RT9209 RICHTEK [Richtek Technology Corporation], RT9209 Datasheet - Page 13

• Manufacturer Part Number: RT9209
• Description: Synchronous Buck PWM DC-DC with Enable & PGOOD
• Manufacturer: RICHTEK [Richtek Technology Corporation]

PWM Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. The voltage
spikes can degrade efficiency and radiate noise, that
results in over-voltage stress on devices. Careful
component placement layout and printed circuit design
can minimize the voltage spikes induced in the converter.
Consider, as an example, the turn-off transition of the
upper MOSFET prior to turn-off, the upper MOSFET was
carrying the full load current. During turn-off, current stops
flowing in the upper MOSFET and is picked up by the low
side MOSFET or Schottky diode. Any inductance in the
switched current path generates a large voltage spike
during the switching interval. Careful component
selections, layout of the critical components, and use
shorter and wider PCB traces help in minimizing the
magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using the RT9209/A. The switching power
components are most critical because they switch large
amounts of energy, and as such, they tend to generate
equally large amounts of noise. The critical small signal
components are those connected to sensitive nodes or
those supplying critical bypass current.
The power components and the PWM controller should
be placed firstly. Place the input capacitors, especially the
high-frequency ceramic decoupling capacitors, close to
the power switches. Place the output inductor and output
capacitors between the MOSFETs and the load. Also
locate the PWM controller near by MOSFETs.
A multi-layer printed circuit board is recommended. Figure
9 shows the connections of the critical components in
the converter. Note that the capacitors C
of them represents numerous physical capacitors. Use a
dedicated grounding plane and use vias to ground all
critical components to this layer. Apply another solid layer
as a power plane and cut this plane into smaller islands
of common voltage levels. The power plane should
support the input power and output power nodes. Use
the remaining printed circuit layers for small signal routing.
DS9209/A-08 March 2007
IN
and C
OUT
each
GND
5V
The PCB traces between the PWM controller and the gate
of MOSFET and also the traces connecting source of
MOSFETs should be sized to carry 2A peak currents.
C
IN
IQ1
Q1
UGATE
LGATE
Figure 9
Q2
A
IQ2
VCC
RT9209/A
IL
RT9209/A
GND
FB
www.richtek.com
C
V
OUT
OUT
LOAD
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