Datasheet AP3917C (Diodes) - 8

AP3917C Operation Description
(continued) Figure 4 describes the driver time sequence. 60ms 30ms 15ms Driver Stage I Stage II Stage III
T C
128 switching cycles 128 switching cycles 128 switching cycles
U D
Figure 4. Driver Time Sequence in Startup Process
O R Operation Frequency and Peak Current Characteristics P
In order to achieve excellent efficiency performance at light load and improve the overall system efficiency, the peak-current limitation value and
W
switching frequency continuously reduce as the load decreases.
E N
The switching-frequency equation is as follows: V  V V f IN O O  ( )  , for CCM s V 2  L  I (  I ) IN PK O V  V 2  V  I f IN O O O  ( )  , for DCM s V 2 IN L  I PK The peak-current limitation changes with the different load. When the load decreases, the OFF time increases. At the same time, the peak- current limitation becomes lower with the longer OFF time. The peak-current limitation changes in the following equation: I  I  4 mA s m  (t  t ) PK PK _ MAX OFF MINOFF
Overload Protection (OLP)
With the increase of load, the peak current and the switching frequency increase. When the peak current reaches the maximum limitation, and the OFF time is the minimum OFF time, the output voltage drops if the load continues to increase. Similarly, the FB voltage decreases as the output voltage drops. When FB voltage drops below OLP threshold VFB_OLP (1.7V), the internal timer of overload starts to count. Once the overload duration lasts more than the OLP delay time tOLP (170ms), the OLP occurs. The time delay setting of OLP should avoid triggering OLP when the system starts up or enters a load transition phase. Therefore it requires that the system startup time must be less than tOLP. The 170ms time delay of tOLP is calculated under the condition of 36kHz operating frequency. The different operating frequency corresponds to different time delay, the time delay calculation under different operating frequency ( f ) as S follows: 36kHz DELAY t  170ms  ( ) fS
Short-Circuit Protection (SCP)
The AP3917C shuts down when the peak current exceeds short-circuit threshold, and the AP3917C resumes operation when the fault is removed.
Over Temperature Protection (OTP)
The AP3917C integrates an internal over temperature protection function. The AP3917C shuts down when the inner junction temperature exceeds thermal shutdown threshold TOTP (+150°C). After exceeding the threshold, the BP voltage begins to drop, and when BP voltage drops to VBP_RESTART (4.5V), the internal high-voltage regulator turns on to charge the external BP capacitor. AP3917C 8 of 11 May 2019 Document number: DS41542 Rev. 2 - 2
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