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參數(shù)資料
| 型號： | IR3651STRPBF |
| 廠商： | International Rectifier |
| 英文描述： | HIGH VOLTAGE SYNCHRONOUS PWM BUCK CONTROLLER |
| 中文描述： | 高壓同步PWM降壓控制器 |
| 文件頁數(shù)： | 10/19頁 |
| 文件大小： | 397K |
| 代理商： | IR3651STRPBF |
IR3651SP
b
F
10/11/2006
Soft-Start
The IR3651 has programmable soft-start to
control the output voltage rise and limit the inrush
current during start-up.
To ensure correct start-up, the soft-start
sequence initiates when Vcc rises above its
threshold and generate the Power On Ready
(POR) signal. The soft-start function operates by
sourcing current to charge an external capacitor
to about 3V.
Initially, the soft-start function clamps the output
of error amplifier by injecting a current (64uA)
into the Fb pin and generates a voltage about
1.6V (64ux25K) across the negative input of error
amplifier (see figure 4).
The magnitude of the injected current is inversely
proportional to the voltage at the soft-start pin. As
the soft-start voltage ramps up, the injected
current decreases linearly and so does the
voltage at negative input of error amplifier.
When the soft-start capacitor is around 1V, the
voltage at the positive input of the error amplifier
is approximately 1.25V.
The output of error amplifier will start increasing
and generating the first PWM signal. As the soft-
start capacitor voltage continues to go up, the
current flowing into the Fb pin will keep
decreasing.
The feedback voltage increases linearly as the
soft start voltage ramps up. When soft-start
voltage is around 2V the output voltage is
reached the steady state and the injected current
is zero.
Figure 5 shows the theoretical operational
waveforms during soft-start.
The output voltage start-up time is the time
period
when
soft-start
increases from 1V to 2V.
The start-up time will be dependent on the size of
the external soft-start capacitor and can be
estimate by:
capacitor
voltage
Fig. 4: Soft-Start circuit for IR3651
Fig. 5: Theoretical operation waveforms
during soft-start
20uA
64uA
POR
Error Amp
SS/SD
Fb
Comp
25K
1.25V
25K
3V
Soft-Start
Voltage
Voltage at negative input
of Error Amp
Voltage at Fb pin
Current flowing
into Fb pin
64uA
0uA
0V
1.25V
1.6V
1.25V
0V
3V
2V
1V
Output of UVLO
POR
For a given start-up time, the soft-start capacitor
(nF) can be estimated as:
V
1
V
2
C
T
μA
20
ss
start
=
)
1
--(
)
ms
(
T
A
20
C
start
SS
*
μ
10
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