NCP1205 ON Semiconductor, NCP1205 Datasheet - Page 13

• Manufacturer Part Number: NCP1205
• Description: Single Ended Pwm Controller Featuring Qr Operation And Soft Frequency Foldback
• Manufacturer: ON Semiconductor

we have entered the above circuit into a SPICE simulator
and observed the output waveforms. When FB is within
regulation, the error flag is low. However, as soon as FB
leaves its normal operating area, the OPAMP can no longer
keep the V(−) point and either goes to the positive top or
down to zero: the error flag goes high.
128 ms operation, the capacitor used for the timing is Ct,
To illustrate how the system reacts to a variable FB level,
Because of the large amount of delay necessary for this
Figure 15. By Monitoring the Internal Virtual Ground, the System can Detect the Presence of a Fault
Figure 14. This Typical Arrangement Allows for an Easy Fault Detection Management
500.0 M
V
6.500
4.500
2.500
fb
+
+
V
V
1.5 V
fb
low
Ri
50 k
1.000 M
1.5 V
V(−)
Monitor
OCP Condition
3.000 M
http://onsemi.com
V
V
Regulation Area
HIGH
LOW
Rf
150 k
3
1
+
5
13
V1
2.5 V
= 5 mV
5.000 M
= 3 V
+
−
−
+
connected from ground to pin 5. In normal VFM operation,
this timing capacitor serves as the VCO capacitor and the
error management circuit is transparent. As soon as an error
is detected (error flag goes high), an internal switch routes
Ct to the 128 ms generator. As a first effect, the switching
frequency is no longer controlled by the VCO (if the error
appears during VFM) and the system is relaxed to natural
BCM. The capacitor now ramps up and down to be further
divided and finally create the 128 ms delay.
Error Flag
7
2
7.000 M
OCP
Circuitry
FB
2R
Virtual Point
6
9.000 M
R
Current
Setpoint