Datasheet

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     
  
SLOS336C − DECEMBER 2000 − REVISED OCTOBER 2002
19
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
headroom and thermal considerations
Linear power amplifiers dissipate a significant amount of heat in the package under normal operating conditions.
A typical music CD requires 12 dB to 15 dB of dynamic headroom to pass the loudest portions without distortion
as compared with the average power output. From the TPA751 data sheet, one can see that when the TPA751
is operating from a 5-V supply into an 8-Ω speaker that 700 mW peaks are available. Converting watts to dB:
P
dB
+ 10Log
P
W
P
ref
+ 10Log
700 mW
1W
+ –1.5 dB
Subtracting the headroom restriction to obtain the average listening level without distortion yields:
−1.5 dB − 15 dB = −16.5 (15 dB headroom)
−1.5 dB − 12 dB = −13.5 (12 dB headroom)
−1.5 dB − 9 dB = −10.5 (9 dB headroom)
−1.5 dB − 6 dB = −7.5 (6 dB headroom)
−1.5 dB − 3 dB = −4.5 (3 dB headroom)
Converting dB back into watts:
P
W
+ 10
PdBń10
xP
ref
= 22 mW (15 dB headroom)
= 44 mW (12 dB headroom)
= 88 mW (9 dB headroom)
= 175 mW (6 dB headroom)
= 350 mW (3 dB headroom)
This is valuable information to consider when attempting to estimate the heat dissipation requirements for the
amplifier system. Comparing the absolute worst case, which is 700 mW of continuous power output with 0 dB
of headroom, against 12 dB and 15 dB applications drastically affects maximum ambient temperature ratings
for the system. Using the power dissipation curves for a 5-V, 8-Ω system, the internal dissipation in the TPA751
and maximum ambient temperatures is shown in Table 1.
Table 1. TPA751 Power Rating, 5-V, 8-Ω, BTL
PEAK OUTPUT
POWER
AVERAGE
POWER
DISSIPATION
D PACKAGE
(SOIC)
DGN PACKAGE
(MSOP)
GQS PACKAGE
(MicroStar Junior)
POWER
(mW)
AVERAGE
OUTPUT POWER
DISSIPATION
(mW)
MAXIMUM AMBIENT
TEMPERATURE
MAXIMUM AMBIENT
TEMPERATURE
MAXIMUM AMBIENT
TEMPERATURE
700 700 mW 675 34°C 110°C 99°C
700 350 mW (3 dB) 595 47°C 115°C 105°C
700 176 mW (6 dB) 475 68°C 122°C 114°C
700 88 mW (9 dB) 350 89°C 125°C 123°C
700 44 mW (12 dB) 225 111°C 125°C 125°C
Table 1 shows that the TPA751 can be used to its full 700-mW rating without any heat sinking in still air up to
110°C, 34°C, and 99°C for the DGN package (MSOP), D package (SOIC), and GQS (MicroStar Junior)
package, respectively.