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IRF7811 Datasheet(PDF) 35 Page - Analog Devices

Part No. IRF7811
Description  Synchronous Buck Controller
Download  40 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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IRF7811 Datasheet(HTML) 35 Page - Analog Devices

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Data Sheet
ADP1878/ADP1879
Rev. A | Page 35 of 40
Figure 90. Layer 4 (Bottom Layer) of Evaluation Board
IC SECTION (LEFT SIDE OF EVALUATION BOARD)
A dedicated plane for the analog ground plane (GND) should
be separate from the main power ground plane (PGND). With
the shortest path possible, connect the analog ground plane to
the GND pin (Pin 5). Place this plane on the top layer only of
the evaluation board. To avoid crosstalk interference, do not
allow any other voltage or current pathway directly below this
plane on Layer 2, Layer 3, or Layer 4. Connect the negative
terminals of all sensitive analog components to the analog
ground plane. Examples of such sensitive analog components
include the bottom resistor of the resistor divider, the high
frequency bypass capacitor for biasing (0.1 μF), and the
compensation network.
Mount a 1 μF bypass capacitor directly across the VREG pin
(Pin 7) and the PGND pin (Pin 11). In addition, tie a 0.1 μF
across the VREG pin (Pin 7) and the GND pin (Pin 5).
POWER SECTION
As shown in Figure 87, an appropriate configuration to localize
large current transfer from the high voltage input (VIN) to the
output (VOUT) and then back to the power ground is to put the
VIN plane on the left, the output plane on the right, and the main
power ground plane in between the two. Current transfers from
the input capacitors to the output capacitors, through Q1/Q2,
during the on state (see Figure 91). The direction of this current
(yellow arrow) is maintained as Q1/Q2 turns off and Q3/Q4 turns
on. When Q3/Q4 turns on, the current direction continues to be
maintained (red arrow) as it circles from the power ground
terminal of the bulk capacitor to the output capacitors, through
the Q3/Q4. Arranging the power planes in this manner minimizes
the area in which changes in flux occur if the current through
Q1/Q2 stops abruptly. Sudden changes in flux, usually at the
source terminals of Q1/Q2 and the drain terminal of Q3/Q4,
cause large dV/dt at the SW node.
The SW node is near the top of the evaluation board. The SW
node should use the least amount of area possible and be away
from any sensitive analog circuitry and components. This is
because the SW node is where most sudden changes in flux
density occur. When possible, replicate this pad onto Layer 2
and Layer 3 for thermal relief and eliminate any other voltage and
current pathways directly beneath the SW node plane. Populate
the SW node plane with vias, mainly around the exposed pad of
the inductor terminal and around the perimeter of the source of
Q1/Q2 and the drain of Q3/Q4.
The output voltage power plane (VOUT) is at the rightmost end of
the evaluation board. This plane should be replicated, descending
down to multiple layers with vias surrounding the inductor
terminal and the positive terminals of the output bulk capacitors.
Ensure that the negative terminals of the output capacitors are
placed close to the main power ground (PGND), as previously
mentioned. All of these points form a tight circle (component
geometry permitting) that minimizes the area of flux change as
the event switches between D and 1 − D.
BOTTOM
RESISTOR TAP
TO ANALOG
GROUND PLANE
PGND SENSE TAP FROM
NEGATIVE TERMINALS OF
THE OUTPUT BULK
CAPACITORS. THIS
TRACK PLACEMENT
SHOULD BE DIRECTLY
BELOW THE VOUT SENSE
LINE OF LAYER 3.


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