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参数资料
| 型号： | IRU3146CFTR |
| 厂商： | International Rectifier |
| 文件页数： | 7/30页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 28TSSOP |
| 标准包装： | 1 |
| PWM 型： | 电压模式 |
| 输出数： | 2 |
| 频率 - 最大： | 345kHz |
| 占空比： | 85% |
| 电源电压： | 4.5 V ~ 16 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 ~ 70°C |
| 封装/外壳： | 28-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 剪切带 (CT) |
| 产品目录页面： | 1384 (CN2011-ZH PDF) |
| 其它名称： | *IRU3146CFTR IRU3146CFCT |
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IRU3146
Dual Soft-Start
The IRU3146 has programmable soft-start to control the
output voltage rise and limit the inrush current during
start-up. It provides a separate Soft-Start function for each
outputs. This will enable to sequence the outputs by
SS2 / SD
SS1 / SD
8
20
25uA 25uA
64uA
Max
64uA
controlling the rise time of each output through selection
of different value soft-start capacitors. The soft-start pins
will be connected together for applications where, both
outputs are required to ramp-up at the same time.
To ensure correct start-up, the soft-start sequence ini-
tiates when the VCC, VCH1 and VCH2 rise above their
POR
0.8V
Error Amp1
threshold (4.2V and 3.5V respectively) and generate the
Power On Reset (POR) signal. Soft-start function oper-
Fb1
22
ates by sourcing an internal current to charge an exter-
nal capacitor to about 3V. Initially, the soft-start function
clamps the E/A’s output of the PWM converter. During
power up, the converter output starts at zero and thus
the voltage at Fb is about 0V. A current (64 μ A) injects
into the Fb pin and generates a voltage about 1.6V
(64 μ A × 25K) across the negative input of E/A and (see
Figure6).
Comp1
V P2
Fb2
Comp2
21
26
6
7
Error Amp2
The magnitude of this current is inversely proportional to
the voltage at soft-start pin. The 25 μ A current source
Figure 6 -Soft-start circuit for IRU3146
starts to charge up the external capacitor. In the mean
time, the soft-start voltage ramps up, the current flowing
into Fb pin starts to decrease linearly and so does the
Output of POR
3V
voltage at negative input of E/A.
When the soft-start capacitor is around 1V, the current
flowing into the Fb pin is approximately 32 μ A. The volt-
age at the positive input of the E/A is approximately:
32 μ A × 25K = 0.8V
The E/A will start to operate and the output voltage starts
Soft-Start
Voltage
Current flowing
into Fb pin
0V
64uA
0uA
? 2V
? 1V
to increase. As the soft-start capacitor voltage contin-
ues to go up, the current flowing into the Fb pin will keep
decreasing. Because the voltage at pin of E/A is regu-
lated to reference voltage 0.8V, the voltage at the Fb is:
Voltage at negative input ? 1.6V
of Error Amp
0.8V
0.8V
V FB = 0.8-(25K × Injected Current)
The feedback voltage increases linearly as the injecting
Voltage at Fb pin
0V
current goes down. The injecting current drops to zero
when soft-start voltage is around 2V and the output volt-
age goes into steady state. Figure 7 shows the theoreti-
cal operational waveforms during soft-start.
Figure 7 - Theoretical operational waveforms
during soft-start.
The output start-up time is the time period when soft-
start capacitor voltage increases from 1V to 2V. The
start-up time will be dependent on the size of the exter-
nal soft-start capacitor. The start-up time can be esti-
mated by:
25 μ A × T START /C SS = 2V-1V
Rev. 1.1
6/25/04
www.irf.com
7
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