NCP1250ASN65T1G ON Semiconductor, NCP1250ASN65T1G Datasheet - Page 19

• Manufacturer Part Number: NCP1250ASN65T1G
• Description: IC PWM CTLR OCP LATCH 6-TSOP
• Manufacturer: ON Semiconductor

Specifications of NCP1250ASN65T1G

• Output Isolation: Isolated
• Frequency Range: 61kHz ~ 71kHz
• Voltage - Input: 9.4 V ~ 28 V
• Voltage - Output: 12V
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 6-TSOP (0.063", 1.60mm Width)
• Number Of Outputs: 1
• Duty Cycle (max): 84 %
• Output Current: 300 mA
• Mounting Style: SMD/SMT
• Switching Frequency: 65 KHz
• Operating Supply Voltage: 28 V
• Supply Current: 2.1 mA
• Maximum Operating Temperature: + 125 C
• Fall Time: 30 ns
• Minimum Operating Temperature: - 40 C
• Rise Time: 40 ns
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

blanking delay at turn off. This spike is energetic enough to
charge the added capacitor C
could make it discharge slower, potentially disturbing the
blanking circuit. When implementing the Zener option, it is
important to carefully observe the OPP pin voltage (short
probe connections!) and check that enough margin exists to
that respect.
Over Temperature Protection
thermal runaways, e.g. when the temperature inside the
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 48. The Internal Circuitry Hooked to Pin 3 Can Be Used to Implement Over Temperature Protection (OTP)
In a lot of designs, the adapter must be protected against
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
and given the time constant,
[ 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)
http://onsemi.com
19
adapter box increases above a certain value. Figure 48
shows how to implement a simple OTP using an external
NTC and a series diode. The principle remains the same:
make sure the OPP network is not affected by the additional
NTC hence the presence of this isolation diode. When the
NTC resistance decreases as the temperature increases, the
voltage on the OPP pin during the off time will slowly
increase and, once it passes 3 V for 4 consecutive clock
cycles, the controller will permanently latch off.
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 49.
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