AN31 SILABS [Silicon Laboratories], AN31 Datasheet - Page 2

• Manufacturer Part Number: AN31
• Description: INDUCTOR DESIGN FOR THE Si41XX SYNTHESIZER FAMILY
• Manufacturer: SILABS [Silicon Laboratories]

AN31
3.2. Using a Printed Trace Inductor
If the required value of L
recommended that a PCB trace be used as shown in
Figure 2.
Table 1 lists the dimensions to be used with a micro
leadframe package (MLP), and Table 2 lists the
dimensions to be used with a thin shrink small outline
package (TSSOP).
The inductance of the shape shown in Figure 2 is
directly proportional to the D dimension.
2
Table 2. Dimensions to be used with TSSOP
Table 1. Dimensions to be used with MLP
B
Figure 2. Printed Trace Inductor
C
device pad
Dimension
A
Dimension
G
A
B
C
D
E
F
A
B
C
D
E
G
F
EXT
F
Value (mm)
Value (mm)
inductor trace
(calculated)
(calculated)
is less than 3 nH, it is
D
1.50
0.30
0.35
0.30
0.35
0.95
0.80
0.30
0.20
0.30
0.20
0.80
E
E
G
Rev. 1.3
The constant of proportionality, X
by Equations 4 and 5 for an MLP and a TSSOP,
respectively:
where
In each of these equations, H is the thickness of the
dielectric between the “top” layer metal and the layer
containing the ground plane measured in µm. Figure 3
illustrates the dimension H.
It is recommended that the H dimension be greater than
100 µm to reduce the sensitivity of the printed trace
inductance to thickness variation in the PCB dielectric.
To accomplish this, it may be necessary to remove
copper from layer 2 and locate the ground plane on
layer 3. In any case, H is the distance from the bottom
of the inductor trace to the top of the ground plane. The
thickness of the ground plane, T2, and the trace on
layer 1, T1, do not have a material effect on the
calculations and should be ignored.
Once the constant of proportionality (X) has been
calculated using Equation 4 (MLP) or Equation 5
(TSSOP), it is necessary to calculate the length of the
inductor trace. This is accomplished using Equation 6.
where D
With this calculation complete, the trace can be
implemented as shown in Figure 2.
Figure 3. Side View of Printed Circuit Board
X
X
dielectric
TSSOP
MLP
ground
traces
L
X
EXT
H = thickness of dielectric between inductor
trace and ground plane in µm
X
X
=
MLP
TSSOP
=
0.620 1 0.823e
= trace length shown in Figure 2 in mm
= calculated value of external inductance
= constant of proportionality for MLP (X
0.700 1 0.857e
or TSSOP (X
= constant of proportionality for MLP
required
D
= constant of proportionality for TSSOP
⎛
⎜
⎝
=
⎛
⎜
⎝
–
L
------------ -
–
EXT
X
--------- -
130
TSSOP
–
(Equation 6)
H
--------- -
140
–
⎞ nH
⎟
⎠
H
--------- -
mm
⎞ nH
⎟
⎠
MLP
--------- -
mm
)
or X
E
(Equation 4)
(Equation 5)
TSSOP
H
T1
, is given
T2
MLP
)