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參數資料
| 型號： | NCP1351 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Variable Off Time PWM Controller |
| 中文描述： | 變關閉時間PWM控制器 |
| 文件頁數： | 12/19頁 |
| 文件大小： | 222K |
| 代理商： | NCP1351 |
NCP1351
http://onsemi.com
12
Figure 13. The Feedback Section Inside the NCP1351
C1
100n
+
Reset
V
CC
R
FB
45k
V
CC
D
FB
V
offset
500mV
+
I
FB
I
FB
I
FB
FB
IC
t
10
C
t
R1
2.5k
C
t
270p
Clock
I
diff
CS
R
offset
3.9k
I
diff
= ICS
max
ICS
min
f
(IFB)
I
diff
ICS
min
V
CC
C3
22pF
to R
sense
The FB pin can actually be seen as a diode, forward biased
by the optocoupler current. The feedback current, I
FB
on
Figure 13, enter an internal 45 k resistor which develops
a voltage. This voltage becomes the variable threshold point
for the capacitor charge, as indicated by Figure 8. Thus, in
lack of feedback current (startup or shortcircuit), there is
no voltage across the 45 k and the series offset of 500 mV
clamps the capacitor swing. If a 270 pF capacitor is used, the
maximum switching frequency is 65 kHz.
Folding the frequency back at a rather high peak current
can obviously generate audible noise. For this reason, the
NCP1351 uses a patented current compression technique
which reduces the peak current in lighter load conditions. By
design, the peak current changes from 100% of its full load
value, to 30% of this value in light load conditions. This is
the block placed on the lower left corner of Figure 13. In full
load conditions, the feedback current is weak and all the
current flowing through the external offset resistor is:
ICS
ICS_min
Idif
ICS_max
ICS_max
ICS_min
(eq. 12)
As the load goes lighter, the feedback current increases and
starts to steal current away from the generators. Equation 12
can thus be updated by:
ICS
ICS_max
kIFB
(eq. 13)
Equation 13 testifies for the current reduction on the offset
generator, k represents an internal coefficient. When the
feedback current equals I
dif
, the offset becomes:
ICS
ICS_min
(eq. 14)
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| NCP1351ADR2G | 功能描述:電流型 PWM 控制器 OSPI PWM CONTROLLER RoHS:否 制造商:Texas Instruments 開關頻率:27 KHz 上升時間: 下降時間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數量:1 最大工作溫度:+ 105 C 安裝風格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
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| NCP1351BDR2G | 功能描述:電流型 PWM 控制器 OSPI PWM CONTROLLER RoHS:否 制造商:Texas Instruments 開關頻率:27 KHz 上升時間: 下降時間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數量:1 最大工作溫度:+ 105 C 安裝風格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
| NCP1351BPG | 功能描述:電流型 PWM 控制器 VRIABLE OFF TM CNTRL RoHS:否 制造商:Texas Instruments 開關頻率:27 KHz 上升時間: 下降時間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數量:1 最大工作溫度:+ 105 C 安裝風格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
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