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NCP1377 Datasheet(PDF) 11 Page - ON Semiconductor

Part # NCP1377
Description  PWM Current?묺ode Controller for Free?뭃unning Quasi?뭃esonant Operation
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1377 Datasheet(HTML) 11 Page - ON Semiconductor

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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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