參數(shù)資料
型號: NCP1381DR2G
廠商: ON SEMICONDUCTOR
元件分類: 穩(wěn)壓器
英文描述: Low−Standby High Performance PWM Controller
中文描述: 1 A SWITCHING CONTROLLER, 125 kHz SWITCHING FREQ-MAX, PDSO14
封裝: LEAD FREE, SOIC-14
文件頁數(shù): 12/26頁
文件大小: 559K
代理商: NCP1381DR2G
NCP1381
http://onsemi.com
12
Figure 12 depicts the internal comparator arrangement.
The FB pin level is permanently compared to a fixed level,
V
skip
, also available on Pin 5 for adjustment. As a result, the
user can wire a resistor to ground and alter the skip level in
case of noise problems. When the FB pin is above V
skip
, the
comparator is transparent to the operation. When the load
becomes lighter, the FB level goes down too. When it
reaches V
skip
, the comparator goes high and resets the
internal flipflop: the driving pulses are stopped. As a result,
V
out
starts to also decrease since no energy transfer is
ensured. Detecting a decay in the output voltage, the FB loop
will react by increasing its level. When the level crosses
V
skip
plus a slight hysteresis, pulses restart again: a ripple
occurs on the FB pin. Please note that the softstart 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.
Figure 11. The Skip Cycle Takes Place at Low Peak
Currents Which Guaranties Noise Free Operation
0
100
200
Width
C
Recurrence
Skip Cycle
Current Limit
Maximum
Peak Current
Figure 12. A Resistor to GND can Adjust the Skip
Level
+
V
skip
V
DD
V
DD
5
8
6
30 A
R
25 k
Reset
SoftStart
Activation
Hysteresis = 50 mV
As soon as the feedback voltage goes up again, there can
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
primary
C
parasitic
network damping factor. The situation
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 s 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 s, 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:
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