NCP1239
http://onsemi.com
31
Soft−Start
The NCP1239 features an internal soft−start activated
during the Power On sequence (PON). As soon as V
CC
reaches 16.4 V, the current setpoint is gradually increased
from nearly zero up to the maximum clamping level (e.g.
0.9 V/Rsense). This situation lasts a programmable time that
is adjusted by the Pin 6 capacitor (7.5 ms typically with
C
pin6
= 390 nF). Further to that time period, the current
setpoint is blocked to 0.9 V/Rsense until the supply enters
regulation. The soft−start is also activated at each start of the
active phase of fault burst operation. Every restart attempt
is followed by a soft−start activation.
Generally speaking, the soft−start will be activated when
V
CC
ramps up either from zero (fresh power−on sequence)
or 6.9 V, the latch−off threshold after an overload detection
(OVL) for instance. Figure 51 shows the soft−start behavior.
The time scales are purposely shifted to offer a better zoom
portion.
Figure 51.
Soft−start is activated during a startup sequence or an OVL condition
16.4V
6.9V
Internal Ramp Compensation
Ramp compensation is a known mean to cure
sub−harmonic oscillations. These oscillations take place at
half the switching frequency and occur only during
Continuous Conduction Mode (CCM) with a duty−cycle
greater than 50%. To lower the current loop gain, one usually
injects between 50 and 100% of the inductor down−slope.
Figure 52 depicts how internally the ramp is generated:
Figure 52.
Inserting a resistor in series with the current sense
information brings ramp compensation
+
−
CS
LEB
32k
Rramp
Rsense
from
setpoint
3.2V
0V
