MIC2174 Datasheet, PDF (18/24 Pages) Micrel Semiconductor – 300kHz, Synchronous Buck Controller 300kHz, Synchronous Buck Controller

Micrel, Inc.
PCB Layout Guideline
Warning!!! To minimize EMI and output noise, follow
these layout recommendations.
PCB Layout is critical to achieve reliable, stable and
efficient performance. A ground plane is required to
control EMI and minimize the inductance in power,
signal and return paths.
The following guidelines should be followed to insure
proper operation of the MIC2174 converter.
IC
• Place the IC and MOSFETs close to the point of
load (POL).
• Use fat traces to route the input and output power
lines.
• Signal and power grounds should be kept separate
and connected at only one location.
Input Capacitor
• Place the HSD input capacitor next.
• Place the HSD input capacitors on the same side of
the board and as close to the MOSFETs as
possible.
• Keep both the HSD and PGND connections short.
• Place several vias to the ground plane close to the
HSD input capacitor ground terminal.
• Use either X7R or X5R dielectric input capacitors.
Do not use Y5V or Z5U type capacitors.
• Do not replace the ceramic input capacitor with any
other type of capacitor. Any type of capacitor can be
placed in parallel with the input capacitor.
• If a Tantalum input capacitor is placed in parallel
with the input capacitor, it must be recommended for
switching regulator applications and the operating
voltage must be derated by 50%.
• In “Hot-Plug” applications, a Tantalum or Electrolytic
bypass capacitor must be used to limit the over-
voltage spike seen on the input supply with power is
suddenly applied.
• An additional Tantalum or Electrolytic bypass input
capacitor of 22µF or higher is required at the input
power connection.
• The 1µF and 0.1µF capacitors, which connect to the
V_IN terminal, must be located right at the IC. The V_IN
terminal is very noise sensitive and placement of the
capacitor is very critical. Connections must be made
with wide trace.
MIC2174
Inductor
• Keep the inductor connection to the switch node
(LX) short.
• Do not route any digital lines underneath or close to
the inductor.
• Keep the switch node (LX) away from the feedback
(FB) pin.
• The LX pin should be connected directly to the drain
of the low-side MOSFET to accurate sense the
voltage across the low-side MOSFET.
• To minimize noise, place a ground plane underneath
the inductor.
Output Capacitor
• Use a wide trace to connect the output capacitor
ground terminal to the input capacitor ground
terminal.
• Phase margin will change as the output capacitor
value and ESR changes. Contact the factory if the
output capacitor is different from what is shown in
the BOM.
• The feedback trace should be separate from the
power trace and connected as close as possible to
the output capacitor. Sensing a long high current
load trace can degrade the DC load regulation.
Schottky Diode (Optional)
• Place the Schottky diode on the same side of the
board as the MOSFETs and HSD input capacitor.
• The connection from the Schottky diode’s Anode to
the input capacitors ground terminal must be as
short as possible.
• The diode’s Cathode connection to the switch node
(LX) must be keep as short as possible.
RC Snubber
• Place the RC snubber on the same side of the board
and as close to the MOSFETs as possible.
September 2009
18
M9999-090409-B