RT7251A/B
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WDFN-8L 2x2 package, the thermal resistance, θJA, is
120°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formula :
Layout Consideration
Follow the PCB layout guidelines for optimal performance
of the RT7251A/B
` Keep the traces of the main current paths as short and
wide as possible.
` Put the input capacitor as close as possible to the device
pins (VINandGND).
PD(MAX) = (125°C − 25°C) / (120°C/W) = 0.833W for
` SW node is with high frequency voltage swing and should
be kept at small area. Keep sensitive components away
from the SW node to prevent stray capacitive noise
pickup.
WDFN-8L 2x2 package
The maximum power dissipation depends on the operating
ambient temperature for fixed TJ(MAX) and thermal
resistance, θJA. The derating curve in Figure 3 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
` Place the feedback components to the FB pin as close
as possible.
0.9
` The GND and Exposed Pad should be connected to a
Four-Layer PCB
strong ground plane for heat sinking and noise protection.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
SW should be connected to
V
OUT
inductor by wide and short trace.
Keep sensitive components
away from this trace.
C
OUT
L
C
IN
1
2
3
4
8
7
6
5
SW
VIN
BOOT
EN
GND
GND
PGOOD
FB
Input capacitor must
be placed as close
to the IC as possible.
R1
9
C
BOOT
V
OUT
R2
SW
GND
The resistor divider must be
connected as close to the
device as possible.
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 4. PCB Layout Guide
Figure 3. Derating Curve of Maximum PowerDissipation
Table 3. Suggested Capacitors for CIN and COUT
Location
Component Supplier
MURATA
Part No.
Capacitance (μF)
Case Size
1206
GRM31CR61E106K
C3225X5R1E106K
TMK316BJ106ML
GRM32ER61E226M
GRM21BR60J226M
C3225X5R0J226M
EMK325BJ226MM
10
10
10
22
22
22
22
C
IN
C
IN
C
IN
TDK
1206
TAIYO YUDEN
MURATA
1206
1210
C
OUT
C
OUT
C
OUT
C
OUT
MURATA
0805
TDK
1210
TAIYO YUDEN
1210
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
www.richtek.com
13
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