NCP1212 ON Semiconductor, NCP1212 Datasheet - Page 12

• Manufacturer Part Number: NCP1212
• Description: Current Mode Pwm Controller For Both Forward And Flyback Converters
• Manufacturer: ON Semiconductor

Current Sense Section
a current sensing resistor, R
power MOSFET and Ground. In most cases, a narrow spike
on the leading edge of the current waveform can be observed
and may cause the power supply to exhibit an instability
when the output is lightly loaded. The spike is due to the
power transformer inter−winding capacitance and output
rectifier recovery time which are unavoidable. NCP1212
provides a 300 ns Leading Edge Blanking block to shield off
the spike. With the Leading Edge Blanking function, the CS
pin is not sensitive to the power switch turn−on noise and
spikes, practically in most applications, no filtering network
is required.
sense input is compared with the level shifted control input
voltage and an internal Current Limit Threshold, V
case the CS input exceeds the Current Limit Threshold,
which is 1.0 V (typ.) in NCP1212, the gate driver output will
be forced to turn off immediately.
Overload
Enable
The current sense pin, CS detects the voltage drop across
In normal operation, voltage developed at the current
Reset
Duty Cycle Control
C
T
Figure 35. Output Pulse Duty Cycle Depends on the SS/DMAX Pin Voltage
sense
F/F
Q
Q
connected in between the
S
−
+
−
+
Figure 34. Soft−Start Operation
From C
http://onsemi.com
CS
3.2 V
T
. In
NCP1212
12
Vsst
following equation:
Soft−Start and Maximum Duty Selection
simplify designer’s job hence make this device easy to use.
During the startup phase, a constant current source of 8.0 mA
flows out of the SS/DMAX pin once V
minimum startup voltage. The capacitor connected at
SS/DMAX pin is slowly charged up and the voltage
developed plus one diode drop, V
saw−tooth waveform, C
shown in Figure 34. Whenever C
V
slowly and it controls the output duty gradually increases as
shown in Figure 35. The minimum C
hence there is no output before SS/DMAX pin attains about
0.4 V (1.0 V–1 diode drop). Soft−Start block will have no
effect to the PWM operation once V
SST
Thus the maximum allowable peak current is given by the
NCP1212 includes an internal Soft−Start function to
, gate driver output will be turned off. Since V
+5 V
8 mA
I pk(max) +
Shutdown
T
from the internal oscillator as
NCP1212
R sense
1 V
SST
T
voltage is higher than
SST
is compared with the
T
voltage is at 1.0 V,
reaches 3.2 V.
CC
SS/DMAX
V
V
SST
DRV
C
attains the
SS
SST
rises