ncp1251 ON Semiconductor, ncp1251 Datasheet - Page 21

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

voltage on the auxiliary diode was 13 V in nominal
conditions. We have selected an NTC which offers a
resistance of 470 kW at 25°C and drops to 8.8 kW at 110°C.
If our auxiliary winding plateau is 14 V and we consider a
0.6 V forward drop for the diode, then the voltage across the
NTC in fault mode must be:
through the device must be:
calculated:
calculate the upper resistor value R
Figure 49. The Internal Circuitry Hooked to Pin 3 Can Be Used to Implement Over Temperature Protection (OTP)
Back to our 19 V adapter, we have found that the plateau
Based on the 8.8 kW NTC resistor at 110 °C, the current
As such, the bottom resistor R
Now that the pulldown OPP resistor is known, we can
ROPPL
2.5k
V
NTC
R
OP P
OPPL
I
NTC
+ 14 * 3 * 0.6 + 10.4 V
Vlatch
+
+
10.4
8.8k
1.2m
3
NT C
[ 1.2 mA
+ 2.5 kW
ROPPU
OPPU
841k
OPPL
1N4148
D2
to adjust the power
, can easily be
full−latch
(eq. 17)
(eq. 18)
(eq. 19)
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21
limit at the chosen output power level. Suppose we need a
200 mV decrease from the 0.8 V set point and the on−time
swing on the auxiliary anode is −67.5 V, then we need to drop
over R
The current flowing in the pulldown resistor R
condition will be:
The R
Combining OVP and OTP
together as illustrated by Figure 50.
The OTP and Zener−based OVP can be combined
OPPU
OPPU
OPP
value is therefore easily derived:
a voltage of:
V
ROPPU
I
R
ROPPU
OPPU
+ 67.5 * 0.2 + 67.3 V
+
+
VCC
67.3
200m
80m
2.5k
+ 841 kW
+ 80 mA
OPPL
au x.
winding
(eq. 20)
(eq. 21)
(eq. 22)
in this