ADD5205 Data Sheet
Rev. 0 | Page 12 of 16
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. 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 printed circuit
board (PCB) to make the traces an absolute minimum of 15 mil
(0.381 mm) per ampere. Place the inductor, output capacitors,
and output diode 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 reduces 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 PCB. This reduces noise by reducing
ground loop errors and by absorbing more of the EMI radiated
by the inductor.
Due to how switching regulators operate, there are two power
states: on and 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.
Layout Procedure
Use the following general guidelines when designing PCBs:
• Keep C
IN
close to the VIN and GND leads of the ADD5205.
• Keep the high current path from C
IN
(through L1) to the
SW and GND leads as short as possible.
• Keep the high current path from C
IN
(through L1), D1, and
C
OUT
as short as possible.
• Keep high current traces as short and wide as possible.
• Place the C
OVP
as close as possible to the OVP pin.
• Place the LED current setting resistors as close as possible
to each pin to prevent noise pickup.
• Avoid routing noise sensitive traces near high current
traces and components, especially the LED current setting
node (ISET).
• Use a thermal pad size that is the same dimension as the
exposed pad on the bottom of the package.
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 using the copper area of the PCB to transfer heat from
the device. Users should maximize this area to optimize thermal
performance.
