NXP Semiconductors TDA8932B - Manual (Page 27)

TDA8932B_4 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 04 — 18 December 2008 27 of 48
NXP Semiconductors
TDA8932B
Class-D audio amplifier
The HTSSOP32 package has an exposed die-pad that reduces significantly the overall
R
th(j-a)
. Therefore it is required to solder the exposed die-pad (at V
SSD
level) to a copper
plane for cooling. The HTSSOP package will have a low thermal resistance when used on
a multi-layer PCB with sufficient space for one or two thermal planes.
Increasing the area of the thermal plane, the number of planes or the copper thickness
can reduce further the thermal resistance R
th(j-a)
of both packages.
Typical thermal resistance R
th(j-a)
of the SO32 package soldered at a small 2-layer
application board (55 mm
×
45 mm), 35
µ
m copper, FR4 base material is 44 K/W.
Typical thermal resistance R
th(j-a)
of the HTSSOP32 package soldered at a small 2-layer
application board (55 mm
×
40 mm), 35
µ
m copper, FR4 base material is 48 K/W.
Equation 10 shows the relation between the maximum allowable power dissipation P and
the thermal resistance from junction to ambient.
(10)
Where:
R
th(j-a)
= thermal resistance from junction to ambient
T
j(max)
= maximum junction temperature
T
amb
= ambient temperature
P = power dissipation which is determined by the efficiency of the TDA8932B
The power dissipation is shown in Figure 22 (SE) and Figure 34 (BTL).
The thermal foldback will limit the maximum junction temperature to 140
°
C.
14.8 Pumping effects
When the amplifier is used in a SE configuration, a so-called 'pumping effect' can occur.
During one switching interval, energy is taken from one supply (e.g. V
DDP1
), while a part of
that energy is delivered back to the other supply line (e.g. V
SSP1
) and visa versa. When
the power supply cannot sink energy, the voltage across the output capacitors of that
power supply will increase.
The voltage increase caused by the pumping effect depends on:
• Speaker impedance
• Supply voltage
• Audio signal frequency
• Value of decoupling capacitors on supply lines
• Source and sink currents of other channels
The pumping effect should not cause a malfunction of either the audio amplifier and/or the
power supply. For instance, this malfunction can be caused by triggering of the
undervoltage or overvoltage protection of the amplifier.
Pumping effects in a SE configuration can be minimized by connecting audio inputs in
anti-phase and changing the polarity of one speaker. This is illustrated in
Figure 12 .
R
th j a –
( )
T
j max
( )
T
amb
–
P
------------------------------------ =