MIC2169B Datasheet, PDF (18/24 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

Micrel, Inc.
Design and 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 MIC2169B 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 V_IN input capacitor next.
• Place the V_IN input capacitors on the same side of the
board and as close to the MOSFETs as possible.
• Keep both the V_IN and power GND connections short.
• Place several vias to the ground plane close to the V_IN
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.
• Use a 5Ω resistor from the input supply to the VDD pin
on the MIC2169B. Also, place a 1µF ceramic
capacitor from this pin to GND, preferably not through
a via. The capacitor must be located right at the IC.
The V_DD terminal is very noise sensitive and
placement of the capacitor is very critical. Connections
must be made with wide trace.
MIC2169B
Inductor
• Keep the inductor connection to the switch node (SW)
short.
• Do not route any digital lines underneath or close to
the inductor.
• Keep the switch node (SW) away from the feedback
(FB) pin.
• 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.
MOSFETs
• Low gate charge MOSFETs should be used to
maximize efficiency, such as Si4800, Si4804BDY,
IRF7821, IRF8910, FDS6680A and FDS6912A, etc.
RC Snubber
• Add a RC snubber of 1.4Ω resistor and a 1000pF
capacitor from the switch node to ground pin. Place
the snubber on the same side of the board and as
close to the MOSFETs as possible. See page 8,
Current Limiting section for more detail.
Schottky Diode (Optional)
• Place the Schottky diode on the same side of the
board as the MOSFETs and V_IN 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
(SW) must be keep as short as possible.
Others
• Connect the current limiting (R2) resistor directly to
the drain of top MOSFET Q1.
• The feedback resistors R3 and R4/R5/R6 should be
placed close to the FB pin. The top side of R3 should
connect directly to the output node. Run this trace
away from the switch node (junction of Q1, Q2, and
L1). The bottom side of R3 should connect to the
GND pin on the MIC2169B.
June 2009
18
M9999- 060309-A
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