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IR3621MPBF Datasheet(PDF) 8 Page - International Rectifier

Part # IR3621MPBF
Description  2-PHASE / DUAL SYNCHRONOUS PWM CONTROLLER WITH OSCILLATOR SYNCHRONIZATION AND PRE-BIAS STARTUP
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Manufacturer  IRF [International Rectifier]
Direct Link  http://www.irf.com
Logo IRF - International Rectifier

IR3621MPBF Datasheet(HTML) 8 Page - International Rectifier

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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
µA) injects into the Fb pin and generates a
voltage about 1.6V (64
µA×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
µA 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
VP2 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
µA. 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:
VFB = 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
µA×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
µA×TSTART/CSS = 1.8V-1V
Low Temperature Start-Up
The controller is capable of starting at -40
!C ambient
temperature.


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