RT6908
curve in Figure 5 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
Layout Consideration
For high frequency switching power supplies, the PCB
layout is important to get good regulation, high efficiency
and stability. The following descriptions are the guidelines
for better PCB layout.
3.2
Four-Layer PCB
2.8
` For good regulation, place the power components as
close as possible. The traces should be wide and short
enough especially for the high current loop.
2.4
2.0
1.6
1.2
0.8
0.4
0.0
` The compensation circuit should be kept away from
the power loops and be shielded with a ground trace to
prevent any noise coupling.
` Minimize the size of all LX nodes and keep them wide
and shorter.
0
25
50
75
100
125
` The exposed pad of the chip should be connected to a
Ambient Temperature (°C)
strong ground plane for maximum thermal consideration.
Figure 5.Derating Curve for the RT6908 Package
Place the power components as close as
possible. The traces should be wide and
short especially for the high current loop.
Separate power ground (PGND) and analog ground
(AGND). Connect the AGND and the PGND islands
at a single end. Make sure that there are no other
connections between these separate ground planes.
C11
L2
V
I/O
V
IN
C21
C13
C12
R3
R2
C20
Place the power components
as close as possible.
40 39 38 37 36 35 34 33 32 31
AGND
1
2
PGND
FBB1
COMP1
RSTB
SDA
30
29
28
27
26
25
BOOT2
LXB2
DHB2
NC
DLB2
ILIMIT2
FBB2
COMP2
CRST
DRVN
V
CORE
R4
C19
M1
M2
3
L3
4
5
C24
The compensation circuit
PGND
6
SCL
should be kept away from the
power loops and should be
shielded with a ground trace
to prevent any noise coupling.
PGND
R11
R23
7
VDET
AVDD
GD
24
23
22
21
8
AVDD
41
R12
C25
AGND
9
10
LXI
The compensation circuit should be
kept away from the power loops and
should be shielded with a ground
trace to prevent any noise coupling.
11 12 13 14 15 16 17 18 19 20
C28
C54
AGND
R38
C26
The exposed pad of the chip
should be connected to ground
plane for thermal consideration.
L1
D1
Q1
C5
AVDD
Place the power components as
close as possible. The traces should
be wide and short especially for the
high-current loop.
V
IN
C2
C1
PGND
V
IN
The power ground (PGND) consists of input and output
capacitor grounds, the components's ground of charge
pump. The PGND should be wide and short enough to
connect to a ground plane.
Figure 6. PCB Layout Guide
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6908-01 March 2013
www.richtek.com
21
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
