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LTC3766 数据表(PDF) 34 Page - Linear Technology |
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LTC3766 数据表(HTML) 34 Page - Linear Technology |
34 / 48 page LT8311 34 8311f For more information www.linear.com/LT8311 APPLICATIONS INFORMATION Setting RTIMER in Preactive Mode In preactive mode, the TIMER pin resistor, RTIMER, pro- grams the maximum period that can elapse between two CSW rising edges before a timeout period is triggered. Timeout allows the LT8311 to stop all synchronous activ- ity in the event that the primary-side IC stops switching. Since CSW rising edges represent primary-side switching activity, timeout of CSW rising edges is interpreted as stoppage of switching—at which point the LT8311 ceases all secondary-side synchronous switching, and starts its evaluation period. Refer to the Operation section for de- tails on the evaluation period. Secondary-side switching resumes when all conditions within the evaluation period are satisfied. Timeout also ensures that switching activity within preactive mode occurs at a frequency that is within preactive mode’s operating frequency range. As shown in Figure 18, every time the CSW pin voltage is detected to rise past 1.2V from a voltage level below –150mV, the LT8311 resets its internal timeout signal. The gate of the catch MOSFET, CG turns on (after some propagation delay) when CSW is detected to fall below –150mV. Upon CG going high, the catch MOSFET turns on and pulls its drain voltage (CSW) close to its source voltage, which is tied to GND. CG turns off predictively in CCM before an anticipated CSW rising edge. If a CSW rising edge (rising from below –150mV to above 1.2V) does not come along in time to reset the timeout signal, the signal eventually charges up to voltage VREF_TIMEOUT and triggers an internal timeout condition. Consequently, the LT8311 shuts down all synchronous conduction and startstheevaluationperiod.Theevaluationperiodendsonly when the four conditions listed in the Operation section, including the timely reset of the internal timeout signal, aresatisfiedforthreeconsecutiveCSWrisingedges.Upon completion of the evaluation period, the LT8311 restarts synchronous control. Figure 18. If Timeout Is Triggered in Preactive Mode, LT8311 Shuts Down All Synchronous Conduction and Starts the Evaluation Period (Note: CSW's ringing waveform is caused by the inductor current getting to 0A.) 0V 8311 F18 LT8311 INTERNAL TIMEOUT SIGNAL CG PREDICTIVE TURN-OFF TIMEOUT RESETS ONLY WHEN CSW RISES PAST 1.2V, PRECEDED BY FALLING BELOW –150mV TIMEOUT CSW (PREACTIVE MODE) CG TIME –150mV 1.2V CG TURNS ON WHEN CSW < –150mV EVALUATION PERIOD VREF_TIMEOUT |
类似零件编号 - LTC3766 |
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类似说明 - LTC3766 |
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