add8754dacpz-reel7 Analog Devices, Inc., add8754dacpz-reel7 Datasheet - Page 19

• Manufacturer Part Number: add8754dacpz-reel7
• Description: Lcd Panel Power, Vcom, And Gate Modulation
• Manufacturer: Analog Devices, Inc.

LAYOUT GUIDELINES
When designing a high frequency, switching, regulated power
supply, layout is very important. Using a good layout can solve
many problems associated with these types of supplies. Some of
the main problems are loss of regulation at high output current
and/or large input-to-output voltage differentials, excessive
noise on the output and switch waveforms, and instability.
Using the following guidelines can help minimize these
problems.
Make all power (high current) traces as short, direct, and thick
as possible. It is good practice on a standard PCB board to make
the traces an absolute minimum of 15 mil (0.381 mm) per
Ampere. The inductor, output capacitors, and output diode
should be as close to each other as possible. This helps reduce
the EMI radiated by the power traces that is due to the high
switching currents through them. This also reduces lead
inductance and resistance, which in turn reduce noise spikes,
ringing, and resistive losses that produce voltage errors.
The grounds of the IC, input capacitors, output capacitors, and
output diode (if applicable), should be connected close together,
directly to a ground plane. It is also a good idea to have a ground
plane on both sides of the printed circuit board (PCB). This
reduces noise by reducing ground-loop errors and absorbing
more of the EMI radiated by the inductor.
For multilayer boards of more than two layers, a ground plane
can be used to separate the power plane (power traces and
components) and the signal plane (feedback, compensation,
and components) for improved performance. On multilayer
boards, the use of vias is required to connect traces and different
planes. If a trace needs to conduct a significant amount of current
from one plane to the other, it is good practice to use one standard
via per 200 mA of current. Arrange the components so that the
switching current loops curl in the same direction.
Due to the how switching regulators operate, there are two
power states: one state when the switch is on, and one when the
switch is off. During each state, there is a current loop made by
the power components currently conducting. Place the power
components so that the current loop is conducting in the same
direction during each of the two states. This prevents magnetic
field reversal caused by the traces between the two half cycles
and reduces radiated EMI.
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Layout Procedure
To achieve high efficiency, good regulation, and stability, a good
PCB layout is required. It is recommended that the reference
board layout be followed as closely as possible because it is
already optimized for high efficiency and low noise.
Use the following general guidelines when designing PCBs:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Heat Sinking
When using a surface-mount power IC or external power
switches, the PCB can often be used as the heat sink. This is
done by simply using the copper area of the PCB to transfer
heat from the device.
Keep CIN close to the IN and GND leads of the ADD8754.
Keep the high current path from CIN (through L1) to the
SW and PGND leads as short as possible.
Keep the high current path from CIN (through L1), D1,
and COUT as short as possible.
Keep high current traces as short and wide as possible.
Keep nodes connected to SW away from sensitive traces
such as FB or COMP to prevent coupling of the traces. If
these traces need to be run near each other, place a ground
trace between the two as a shield.
Place the feedback resistors as close as possible to the FB pin
to prevent noise pickup.
Place the compensation components as close as possible to
the COMP pin.
Avoid routing noise-sensitive traces near the high current
traces and components.
Use a thermal pad size that is the same as the dimension of
the exposed pad on the bottom of the package.
ADD8754