ncp1377b ON Semiconductor, ncp1377b Datasheet - Page 11

• Manufacturer Part Number: ncp1377b
• Description: Pwm Current-mode Controller For Free-running Quasi-resonant Operation
• Manufacturer: ON Semiconductor

Shutting Off the NCP1377
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).
Overload Operation
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
Thermistor
ON/OFF
Shutdown can easily be implemented through a simple
In applications where the output current is purposely not
Figure 25. A simple arrangement triggers
Figure 27. A Simple Bipolar Transistor Totally
the latchoff as soon as the temperature
3
exceeds a given setpoint.
10 k
1
2
3
4
Disables the IC
10 nF
Rdem
8
7
6
5
2
Q1
CV
1
CC
1
2
3
4
NCP1377
Aux.
Winding
NCP1377, NCP1377B
http://onsemi.com
8
7
6
5
ON/OFF
11
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.
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.
During the startup phase, the peak current is pushed to
Figure 26. A simple transistor arrangement
temperature exceeds a given setpoint.
triggers the latchoff as soon as the
1
2
3
4
NCP1377
8
7
6
5
VCCcap
Aux.