rt9212 Richtek Technology Corporation, rt9212 Datasheet - Page 13

• Manufacturer Part Number: rt9212
• Description: Dual Synchronous Buck Dc-dc Linear Power Controller
• Manufacturer: Richtek Technology Corporation

Reference Voltage
Because one of the RT9212 regulators uses a low 35dB
gain error amplifier, shown in Figure 4. The voltage
regulation is dependent on V
reference voltage of 0.8V is trimmed at V
V
FB reference voltage vs. V
as Figure 5.
Layout Consideration
The layout is very important when designing high frequency
switching converters. Layout will affect noise pickup and
can cause a good design to perform with less than expected
results.
1. Even though double-sided PCB is usually sufficient for
a good layout, four-layer PCB is the optimum approach to
reducing the noise. Use the two internal layers as the power
and GND planes, the top layer for power connections with
wide, copper filled areas, and the bottom layer for the noise
sensitive traces.
DS9212-05 March 2007
OUT
FB
= 2.5V condition. In a fixed V
0.815
0.805
0.795
0.785
0.775
0.81
0.79
0.78
0.8
+
-
1
1.5
REF
0.8V
1K
2
R1
Figure 4
Figure 5
2.5
OUT
V
1.9V
OUT
IN
voltage can be calculated
RAMP
-
+
3
EA
56K
R2
(V)
& V
IN
3.5
= 5V application, the
OUT
setting.The FB
4
+
-
PWM
IN
4.5
= 5V &
Preliminary
5
2. There are two sets of critical components in a DC-DC
converter. The switching components are the most critical
because they switch large amounts of energy, and therefore
tend to generate large amounts of noise. The others are
the small signal components that connect to sensitive
nodes or supply critical bypass current and signal coupling.
Make all critical component ground connections with vias
to GND plane.
3. Use fewer, but larger output capacitors, keep the
capacitors clustered, and use multiple layer traces with
heavy copper to keep the parasitic resistance low. Place
the output capacitors as close to the load as possible.
4. The inductor, output capacitor and the MOSFET should
be as close to each other as possible. This helps to reduce
the EMI radiated.
5. Place the switching MOSFET as close to the input
capacitors as possible. The MOSFET gate traces to the
IC must be as short, straight, and wide as possible. Use
copper filled polygons on the top and bottom layers for the
PHASE nodes.
6. Place the C
PHASE pins.
7. The feedback part of the system should be kept away
from the inductor and other noise sources, and be placed
close to the IC. Connect to the GND pin with a single
trace, and connect this local GND trace to the output
capacitor GND.
8. Minimize the leakage current paths on the OCSET/SD
pin and locate the resistor as close to the OCSET/SD pin
as possible because the internal current source is only
40μA.
9. In multilayer PCB, use one layer as ground plane and
have a control circuit ground (analog ground), to which all
signals are referenced. The goal is to localize the high
current path to a separate loop that does not interfere with
the more sensitive analog control function. These two
grounds must be connected together on the PC board
layout at a single point.
BOOT
as close as possible to the BOOT and
RT9212
www.richtek.com
13