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IP1206PBF 데이터시트(PDF) 13 Page - International Rectifier

부품명 IP1206PBF
상세설명  Synchronous Buck Multiphase Optimized LGA Power Block
PDF  30 Pages
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제조업체  IRF [International Rectifier]
홈페이지  http://www.irf.com
Logo IRF - International Rectifier

IP1206PBF 데이터시트(HTML) 13 Page - International Rectifier

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2/26/2008
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13
iP1206PbF
Soft-Start (cont.)
During power up, the converter output starts at
zero and thus the voltage at Fb is about 0V. A
current (64µA) injects into the Fb pin and
generates a voltage about 1.6V (64µAx25K)
across the negative input of error amplifier, see
Fig. 10.
The magnitude of this current is inversely
proportional to the voltage at the soft-start pin.
The 28µA 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 error amplifier.
When the soft-start capacitor is around 1V, the
voltage at the negative input of the error amplifier
is approximately 0.8V.
As the soft-start capacitor voltage charges up,
the current flowing into the Fb pin keeps
decreasing.
The feedback voltage increases linearly as the
injecting current decreases. The injecting current
drops to zero when soft-start voltage is around
1.8V and the output voltage goes into steady
state. Fig. 11 shows the 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 external soft-start capacitor. The start-
up time can be estimated by:
For a given start up time, the soft-start capacitor
(nF) can be estimated as:
For normal start up the Seq pin should be pulled
high (usually can be connected to Vout3).
Fig. 10: Soft-Start circuit for iP1206
Fig. 11: Theoretical operation waveforms
during soft-start
)
5
(
-
-
-
-
V
8
0
ms
T
A
20
C
start
SS
)
(
.
)
(
*
)
(
μ
≅
V
1
V
8
1
C
T
μA
28
ss
start
−
=
∗
.
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
E/A2
VP2
Fb2
SS2/SD2
ISS2 = 28uA
Track
POR
OCP2
Ihiccup2 = 3uA
64uA
3V
E/A1
VP1
Fb1
SS1/SD1
ISS1 = 28uA
POR
OCP1
Ihiccup1 = 3uA
64uA
3V
Seq



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