NCP1378 ON Semiconductor, NCP1378 Datasheet - Page 11

• Manufacturer Part Number: NCP1378
• Description: PWM Current-Mode Controller
• Manufacturer: ON Semiconductor

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soon as the temperature exceeds a given setpoint.
Shutting Off the NCP1378
NPN bipolar transistor as depicted by Figure 6. 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 26).
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
a low level, preventing a bias current to circulate in the
ON/OFF
Figure 24. A simple CTN triggers the latchoff as
Shutdown can easily be implemented through a simple
In applications where the output current is purposely not
Figure 26. A Simple Bipolar Transistor Totally
3
10 k
CTN
1
2
3
4
NCP1378
Disables the IC
10 nF
2
8
7
6
5
Q1
Aux.
1
1
2
3
4
NCP1378
http://onsemi.com
8
7
6
5
NCP1378
ON/OFF
11
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,
NCP1378 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,
NCP1378 immediately stops the switching pulses and
unbiases all unnecessary logical blocks. The overall
consumption drops, while keeping the gate grounded, and
the V
typically 5.5 V, the startup source turns−on again and a new
startup sequence occurs, bringing V
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 27 portrays the typical operating signals in short
circuit.
During the startup phase, the peak current is pushed to
Figure 25. A simple transistor arrangement allows
CC
to trigger the latchoff by an external signal.
slowly falls down. As soon as V
1
2
3
4
NCP1378
8
7
6
5
CC
toward 8.4 V as an
VCCcap
CC
reaches
Aux.