NCP1253_14 Datasheet, PDF (13/15 Pages) ON Semiconductor – Current-Mode PWM Controller for Off-line Power Supplie

FB
Frequency
F_sw
65 kHz
NCP1253
Peak current setpoint
V_CS
max
0.8 V
max
26 kHz ^min
350 mV 1.5 V
V_fold,end V_fold
V_FB
3.4 V
[0.36 V
[0.25 V ^min
V_freeze V_fold
1.05 V 1.5 V
V_FB
3.4 V
Figure 27. By Observing the Voltage on the Feedback Pin, the Controller Reduces its Switching Frequency for an
Improved Performance at Light Load
Auto−recovery Short−Circuit Protection
In case of output short−circuit or if the power supply
experiences a severe overloading situation, an internal error
flag is raised and starts a countdown timer. If the flag is
asserted longer than 100 ms, the driving pulses are stopped
and V_CC falls down as the auxiliary pulses are missing.
When it crosses VCC_(min), the controller consumption is
down to a few mA and the V_CC slowly builds up again thanks
to the resistive starting network. When V_CC reaches
VCC_ON, the controller attempts to re−start, checking for the
absence of the fault. If the fault is still there, the supply enters
another cycle of so−called hiccup. If the fault has
disappeared, the power supply resumes operations. Please
note that the soft−start is activated during each of the re−start
sequence.
1 vcc 2 vdrv 3 ilprim
23.6 15.9 4.32
1
V_cc (t)
14.8 9.90 3.35
6.05 3.89 2.38
V_DRV (t)
−2.72 −2.12 1.41
−11.5 −8.13 445m
2
SS
I_Lp (t)
500u
1.50m
2.50m
time in seconds
3.50m
3
4.50m
Figure 28. An Auto−Recovery Hiccup Mode is Entered in Case a Faulty Event Longer Than 100 ms is
Acknowledged by the Controller
Ramp compensation
The NCP1253 includes an internal ramp compensation
signal. This is the buffered oscillator clock delivered during
the on time only. Its amplitude is around 2.5 V at the
maximum duty−cycle. Ramp compensation is a known
means used to cure sub harmonic oscillations in
CCM−operated current−mode converters. 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 downslope.
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