ISD15D00 Thermal Guidelines
AN-15D.0001
Publication Release Date: Apr 19, 2010
Revision 0.1
1
1. Introduction
The scope of this document is to provide a brief outline of layout design considerations
for the ISD15D00 series devices.
2. Output Power and Dissipated Heat
The ISD15D00 features two 1-watt direct-drive outputs:
Class-D PWM and
Class-AB BTL.
Output power and the corresponding efficiency are shown in the figures below.
Figure 1 Class-D Power vs. V
CCSPK
Figure 2 Class-D Power vs. Efficiency
Figure 3 Class-AB Power vs. V
CCSPK
Figure 4 Class-AB Power vs. Efficiency
0
0.5
1
1.5
2 2.5 3 3.5 4 4.5 5 5.5 6
Class-D PWM Power (W)
V
CCSPK
(V)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0 .5 0.6 0.7 0.8 0.9 1 1.1 1.2
Efficiency
Class-D PWM Power (W)
0
0.5
1
1.5
2 2.5 3 3.5 4 4.5 5 5.5 6
Class-AB BTL Power (W)
V
CCSPK
(V)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
Efficiency
Class-AB BTL Power (W)
ISD15D00 Thermal Guidelines
AN-15D.0001
Publication Release Date: Apr 19, 2010
Revision 0.1
2
The dissipated heat at a certain output power can be calculated as:
Dissipated Heat = Output Power x (1 - Efficiency)
Example of Dissipated Heat:
For the Class-AB BTL output
V
CCSPK
= 5V.
Volume level set to -3dB.
Figure 3 shows that Class-AB BTL delivers 1.0 watt at V
CCSPK
= 5V, given that
volume sets to its maximum (0dB). Therefore, Class-AB BTL delivers 0.5 watt at
V
CCSPK
= 5V, given that volume sets to -3dB.
Figure 3 shows that Efficiency = 0.43 when Class-AB BTL delivers 0.5 watt.
Dissipated Heat = Output Power x (1 - Efficiency)
= 0.5 x (1 - 0.43)
= 0.285 watt
3. Thermal Package Information
The ISD15D00 are available in two packages:
48L-LQFP 7x7mm and
32L-QFN 5x5mm with exposed pad
o The exposed pad is the bottom of the lead-frame pad and can improve
heat dissipation.
o The exposed pad should be connected to analog ground.
Table 1 below shows thermal resistance of the two available packages.
Ѳ
ja
* (C/W)
0 m/s**
1 m/s
2 m/s
Ѳ
jc
(C/W)
48L-LQFP
60
-
-
17
32L-QFN
37.68
35.16
33.29
6.56
Table 1 Theta J of ISD15D00 packages
*note: please see appendix at the bottom for an explanation of theta J.
**note: m/s is meters per second – ambient air speed.
The increased temperature can be calculated as:
Increased Temperature = Dissipated Heat x Ѳ
ja
Example of Increased Temperature:
For the 32L-QFN package
Ambient air is still.
Dissipated heat = 0.285 watt (from the dissipated heat example).
Increased Temperature = Dissipated Heat x Ѳ
ja
= 0.285W x 37.68 C/W
= 10.739 C