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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 EN CLOCK D MAX IDRAIN VDRAIN PI-2749-050301 Figure 6. TinySwitch-II Operation at Near Maximum Loading. EN CLOCK D MAX IDRAIN VDRAIN PI-2667-090700 Figure 7. TinySwitch-II Operation at Moderately Heavy Loading. TNY263-268 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 the appropriate current limit. At high loads, when the EN/UV pin is high (less than 240 μA out of the pin), a switching cycle with the full current limit occurs. At lighter loads, when EN/UV is high 更多：https://www.bmcx.com/ , a switching cycle with a reduced current limit occurs. At near maximum load, TinySwitch-II will conduct during nearly all of its clock cycles (Figure 6). At slightly lower load, it will “skip” additional cycles in order to maintain voltage regulation at the power supply output (Figure 7). At medium loads, cycles will be skipped and the current limit will be reduced (Figure 8). At very light loads, the current limit will be reduced even further (Figure 9). Only a small percentage of cycles will occur to satisfy the power consumption of the power supply. The response time of the TinySwitch-II ON/OFF control scheme is very fast compared to normal PWM control. This provides tight regulation and excellent transient response. Power Up/Down The TinySwitch-II requires only a 0.1 μF capacitor on the BYPASS pin. Because of its small size, the time to charge this capacitor is kept to an absolute minimum, typically 0.6 ms. Due to the fast nature of the ON/OFF feedback, there is no overshoot at the power supply output. When an external resistor (2 M.) is connected from the positive DC input to the EN/UV pin, the power MOSFETswitching will be delayed during power-up until the DC line voltage exceeds the threshold (100 V). Figures 10 and 11 show the power-up timing waveform of TinySwitch-II in applications with and without an external resistor (2 M.) connected to the EN/UV pin. V EN CLOCK D MAX IDRAIN VDRAIN PI-2377-091100 Figure 8. TinySwitch-II Operation at Medium Loading. G 4/05 5 TNY263-268 200 100 PI-2381-1030801 0 10 5 0 400 200 0 V DC-INPUTV BYPASSV DRAIN 01 2 Time (ms) Figure 11. TinySwitch-II Power-up without Optional External UV Resistor Connected to EN/UV Pin. PI-2661-072400VVENCLOCKDIMAXDRAINDRAIN V DC-INPUT V DRAIN PI-2348-030801 Figure 9. TinySwitch-II Operation at Very Light Load. 200 100