NCP1234
www.onsemi.com
18
CURRENT−MODE CONTROL WITH OVERPOWER COMPENSATION AND SOFT−START
Current sensing
NCP1234 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
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.
Figure 32. Current Sense Block Schematic
CS
FB
−
+
t
LEB
blanking
K
FB
R
FB(up)
−
+
−
+
−
+
+
+
V
ILIM
V
CS(stop)
S
R
Q
t
SSTART
Soft−start ramp
Start
Reset
IC Start
IC Stop
Oscillator
Protection
Mode
UVLO
Jitter
Latch
Soft−start
IC stop
TSD
Fault
DRV Stage
blanking
PWM
t
BCS
V
FB(ref)
Each time the controller is starting, i.e. the controller was
off and starts – or restarts – when V
CC
reaches V
CC(on)
, 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
V
ILIM
(after a duration of t
SSTART
), or until the FB loop
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.
