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| 型号： | IR3621FTR |
| 厂商： | International Rectifier |
| 文件页数： | 8/29页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 28TSSOP |
| 产品变化通告： | (EP) Parts Discontinuation 25/May/2012 |
| 标准包装： | 0 |
| PWM 型： | 电压模式 |
| 输出数： | 2 |
| 频率 - 最大： | 345kHz |
| 占空比： | 86.5% |
| 电源电压： | 5.5 V ~ 14.5 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： | -40°C ~ 125°C |
| 封装/外壳： | 28-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 标准包装 |
| 产品目录页面： | 1383 (CN2011-ZH PDF) |
| 配用： | IRDC3621-ND - BOARD EVAL DUAL SYNC BUCK CTRLR |
| 其它名称： | IR3621FDKR |
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IR3621 & (PbF)
Dual Soft-Start
28uA 28uA
The IR3621 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
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 and generate the Power On Reset (POR) sig-
nal. Soft-start function operates by sourcing an internal
current to charge an external capacitor to about 3V. Ini-
Fb1
Comp1
22
21
tially, the soft-start function clamps the E/A’s output of
the PWM converter. During power up, the converter out-
put starts at zero and thus the voltage at Fb is about 0V.
V P2
26
Error Amp2
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).
The magnitude of this current is inversely proportional to
Fb2
Comp2
6
7
the voltage at soft-start pin. The 28 μ A current source
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
voltage at negative input of E/A.
Figure 6 -Soft-start circuit for IR3621
Output of POR
3V
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:
Soft-Start
Voltage
0V
? 1.8V
? 1V
32 μ A × 25K = 0.8V
The E/A will start to operate and the output voltage starts
Current flowing
into Fb pin
64uA
0uA
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 1.8V and the output
voltage goes into steady state. Figure 7 shows the theo-
retical operational waveforms during soft-start.
Low Temperature Start-Up
The controller is capable of starting at -40 C ambient
temperature.
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 1.8V. 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:
28 μ A × T START /C SS = 1.8V-1V
8
www.irf.com
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