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The current limit state machine reduces the current limit by discrete amounts at light loads when TinySwitch-II is likely to switch in the audible frequency range. The lower current limit raises the effective switching frequency above the audio range and reduces the transformer flux density, including the associated audible noise. The state machine monitors the sequence of EN/UV pin voltage levels to determine the load condition and adjusts the current limit level accordingly in discrete amounts. Under most operating conditions(except when close to no-load), the low impedance of the source follower keeps the voltage on the EN/UV pin from going much below 1.0 V in the disabled state. This improves the response time of the optocoupler that is usually connected to this pin. 5.8 V Regulator and 6.3 V Shunt Voltage Clamp The 5.8 V regulator charges the bypass capacitor connected to the BYPASS pin to 5.8 V by drawing a current from the voltage on the DRAIN pin whenever the MOSFET is off. The BYPASS pin is the internal supply voltage node for the TinySwitch-II. When the MOSFET is on, the TinySwitch-II operates from the energy stored in the bypass capacitor. Extremely low power consumption of the internal circuitry allows TinySwitch-II to operate continuously from current it takes from the DRAIN pin. A bypass capacitor value of 0.1 μF is sufficient for both high frequency decoupling and energy storage. In addition, there is a 6.3 V shunt regulator clamping the BYPASS pin at 6.3 V when current is provided to the BYPASS 136 kHz 128 kHz VDRAIN PI-2741-041901 BYPASS Pin Under-Voltage 300 The BYPASS pin under-voltage circuitry disables the power MOSFET when the BYPASS pin voltage drops below 4.8 V. 200 Once the BYPASS pin voltage drops below 4.8 V, it must rise 100 back to 5.8 V to enable (turn-on) the power MOSFET. 0 pin through an external resistor. This facilitates powering of TinySwitch-II externally through a bias winding to decrease the no-load consumption to about 50 mW. Over Temperature Protection The thermal shutdown circuitry senses the die temperature. The threshold is typically set at 135 °C with 70 °C hysteresis. When the die temperature rises above this threshold the power MOSFET 5 10 is disabled and remains disabled until the die temperature falls Time (μs) by 70 °C, at which point it is re-enabled. A large hysteresis of Figure 4. Frequency Jitter. G 4/05 3 TNY263-268 70 °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 (ILIMIT) 更多：https://www.bmcx.com/ , 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 (tLEB) 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 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 850 ms time until the line under-voltage condition ends. Line Under-Voltage Sense Circuit