rt9259 Richtek Technology Corporation, rt9259 Datasheet - Page 12

• Manufacturer Part Number: rt9259
• Description: Synchronous Buck Dc-dc Linear Power Controller
• Manufacturer: Richtek Technology Corporation

RT9259
PCB 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 RT9259. 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.
www.richtek.com
12
Figure 11. Derating Curves for RT9259 Packages
1.75
1.25
0.75
0.25
1.5
0.5
2
1
0
0
SOP-14
SSOP-16
Ambient Temperature
25
QFN-16L 4x4
50
75
(°C)
4-Layers PCB
100
125
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 12 shows the
connections of the critical components in the converter.
Note that the capacitors C
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 copper filled
polygons on the top and bottom circuit layers for the PHASE
node, but it is not necessary to oversize this particular
island. Since the PHASE node is subjected to very high
dV/dt voltages, the stray capacitance formed between
these islands and the surrounding circuitry will tend to couple
switching noise. Use the remaining printed circuit layers
for small signal routing. 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.
Figure 12. The connections of the critical components in
5V/12V
GND
IQ1
the converter
Q1
UGATE
LGATE
Q2
DS9259-03T00 August 2007
IN
IQ2
and C
VCC
RT9259
IL
GND
OUT
FB
each of them
V
OUT
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