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NCP1377 Datasheet(PDF) 11 Page - ON Semiconductor |
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NCP1377 Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 16 page NCP1377, NCP1377B http://onsemi.com 11 Figure 25. A simple arrangement triggers the latchoff as soon as the temperature exceeds a given setpoint. 1 8 2 3 4 6 5 7 Thermistor Rdem Aux. Winding CVCC 1 8 2 3 4 6 5 7 NCP1377 ON/OFF VCCcap Aux. Figure 26. A simple transistor arrangement triggers the latchoff as soon as the temperature exceeds a given setpoint. Shutting Off the NCP1377 Shutdown can easily be implemented through a simple NPN bipolar transistor as depicted by Figure 27. When OFF, Q1 is transparent to the operation. When forward biased, the transistor pulls the FB pin to ground (Vcesat ≈ 200 mV) and permanently disables the IC. A small time constant on the transistor base will avoid false triggering (Figure 27). 1 8 2 3 4 6 5 7 NCP1377 ON/OFF Figure 27. A Simple Bipolar Transistor Totally Disables the IC Q1 10 nF 10 k 32 1 Overload Operation In applications where the output current is purposely not controlled (e.g. wall adapters delivering raw DC level), it is interesting to implement a true short−circuit protection. A short−circuit actually forces the output voltage to be at a low level, preventing a bias current to circulate in the optocoupler LED. As a result, the auxiliary voltage also decreases because it also operates in Flyback and thus duplicates the output voltage, providing the leakage inductance between windings is kept low. To account for this situation and properly protect the power supply, NCP1377 hosts a dedicated overload detection circuitry. Once activated, this circuitry imposes to deliver pulses in a burst manner with a low Duty Cycle. The system auto−recovers when the fault condition disappears. During the startup phase, the peak current is pushed to the maximum until the output voltage reaches its target and the feedback loop takes over. The auxiliary voltage takes place after a few switching cycles and self−supplies the IC. In presence of a short circuit on the output, the auxiliary voltage will go down until it crosses the undervoltage lockout level of typically 7.5 V. When this happens, NCP1377 immediately stops the switching pulses and unbiases all unnecessary logical blocks. The overall consumption drops, while keeping the gate grounded, and the Vcc slowly falls down. As soon as Vcc reaches typically 5.6 V, the startup source turns−on again and a new startup sequence occurs, bringing Vcc toward 12.5 V as an attempt to restart. If the default has gone, then the power supply normally restarts. If not, a new protective burst is initiated, shielding the SMPS from any runaway. Figure 28 portrays the typical operating signals in short circuit. |
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