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參數資料
| 型號： | NCP1560 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Full Featured Voltage Mode PWM Controller(全性能電壓模式PWM控制器) |
| 中文描述： | 全功能電壓模式PWM控制器（全性能電壓模式的PWM控制器） |
| 文件頁數： | 14/18頁 |
| 文件大小： | 122K |
| 代理商： | NCP1560 |
NCP1560
http://onsemi.com
14
Once the UV or OV condition is removed and V
AUX
reaches 11 V, the controller initiates a soft start cycle.
Figure 31 shows the relationship between the UV/OV
voltage, the outputs and the soft start voltage.
The UV/OV pin can also be used to implement a remote
enable/disable function. Biasing the UV/OV pin below its
UV threshold disables the converter.
Figure 31. Soft Start Timing Diagram (Using Auxiliary Winding)
V
AUX(off)
V
AUX(on)
V
AUX
0 V
2 V
0 V
0 V
0 V
0 V
OUT2
OUT1
Soft Start Voltage
UV/OV Voltage
SOFT START
Feedforward Ramp Generator
The NCP1560 incorporates line feedforward (FF) to
compensate for changes in line voltage. A FF Ramp
proportional to V
in
is generated and compared to V
EA
. If the
line voltage changes, the FF Ramp slope changes
accordingly. The duty cycle will be adjusted immediately
instead of waiting for the line voltage change to propagate
around the system and be reflected back on V
EA
.
A resistor between V
in
and the FF pin (R
FF
) sets the
feedforward current (I
FF
). The FF Ramp is generated by
charging an internal 10 pF capacitor (C
FF
) with a constant
current proportional to I
FF
. The FF Ramp is finished
(capacitor is discharged) once the Oscillator Ramp reaches
2.0 V. Please refer to Figure 2 for a functional drawing of the
Feedforward Ramp generator.
I
FF
is usually a few hundred microamps, depending on the
operating frequency and the required duty cycle. If the
operating frequency and maximum duty cycle are known,
I
FF
is calculated using the equation below:
CFF
VDC(inv)
6.7 k
IFF
125 k
ton(max)
where V
DC(inv)
is the voltage on the inverting input of the
Max DC Comparator and t
on(max)
is the maximum ON time.
Figure 18 shows the relationship between I
FF
and DC
MAX
.
For example, if a system is designed to operate at 300 kHz,
with a 60% maximum duty cycle at 36 V, the DC
MAX
pin can
be grounded and I
FF
is calculated as follows:
T
f
300 kHz
1
1
3.33 s
ton(max)
DCMAX
T
0.6
3.33 s
2.0 s
IFF
CFF
VDC(inv)
6.7 k
125 k
ton(max)
10 pF
0.888 V
6.7 k
125 k
2.0 s
82.8 A
As the minimum line voltage is 36 V, the required
feedforward resistor is calculated using the equation below:
Vin
IFF
82.8 A
From the above calculations it can be observed that I
FF
is
controlled predominantly by the value of R
FF
, as the
resistance seen into the FF pin is only 12 k . If a tight
maximum duty cycle control over temperature is required,
R
FF
should have a low thermal coefficient.
RFF
12.0 k
36 V
12.0 k
434 k
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