NCP1250 データシートの表示（PDF） - ON Semiconductor
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NCP1250 Datasheet PDF : 23 Pages
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NT C
D2
1N4148
ROPPU
841k
NCP1250
OP P
ROPPL
2.5k
Vlatch
full−latch
VCC
OPP
au x.
winding
Figure 48. 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
voltage on the auxiliary diode was 13 V in nominal
conditions. We have selected an NTC which offers a
resistance of 470 kW at 25C and drops to 8.8 kW at 110C.
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:
VNTC + 14 * 3 * 0.6 + 10.4 V
(eq. 17)
Based on the 8.8 kW NTC resistor at 110 C, the current
through the device must be:
INTC
+
10.4
8.8k
[
1.2
mA
(eq. 18)
As such, the bottom resistor ROPPL, can easily be
calculated:
ROPPL
+
3
1.2m
+
2.5
kW
(eq. 19)
Now that the pulldown OPP resistor is known, we can
calculate the upper resistor value ROPPU to adjust the power
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 ROPPU a voltage of:
VROPPU + 67.5 * 0.2 + 67.3 V
(eq. 20)
The current flowing in the pulldown resistor ROPPL in this
condition will be:
IROPPU
+
200m
2.5k
+
80
mA
(eq. 21)
The ROPPU value is therefore easily derived:
ROPPU
+
67.3
80m
+
841
kW
(eq. 22)
Combining OVP and OTP
The OTP and Zener−based OVP can be combined
together as illustrated by Figure 49.
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