NCP1396B ON Semiconductor, NCP1396B Datasheet - Page 21

• Manufacturer Part Number: NCP1396B
• Description: High Performance Resonant Mode Controller featuring High-voltage Drivers
• Manufacturer: ON Semiconductor

Slow input
comparator. When this input exceeds 1V typical, the
current source Itimer turns on, charging the external
capacitor Ctimer. If the fault duration is long enough,
when Ctimer voltage reaches the VtimerON level (4V
typical), then all pulses are stopped. If the fault input
signal is still present, then the controller permanently
stays off and the voltage on the timer capacitor does not
move (Itimer is on and the voltage is clamped to 5V). If
the fault input signal is removed (because pulses are off
for instance), Itimer turns off and the capacitor slowly
discharges to ground via a resistor installed in parallel
with it. As a result, the designer can easily determine the
time during which the power supply stays locked by
playing on Rtimer. Now, when the timer capacitor voltage
reaches 1V typical (VtimerOFF), the comparator instructs
the internal logic to issues pulses as on a clean soft-start
sequence (soft-start is activated). Please note that the
UVLO
On this circuit, the slow input goes to a
reset
reset
Figure 46. This circuit combines a slow and fast input for improved protection features.
VtimerON
VtimerOFF
DRIVING
LOGIC
Itimer
Vdd
1 = ok
0 = fault
SS
VrefFault
ON/OFF
+
-
NCP1396A, NCP1396B
VrefFault
1 = fault
0 = ok
1 = ok
0 = fault
+
http://onsemi.com
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discharge resistor can not be lower than 4V / Itimer
otherwise the voltage on Ctimer will not reach the turn-
off voltage of 4V.
In both cases, when the fault is validated, both outputs
Mlower and Mupper are internally pulled down to
ground.
On figure 46 example, a voltage proportional to primary
current, once averaged, gives an image of the input power
in case Vin is kept constant via a PFC circuit. If the
output loading increases above a certain level, the voltage
on this pin will pass the 1V threshold and start the timer.
If the overload stays there, after a few tens of milli-
seconds, switching pulses will disappear and a protective
auto-recovery cycle will take place. Adjusting the resistor
R in parallel with the timer capacitor will give the
flexibility to adjust the fault burst mode.
FB
Ctimer
Fast Fault
FB
Slow Fault
A
B
Ctimer
Rtimer
av erage input current
A
B
Vcc
skip
to primary current
sensing circuitry