參數(shù)資料
型號(hào)： IRU3146
廠商： International Rectifier
英文描述： DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND AUTO-RESTART
中文描述： 雙同步PWM控制器，帶有均流電路和自動(dòng)重新啟動(dòng)
文件頁(yè)數(shù)： 7/30頁(yè)
文件大?。?/td> 528K
代理商： IRU3146
IRU3146
7
Rev. 1.1
6/25/04
www.irf.com
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
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 (4.2V and 3.5V respectively) and generate the
Power On Reset (POR) signal. Soft-start function oper-
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).
The magnitude of this current is inversely proportional to
the voltage at soft-start pin. The 25
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.
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:
SS1 / SD
Comp2
Error Amp2
POR
25uA
Fb1
Comp1
Error Amp1
64uA
Max
Fb2
0.8V
SS2 / SD
20
22
21
6
V
P2
26
8
7
25uA
64uA
Figure 6 -Soft-start circuit for IRU3146
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 2V and the output volt-
age goes into steady state. Figure 7 shows the theoreti-
cal 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
2V
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 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
相關(guān)PDF資料
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