NCP1377, NCP1377B

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9

Figure 20. 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.

Demag Restart

Current Sense and Timeout Restart

5

ms

5

ms

Drain

Signal

Timeout

Signal

Drain

Signal

Timeout

Signal

An optocoupler is generally used to transfer the feedback

information to the FB pin while providing the necessary

isolation. It introduces a limitation in how low the skip

level can be adjusted since an optocoupler cannot pull the

FB voltage below its Vce(sat), which is usually around

150 mV. Therefore, in order to take into account

temperature and process variations, it is not recommended

to set up the skip level below 250 mV, which corresponds

to a minimum resistor Rskip of 420

W. The 150 mV is a

much lower level than what will usually be used (it sets the

peak current when entering skip mode at 5% of the

maximum peak current). If anyway a lower skip threshold

is needed, care must be taken to select an optocoupler with

a Vce(sat) guaranteed to be below the chosen skip level

with enough margin.

Demagnetization Detection

The core reset detection is done by monitoring the

voltage activity on the auxiliary winding. This voltage

features a FLYBACK polarity. The typical detection level

is fixed at 50 mV as exemplified by Figure 21.

Figure 21. Core Reset Detection is Done through

a Dedicated Auxiliary Winding Monitoring

Figure 22. Internal Pad Implementation

POSSIBLE

RESTARTS

50 mV

7.0

5.0

3.0

1.0

−1.0

0 V

TO INTERNAL

COMPARATOR

Aux

ESD2

ESD1

4

5

2

4

1

3

1