NCP1381 ONSEMI [ON Semiconductor], NCP1381 Datasheet - Page 12

• Manufacturer Part Number: NCP1381
• Description: Low−Standby High Performance PWM Controller
• Manufacturer: ONSEMI [ON Semiconductor]

The FB pin level is permanently compared to a fixed level,
V
user can wire a resistor to ground and alter the skip level in
case of noise problems. When the FB pin is above V
comparator is transparent to the operation. When the load
becomes lighter, the FB level goes down too. When it
reaches V
internal flip−flop: the driving pulses are stopped. As a result,
V
ensured. Detecting a decay in the output voltage, the FB loop
will react by increasing its level. When the level crosses
V
occurs on the FB pin. Please note that the soft−start will be
activated every time the skip comparator asks to restart.
Therefore, instead of having sharp skip transitions, a smooth
current rampup can be observed on the current envelope.
This option significantly decreases the acoustical noise.
Figure 13 shows a typical shot and Figure 15 portrays
several skip cycles.
out
skip
skip
Figure 11. The Skip Cycle Takes Place at Low Peak
Figure 12 depicts the internal comparator arrangement.
200
100
Currents Which Guaranties Noise Free Operation
0
, also available on Pin 5 for adjustment. As a result, the
starts to also decrease since no energy transfer is
plus a slight hysteresis, pulses restart again: a ripple
skip
Peak Current
, the comparator goes high and resets the
Maximum
Width
Recurrence
Current Limit
Skip Cycle
skip
http://onsemi.com
, the
NCP1381
12
be two situations as we have seen before: in normal
operating conditions, e.g. when the drain oscillations are
generous, the demagnetization comparator can detect the
45 mV crossing and gives the “green light”, alone, to
reactive the power switch. However, when skip cycle takes
place (e.g. at low output power demands), the restart event
slides along the drain ringing waveforms (actually the valley
locations) which decays more or less quickly, depending on
the L
can thus quickly occur where the ringing becomes too weak
to be detected by the demagnetization comparator: it then
permanently stays locked in a given position and can no
longer deliver the “green light” to the controller. To help in
this situation, the NCP1381 implements a 8 ms timeout
generator: each time the 45 mV crossing occurs, the timeout
is reset. So, as long as the ringing becomes too low, the
timeout generator starts to count and after 8 ms, it delivers its
“green light”. If the skip signal is already present then the
controller restarts; otherwise the logic waits for it to release
the reset input and set the drive output high. Figure 14
depicts these two different situations:
As soon as the feedback voltage goes up again, there can
Figure 12. A Resistor to GND can Adjust the Skip
primary
R
−C
parasitic
30 mA
5
8
6
25 k
V
skip
V
network damping factor. The situation
DD
Level
V
DD
Hysteresis = 50 mV
−
+
Soft−Start
Activation
Reset