ncp1251 ON Semiconductor, ncp1251 Datasheet - Page 22

• Manufacturer Part Number: ncp1251
• Description: Ncp1251 Current-mode Pwm Controller For Off-line Power Supplies
• Manufacturer: ON Semiconductor

activated, the NTC can drive the OPP pin and trigger the
adapter in case of an over temperature. During nominal
temperature if the loop is broken, the voltage runaway will
be detected and the controller will shut down the converter.
can simply be grounded.
Filtering the Spikes
to the OPP pin via the parasitic capacitances exhibited by the
Zener diode and the series diode. To prevent an adverse
triggering of the Over Voltage Protection circuitry, it is
possible to install a small RC filter before the detection
In nominal V
In case the OPP pin is not used for either OPP or OVP, it
The auxiliary winding is the seat of spikes that can couple
Figure 51. A Small RC Filter Avoids the Fast Rising Spikes from Reaching the Protection Pin of the NCP1251 in
ROPPL
2.5k
ROPPL
2.5k
Figure 50. With the NTC Back in Place, the Circuit Nicely Combines OVP, OTP and OPP on the Same Pin
10
10
OPP
CC
OP P
/ output conditions, when the Zener is not
Vlatch
Vlatch
ROPPU
ROPPU
841k
NT C
841k
15V
NT C
D3
D3
15V
Presence of Energetic Perturbations Superimposed on the Input Line
11
4
5
11
4
5
1N4148
D2
1N4148
D2
1
1
OVP
OVP
http://onsemi.com
2
22
network. Typical values are those given in Figure 51 and
must be selected to provide the adequate filtering function
without degrading the stand−by power by an excessive
current circulation.
Latched OVP on V
When the V
immediately stopped and the V
latched-level around 7 V typical. The controller remains in
this state as long as a sufficient current flows in the SCR, at
least 30 mA. We recommend to put a design margin there,
with a minimum current around 60 mA at the lowest input
line.
The V
CC
OPP
OPP
pin is permanently monitored by a comparator.
CC
exceeds 25.5 V (typical), all pulses are
CC
ad d ition al fil ter
330pF
C1
VCC
VCC
9
R3
220
CC
9
falls to the SCR
8
3
au x.
winding
aux.
winding