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TOP233G Datasheet(PDF) 7 Page - Power Integrations, Inc. |
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TOP233G Datasheet(HTML) 7 Page - Power Integrations, Inc. |
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7 / 36 page  TOP232-234 B 7/01 7 Figure 7. Synchronization Timing Diagram. voltage reaches the under-voltage threshold. At power down, UV prevents auto-restart attempts after the output goes out of regulation. This eliminates power down glitches caused by the slow discharge of input storage capacitor present in applications such as standby supplies. A single resistor connected from the MULTI-FUNCTION pin to the rectified DC high voltage bus sets UV threshold during power up. Once the power supply is successfully turned on, UV is disabled to allow extended input voltage operating range. Input voltage is not checked again until the power supply loses regulation and attempts another turn-on. This is accomplished by enabling the UV comparator only when the divide-by-8 counter used in auto-restart reaches full count (S7) which is also the state that the counter is reset to at power up (see Figure 5). The UV feature can be disabled independent of OV feature as shown in Figure 16. Line Overvoltage Shutdown (OV) The same resistor used for UV also sets an overvoltage threshold which, once exceeded, will force TOPSwitch-FX output into off-state. The ratio of OV and UV thresholds is preset at 4.5 as can be seen in Figure 8. This feature turns off the TOPSwitch-FX power MOSFET when the rectified DC high voltage exceeds the OV threshold. When the MOSFET is off, the rectified DC high voltage surge capability is increased to the voltage rating of the MOSFET (700 V), due to the absence of the reflected voltage and leakage spikes on the drain. Small amount of hysteresis is provided on the OV threshold to prevent noise triggering. The OV feature can be disabled independent of UV feature as shown in Figure 15. Line Feed Forward with DC MAX Reduction The same resistor used for UV and OV also implements line voltage feed forward which minimizes output line ripple and reduces power supply output sensitivity to line transients. This feed forward operation is illustrated in Figure 4 by the different values of I M. Note that for the same CONTROL pin current, higher line voltage results in smaller operating duty cycle. As an added safety measure, the maximum duty cycle DC MAX is also reduced from 78% (typical) at a voltage slightly higher than the UV threshold to 38% (typical) at the OV threshold (see Figures 4, 8). DC MAX of 38% at the OV threshold was chosen to ensure that the power capability of the TOPSwitch-FX is not restricted by this feature under normal operation. Remote ON/OFF and Synchronization TOPSwitch-FX can be turned on or off by controlling the current into or out from the MULTI-FUNCTION pin (see Figure 8). This allows easy implementation of remote ON/OFF control of TOPSwitch-FX in several different ways. A transistor or an optocoupler output connected between the MULTI- FUNCTION pin and the SOURCE pin implements this function with “active-on” (Figure 19) while a transistor or an optocoupler output connected between the MULTI-FUNCTION pin and the CONTROL pin implements the function with “active-off” (Figure 20). When a signal is received at the MULTI-FUNCTION pin to disable the output through any of the MULTI-FUNCTION pin functions such as OV, UV and remote ON/OFF, TOPSwitch-FX always completes its current switching cycle as illustrated in Figure 7 before the output is forced off. The internal oscillator is stopped slightly before the end of the current cycle and stays there as long as the disable signal exists. When the signal at the MULTI-FUNCTION pin changes state from disable to enable, the internal oscillator starts the next switching cycle. This approach allows the use of this pin to synchronize TOPSwitch-FX to any external signal with a frequency lower than its internal switching frequency. PI-2558-092999 Oscillator (SAW) DMAX Enable from M Pin (STOP) Time |
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