TPA751

700-mW MONO LOW-VOLTAGE AUDIO POWER AMPLIFIER

WITH DIFFERENTIAL INPUTS

SLOS336C – DECEMBER 2000 – REVISED OCTOBER 2002

19

POST OFFICE BOX 655303

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 +

10 Log

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

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.