FAN5070 Datasheet, PDF (11/14 Pages) Fairchild Semiconductor – High Performance Programmable Synchronous DC-DC Controller

PCB Layout Guidelines
• Placement of the MOSFETs relative to the FAN5070 is
critical. Place the MOSFETs such that the trace length of
the HIDRV and LODRV pins of the FAN5070 to the FET
gates is minimized. A long lead length on these pins will
cause high amounts of ringing due to the inductance of the
trace and the gate capacitance of the FET. This noise
radiates throughout the board, and, because it is switching
at such a high voltage and frequency, it is very difficult to
suppress.
• In general, all of the noisy switching lines should be kept
away from the quiet analog section of the FAN5070. That
is, traces that connect to pins 1, 2, 16, and 17 (HIDRV,
SW, LODRV and V_CCP) should be kept far away from the
traces that connect to pins 3, 13 and 14.
• Place the 0.1µF decoupling capacitors as close to the
FAN5070 pins as possible. Extra lead length on these
reduces their ability to suppress noise.
• Each V_CC and GND pin should have its own via to the
appropriate plane. This helps provide isolation between
pins.
• Place the MOSFETs, inductor, and Schottky as close
together as possible for the same reasons as in the first
bullet above. Place the input bulk capacitors as close to
the drains of the high side MOSFETs as possible. In
addition, placement of a 0.1µF decoupling cap right on
the drain of each high side MOSFET helps to suppress
some of the high frequency switching noise on the input
of the DC-DC converter.
• Place the output bulk capacitors as close to the CPU as
possible to optimize their ability to supply instantaneous
current to the load in the event of a current transient.
Additional space between the output capacitors and the
CPU will allow the parasitic resistance of the board traces
to degrade the DC-DC converter’s performance under
severe load transient conditions, causing higher voltage
deviation. For more detailed information regarding
capacitor placement, refer to Application Bulletin AB-5.
• A PC Board Layout Checklist is available from Fairchild
Applications. Ask for Application Bulletin AB-11.
Additional Information
For additional information contact your local Fairchild
Semiconductor representative, or visit us at our web site
www.fairchildsemi.com.
REV. 1.0.1 8/7/01
FAN5070
11