IRDC3831 International Rectifier, IRDC3831 Datasheet - Page 12

• Manufacturer Part Number: IRDC3831
• Description: BOARD EVAL SYNC BUCK CONVERTER
• Manufacturer: International Rectifier

Pre-Bias Startup
IR3831 is able to start up into pre-charged
output,
disturbances of the output voltage.
The output starts in asynchronous fashion and
keeps the synchronous MOSFET off until the first
gate signal for control MOSFET is generated.
Figure 4 shows a typical Pre-Bias condition at
start up.
The synchronous MOSFET always starts with a
narrow pulse width and gradually increases its
duty cycle with a step of 25%, 50%, 75% and
100% until it reaches the steady state value. The
number
synchronous MOSFET is internally programmed.
Figure 5 shows a series of 32, 16, 8 startup
pulses.
11/05/10
Figure 3c. shows the recommended startup
sequence for tracking operation of IR3831 with
Enable used as logic input.
Fig. 3c. Recommended startup sequence,
of
which
Fig. 4. Pre-Bias startup
these
Sequenced operation
prevents
startup
oscillation
pulses
for
and
the
Soft-Start
The IR3831 has a programmable soft-start to
control the output voltage rise and limit the
current surge at the start-up. To ensure correct
start-up, the soft-start sequence initiates when
the Enable and Vcc rise above their UVLO
thresholds and generate the Power On Ready
(POR) signal. The internal current source
(typically 20uA) charges the external capacitor
C
waveforms during the soft start.
The start up time can be estimated by:
During the soft start the OCP is enabled to
protect the device for any short circuit and over
current condition.
ss
T
start
linearly from 0V to 3V. Figure 6 shows the
Fig. 6. Theoretical operation waveforms
=
V
Fig. 5. Pre-Bias startup pulses
p
20
*
μ
C
A
SS
during soft-start
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