TDF8599A_2 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 02 — 30 June 2009 36 of 54
NXP Semiconductors
TDF8599A
I
2
C-bus controlled dual channel class-D power amplifier
14.4 Speaker configuration and impedance
A flat-frequency response (due to a 2
nd
order Butterworth filter) is obtained by changing
the low-pass filter components (L
LC
, C
LC
) based on the speaker configuration and
impedance. Table 22 shows the required values.
Remark: When using a 1 Ω load impedance in Parallel mode, the outputs are shorted
after the low-pass filter switches two 2 Ω filters in parallel.
14.5 Heat sink requirements
In most applications, it is necessary to connect an external heat sink to the TDF8599A.
Thermal foldback activates at T
j
= 140 °C. The expression below shows the relationship
between the maximum power dissipation before activation of thermal foldback and the
total thermal resistance from junction to ambient:
(7)
P
max
is determined by the efficiency (η) of the TDF8599A. The efficiency measured as a
function of output power is given in Figure 43. The power dissipation can be derived as a
function of output power (see Figure 42).
Example 1:
• V
P
= 14.4 V
• P
o
=2× 25 W into 4 Ω (THD = 10 % continuous)
• T
j(max)
= 140 °C
• T
amb
=25°C
• P
max
= 5.8 W (from Figure 42)
• The required R
th(j-a)
= 115 °C / 5.8W=19K/W
The total thermal resistance R
th(j-a)
consists of: R
th(j-c)
+ R
th(c-h)
+ R
th(h-a)
Where:
• Thermal resistance from junction to case (R
th(j-c)
) = 1 K/W
• Thermal resistance from case to heat sink (R
th(c-h)
) = 0.5 K/W to 1 K/W (depending on
mounting)
• Thermal resistance from heat sink to ambient (R
th(h-a)
) would then be
19 − (1+1)=17K/W.
If an audio signal has a crest factor of 10 (the ratio between peak power and average
power = 10 dB) then T
j
will be much lower.
Table 22. Filter component values
Load impedance (Ω) L
LC
(µH) C
LC
(µF)
1 2.5 4.4
2 5 2.2
4101
j-a()
T
j max()
T
amb
–
P
max
------------------------------------
K/W[]=
