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LTC3425 Datasheet(Fiches technique) 8 Page - Linear Technology

Numéro de pièce LTC3425
Description  5A, 8MHz, 4-Phase Synchronous Step-Up DC/DC Converter
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
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LTC3425
8
3425f
PI FU CTIO S
GNDA–D (Pins 1, 2, 7, 8, 17, 18, 23, 24): Power Ground
for the IC and the Four Internal N-channel MOSFETs.
Connect directly to the power ground plane.
SWA–D (Pins 3, 6, 19, 22): Switch Pins. Connect induc-
tors here. Minimize trace length to keep EMI to a mini-
mum. For discontinuous inductor current, a controlled
impedance is internally connected from SW to VIN to
minimize EMI. For applications where VOUT > 4.3V, it is
required to have Schottky diodes from SW to VOUT or a
snubber circuit to stay within absolute maximum rating on
the SW pins.
VOUTA–D (Pins 4, 5, 20, 21): Output of the Four Synchro-
nous Rectifiers. Connect output filter capacitors to these
pins. Connect one low ESR ceramic capacitor directly
from each pin to the ground plane.
REFEN (Pin 9): Pull this pin above 1.4V to enable the REF
output. Grounding this pin turns the REF output off to
reduce quiescent current.
VOUTS (Pin 10): VOUT Sense Pin. Connect VOUTS directly to
an output filter capacitor. The top of the feedback divider
network should also be tied to this point.
SGND (Pin 11): Signal Ground Pin. Connect to ground
plane, near the feedback divider resistor.
FB (Pin 12): Feedback Pin. Connect FB to a resistor divider,
keeping the trace as short as possible. The output voltage
can be adjusted according to the following formula:
V
RR
R
OUT =
+
122
12
1
.•
where R1 is connected from FB to SGND and R2 is
connected from FB to VOUTS.
COMP (Pin 13): Error Amp Output. A frequency compen-
sation network is connected from this pin to ground to
compensate the loop. See the section Closing the Feed-
back Loop for guidelines.
BURST (Pin 14): Burst Mode Threshold Adjust Pin. A
resistor/capacitor combination from this pin to ground
programs the average load current at which automatic
Burst Mode operation is entered.
For manual control of Burst Mode operation, ground
BURST to force Burst Mode operation or connect it to VOUT
to force fixed frequency PWM mode. Note that BURST
must not be pulled higher than VOUT.
REFOUT (Pin 15): Buffered 1.22V Reference Output. This
pin can source up to 100
µA and sink up to 10µA (only
active when REFEN is pulled high). This pin must be
decoupled with a 0.1
µF capacitor for stability.
PGOOD (Pin 16): Open-Drain Output of the Power Good
Comparator. This pin will go low when the output voltage
drops 11% below its regulated value. Maximum sink
current should be limited to 10mA.
SYNCOUT (Pin 25): Sync Output Pin. A clock is provided
at the oscillator frequency, but phase-shifted 180 degrees
to allow for synchronizing two devices for an 8-phase
converter.
CCM (Pin 26): This pin is used to select forced continuous
conduction mode. Normally this pin is grounded to allow
CCM or DCM operation. To force continuous conduction
mode, tie this pin to VOUT. In this mode, a reverse current
of up to about 0.6A will be allowed before turning off the
synchronous rectifier. This will prevent pulse skipping at
light load when Burst Mode operation is disabled, and will
also improve the large-signal transient response when
going from a heavy load to a light load. For Burst Mode
operation, CCM should be low.




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