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参数资料
| 型号： | IR3651STRPBF |
| 厂商： | International Rectifier |
| 文件页数： | 10/19页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 14-SOIC |
| 标准包装： | 2,500 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 460kHz |
| 占空比： | 80% |
| 电源电压： | 4.5 V ~ 13.2 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | 0°C ~ 125°C |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
| 配用： | IRDC3651-ND - KIT REF DESIGN SYNCH BUCK REG |
IR3651SPBF
Soft-Start
The IR3651 has programmable soft-start to
control the output voltage rise and limit the inrush
current during start-up.
20uA
3V
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
SS/SD
POR
64uA
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
Comp
1.25V
Fb
25K
25K
Error Amp
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.
Fig. 4: Soft-Start circuit for IR3651
Output of UVLO
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
POR
3V
? 2V
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
Soft-Start
Voltage
Current flowing
into Fb pin
0V
64uA
0uA
? 1V
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
Voltage at negative input ? 1.6V
of Error Amp
1.25V
1.25V
period when soft-start capacitor voltage
increases from 1V to 2V.
The start-up time will be dependent on the size of
the external soft-start capacitor and can be
Voltage at Fb pin
0V
estimate by:
Fig. 5: Theoretical operation waveforms
20 μ A ?
T start
C ss
= 2 V ? 1 V
during soft-start
For a given start-up time, the soft-start capacitor
(nF) can be estimated as:
C SS ? 20 μ A * T start ( ms )
06/08/2011
--( 1 )
10
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