voltage regulation at the secondary output (Figure 5). At light load or no load, almost all cycles will be skipped (Figure 6). A small |
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