NXP Semiconductors SA58670A - Manual (Page 18)

SA58670A_2 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 02 — 23 October 2008 18 of 24
NXP Semiconductors
SA58670A
2.1 W/channel stereo class-D audio amplifier
11.5 Efficiency and thermal considerations
The maximum ambient operating temperature depends on the heat transferring ability of
the heat spreader on the PCB layout. In
Table 3 “Limiting values” , the power derating
factor is given as 41.6 mW/K. The device thermal resistance, R
th(j-a)
is the reciprocal of the
power derating factor. Convert the power derating factor to R
th(j-a)
by Equation 3 :
(3)
For a maximum allowable junction temperature T
j
= 150
°
C and R
th(j-a)
= 24 K/W and a
maximum device dissipation of 1.5 W (750 mW per channel) and for 2.1 W per channel
output power, 4
Ω
load, 5 V supply, the maximum ambient temperature is calculated using
Equation 4 :
(4)
The maximum ambient temperature is 114
°
C at maximum power dissipation for 5 V
supply and 4
Ω
load. If the junction temperature of the SA58670A rises above 150
°
C, the
thermal protection circuitry turns the SA58670A off; this prevents damage to IC. Using
speakers greater than 4
Ω
further enhances thermal performance and battery lifetime by
reducing the output load current and increasing amplifier efficiency.
11.6 Additional thermal information
The SA58670A HVQFN20 package incorporates an exposed DAP that is designed to
solder the mount directly to the PCB heat spreader. By the use of thermal vias, the DAP
may be soldered directly to a ground plane or special heat sinking layer designed into the
PCB. The thickness and area of the heat spreader may be maximized to optimize heat
transfer and achieve lowest package thermal resistance.
12. Test information
R
th j-a
( )
1
derating factor
------------------------------------------
1
0.0416
---------------- 24 K /W = = =
T
amb max
( )
T
j max
( )
R
th j-a
( )
P
max
Ч ( )
– 150 24 1.5
Ч ( )
– 114
°
C = = =
Fig 17. Test circuit
002aad417
POWER
SUPPLY
+
−
INxP
INxN
OUTxP
OUTxN
15 µ H
15 µ H
DUT RL
AUX0025
30 kHz
LOW-PASS FILTER
AP585
MEASUREMENT
INPUTS
AP585
AUDIO
ANALYZER