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參數(shù)資料
| 型號： | IR3621M |
| 廠商： | International Rectifier |
| 英文描述： | 2-PHASE / DUAL SYNCHRONOUS PWM CONTROLLER WITH OSCILLATOR SYNCHRONIZATION AND PRE-BIAS STARTUP |
| 中文描述： | 2相/雙同步PWM控制器，帶有振蕩器同步操作和預偏置啟動 |
| 文件頁數(shù)： | 8/29頁 |
| 文件大小： | 389K |
| 代理商： | IR3621M |
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8
IR3621 & (PbF)
www.irf.com
Dual Soft-Start
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
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
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-
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.
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
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.
SS1 / SD
Comp2
Error Amp2
POR
28uA
Fb1
Comp1
Error Amp1
64uA
Max
Fb2
0.8V
SS2 / SD
20
22
21
6
V
P2
26
8
7
28uA
64uA
Figure 6 -Soft-start circuit for IR3621
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:
The E/A will start to operate and the output voltage starts
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:
V
FB
= 0.8-(25K
×
Injected Current)
The feedback voltage increases linearly as the injecting
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.
32
μ
A
×
25K = 0.8V
Soft-Start
Voltage
Voltage at negative input
of Error Amp
Voltage at Fb pin
Current flowing
into Fb pin
64uA
0uA
0V
0.8V
1.6V
0.8V
0V
3V
1.8V
1V
Output of POR
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
Low Temperature Start-Up
The controller is capable of starting at -40
!
C ambient
temperature.
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