NCP1234 ONSEMI [ON Semiconductor], NCP1234 Datasheet - Page 18

• Manufacturer Part Number: NCP1234
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Current sensing
the FB voltage sets the peak current flowing in the
inductance and the MOSFET. This is done through a PWM
comparator: the current is sensed across a resistor and the
resulting voltage is applied to the CS pin. It is applied to one
off and starts – or restarts – when V
soft−start is applied: the current sense setpoint is linearly
increased from 0 (the minimum level can be higher than 0
because of the LEB and propagation delay) until it reaches
V
ILIM
NCP1234 is a current−mode controller, which means that
Each time the controller is starting, i.e. the controller was
FB
CS
(after a duration of t
V
FB(ref)
CURRENT−MODE CONTROL WITH OVERPOWER COMPENSATION AND SOFT−START
R
FB(up)
blanking
blanking
t
t
BCS
LEB
K
FB
SSTART
+
), or until the FB loop
CC
+
−
Soft−start ramp
t
SSTART
Figure 32. Current Sense Block Schematic
V
PWM
+
reaches V
−
CS(stop)
+
−
Soft−start
+
Reset
Start
Fault
V
+
−
ILIM
CC(on)
http://onsemi.com
IC Start
IC Stop
, a
18
Protection
input of the PWM comparator through a 250 ns LEB block.
On the other input the FB voltage divided by 5 sets the
threshold: when the voltage ramp reaches this threshold, the
output driver is turned off.
is set by a dedicated comparator.
imposes a setpoint lower than the one imposed by the
soft−start (the 2 comparators outputs are OR’ed). The
soft−start ramp signal is generated by the D/A converter in
the NCP1234, that’s why there are observable 15 discrete
steps instead the truly linearly increasing current setpoint
ramp.
Mode
The maximum value for the current sense is 0.7 V, and it
IC stop
Oscillator
UVLO
Jitter
S
R
Q
Latch
TSD
DRV Stage