744309025 查看數據表（PDF） - International Rectifier
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744309025
International Rectifier
744309025 Datasheet PDF : 53 Pages
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IR3846
PVin=Vin
Vcc
Vp
EN
Intl_SS
Vo
> 1.0V
> 1.2V
Figure 5: Recommended startup for Normal operation
PVin=Vin
Vcc
Vp
EN
Intl_SS
Vo
> 1.2V
Figure 6: Recommended startup for sequencing
operation (ratiometric or simultaneous)
PVin=Vin
Vcc
VDDQ
Vp
EN
Vref
Vo
VDDQ/2
> 1.2V
0V
VTT Tracking
Figure 7: Recommended startup for memory tracking
operation (Vtt-DDR)
Figure 5 shows the recommended startup sequence
for the normal (non-tracking, non-sequencing)
operation of IR3846, when Enable is used as a logic
input. In this operating mode Vref is left floating.
Figure 6 shows the recommended startup sequence
for sequenced operation of IR3846 with Enable used
as logic input. Figure 7 shows the recommended
startup sequence for tracking operation of IR3846 with
Enable used as logic input. For this mode of
operation, Vref should be connected to LGND.
PRE-BIAS STARTUP
IR3846 is able to start up into pre-charged output,
which prevents oscillation and disturbances of the
output voltage.
The output starts in asynchronous fashion and keeps
the synchronous MOSFET (Sync FET) off until the
first gate signal for control MOSFET (Ctrl FET) is
generated. Figure 8 shows a typical Pre-Bias
condition at start up. The sync FET always starts with
a narrow pulse width (12.5% of a switching period)
and gradually increases its duty cycle with a step of
12.5% until it reaches the steady state value. The
number of these startup pulses for each step is 16
and it’s internally programmed. Figure 9 shows the
series of 16x8 startup pulses.
[V]
Vo
Pre-Bias
Voltage
[Time]
Figure 8: Pre-Bias startup
HDRv
LDRv
... ...
12.5% 25% ...
... ...
16
16
...
...
87.5%
...
...
...
End of
PB
Figure 9: Pre-Bias startup pulses
SOFT-START
IR3846 has an internal digital soft-start to control the
output voltage rise and to limit the current surge at the
start-up. To ensure correct start-up, the soft-start
sequence initiates when the Enable and VCC rise
19 www.irf.com © 2013 International Rectifier
August 01, 2013
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