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NCP1308 数据表(PDF) 9 Page - ON Semiconductor |
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NCP1308 数据表(HTML) 9 Page - ON Semiconductor |
9 / 16 page NCP1308 http://onsemi.com 9 The skip level selection is done through a simple resistor inserted between the current sense input and the sense element. Every time the NCP1308 output driver goes low, a 200 mA source forces a current to flow through the sense pin (Figure 15): when the driver is high, the current source is off and the current sense information is normally processed. As soon as the driver goes low, the current source delivers 200 mA and develops a ground−referenced voltage across Rskip. If this voltage is below the feedback voltage, the current sense comparator stays in the high state and the internal latch can be triggered by the next clock cycle. Now, if because of a low load mode the feedback voltage is below Rskip level, then the current sense comparator permanently resets the latch and the next clock cycle (given by the demagnetization detection) is ignored: we are skipping cycles as shown in Figure 15. As soon as the feedback voltage goes up again, there can be two situations: the recurrent period is small and a new demagnetization detection (next wave) signal triggers the NCP1308. To the opposite, in low output power conditions, no more ringing waves are present on the drain and the toggling of the current sense comparator together with the internal 5 ms timeout initiates a new cycle start. In normal operating conditions, e.g. when the drain oscillations are generous, the demagnetization comparator can detect the 50 mV crossing and gives the “green light”, alone, to re−active the power switch. However, when skip cycle takes place (e.g. at low output power demands), the restart event slides along the drain ringing waveforms (actually the valley locations) which decays more or less quickly, depending on the Lprimary−Cparasitic network damping factor. The situation can thus quickly occur where the ringing becomes too weak to be detected by the demagnetization comparator: it then permanently stays locked in a given position and can no longer deliver the “green light” to the controller. To help in this situation, the NCP1308 implements a 5 ms timeout generator: each time the 50 mV crossing occurs, the timeout is reset. So, as long as the ringing becomes too low, the timeout generator starts to count and after 5 ms, it delivers its “green light”. If the skip signal is already present then the controller restarts; otherwise the logic waits for it to set the drive output high. Figure 16 depicts these two different situations: Figure 16. When the primary natural ringing becomes too low, the internal Timeout together with the sense comparator initiates a new cycle when FB passes the skip level. Dmg Restart Current Sense and Timeout Restart 5 ms 5 ms Drain Signal Timeout Signal Drain Signal Timeout Signal |
类似零件编号 - NCP1308 |
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类似说明 - NCP1308 |
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