NCP1236
CURRENT−MODE CONTROL WITH OVERPOWER COMPENSATION AND SOFT−START
Current sensing
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.
The maximum value for the current sense is 0.7 V, and it
is set by a dedicated comparator.
NCP1236 is a current−mode controller, which means that
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
V
FB(ref)
PWM
K
FB
+
−
R
FB(up)
Jitter
Soft−start
+
−
Oscillator
DCMAX
Soft−start ramp
FB
CS
Start
IC Start
IC Stop
t
SSTART
Reset
S
Q
R
DRV Stage
blanking
t
LEB
+
−
+
V
ILIM
IC stop
UVLO
blanking
HV stop
t
BCS
−
+
Protection
Mode
Latch
TSD
+
V
CS(stop)
Fault
Figure 36. Current Sense Block Schematic
Each time the controller is starting, i.e. the controller was
off and starts – or restarts – when V reaches V , a
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
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 NCP1236, that’s why there are observable 15 discrete
steps instead the truly linearly increasing current setpoint
ramp.
CC
CC(on)
V
ILIM
(after a duration of t ), or until the FB loop
SSTART
http://onsemi.com
19
ONSEMI [ ONSEMI ]
