TPA0172

2-W STEREO AUDIO POWER AMPLIFIER

WITH I

C BUS

SLOS327C – AUGUST 2000 – REVISED MAY 2001

POST OFFICE BOX 655303 • DALLAS, TEXAS 75265

APPLICATION INFORMATION

BTL amplifier efficiency (continued)

(8)

= Power delivered to load

SUP

= Power drawn from power supply

LRMS

= RMS voltage on BTL load

= Load resistance

= Peak voltage on BTL load

avg = Average current drawn from the power supply

= Power supply voltage

BTL

= Efficiency of a BTL amplifier

Therefore,

SUP

substituting P

and P

SUP

into equation 7,

Efficiency of a BTL amplifier

BTL

Where:

Therefore,

Table 2 employs equation 8 to calculate efficiencies for four different output power levels. Note that the efficiency

of the amplifier is quite low for lower power levels and rises sharply as power to the load is increased resulting

in a nearly flat internal power dissipation over the normal operating range. Note that the internal dissipation at

full output power is less than in the half power range. Calculating the efficiency for a specific system is the key

to proper power supply design. For a stereo 1-W audio system with 8-Ω loads and a 5-V supply, the maximum

draw on the power supply is almost 3.25 W.

Table 2. Efficiency vs Output Power in 5-V, 8-Ω BTL Systems

OUTPUT POWER

(W)

EFFICIENCY

(%)

PEAK VOLTAGE

(V)

INTERNAL DISSIPATION

(W)

0.25 31.4 2 0.55

0.5 44.4 2.83 0.62

1 62.8 4 0.59

1.25 70.2 4.47

†

0.53

†

High peak voltages cause the THD to increase.

A final point to remember about class-AB amplifiers (either SE or BTL) is how to manipulate the terms in the

efficiency equation to the utmost advantage when possible. Note that in equation 8, V

is in the denominator.

This indicates that as V

goes down, efficiency goes up.

TPA0172

2-W STEREO AUDIO POWER AMPLIFIER

WITH I

C BUS

SLOS327C – AUGUST 2000 – REVISED MAY 2001

POST OFFICE BOX 655303 • DALLAS, TEXAS 75265

APPLICATION INFORMATION

crest factor and thermal considerations

Class-AB 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 range, or headroom above the average power

output, to pass the loudest portions of the signal without distortion. In other words, music typically has a crest

factor between 12 dB and 15 dB. When determining the optimal ambient operating temperature, the internal

dissipated power at the average output power level must be used. When the TPA0172 is operating from a 5-V

supply into a 3-Ω speaker, 4-W peaks are available. To convert watts to dB use equation 9.

10Log

ref

10Log

6dB

(9)

Subtracting the headroom restriction to obtain the average listening level without distortion yields:

6 dB – 15 dB = –9 dB (15-dB crest factor)

6 dB – 12 dB = –6 dB (12-dB crest factor)

6 dB – 9 dB = –3 dB (9-dB crest factor)

6 dB – 6 dB = 0 dB (6-dB crest factor)

6 dB – 3 dB = 3 dB (3-dB crest factor)

To convert dB back to watts use equation 10.

PdB 10

ref

63 mW (18-dB crest factor)

125 mW (15-dB crest factor)

250 mW (12-dB crest factor)

500 mW (9-dB crest factor)

1000 mW (6-dB crest factor)

(10)

2000 mW (3-dB crest factor)

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 2 W of continuous power output with a 3-dB crest

factor, 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, 3-Ω system, the internal dissipation in the TPA0172 and

maximum ambient temperatures are shown in Table 3.

Table 3. TPA0172 Power Rating, 5-V, 3-Ω Stereo

PEAK OUTPUT POWER

(W)

AVERAGE OUTPUT POWER

POWER DISSIPATION

(W/Channel)

MAXIMUM AMBIENT

TEMPERATURE

4 2 W (3 dB) 1.7 –3°C

4 1000 mW (6 dB) 1.6 6°C

4 500 mW (9 dB) 1.4 24°C

4 250 mW (12 dB) 1.1 51°C

4 125 mW (15 dB) 0.8 78°C

4 63 mW (18 dB) 0.6 96°C

TPA0172

2-W STEREO AUDIO POWER AMPLIFIER

WITH I

C BUS

SLOS327C – AUGUST 2000 – REVISED MAY 2001

POST OFFICE BOX 655303 • DALLAS, TEXAS 75265

APPLICATION INFORMATION

crest factor and thermal considerations (continued)

Table 4. TPA0172 Power Rating, 5-V, 8-Ω Stereo

PEAK OUTPUT POWER AVERAGE OUTPUT POWER

POWER DISSIPATION

(W/Channel)

MAXIMUM AMBIENT

TEMPERATURE

2.5 W 1250 mW (3-dB crest factor) 0.55 100°C

2.5 W 1000 mW (4-dB crest factor) 0.62 94°C

2.5 W 500 mW (7-dB crest factor) 0.59 97°C

2.5 W 250 mW (10-dB crest factor) 0.53 102°C

The maximum dissipated power (P

D(max)

) is reached at a much lower output power level for an 8-Ω load than

for a 3-Ω load. As a result, use equation 11 for calculating P

D(max)

for an 8-Ω application.

D(max)

(11)

However, in the case of a 3-Ω load, the P

D(max)

occurs at a point well above the normal operating power level.

The amplifier may therefore be operated at a higher ambient temperature than required by the P

D(max)

formula

for a 3-Ω load.

The maximum ambient temperature depends on the heat sinking ability of the PCB system. The derating factor

for the PWP package is shown in the dissipation rating table. To convert this to Θ

use equation 12.

Derating Factor

0.022

45°C W

(12)

To calculate maximum ambient temperatures, first consider that the numbers from the dissipation graphs are

per channel so the dissipated power needs to be doubled for two channel operation. Given the maximum

allowable junction temperature (Θ

) and the total internal dissipation, the maximum ambient temperature can

be calculated using equation 13. The maximum recommended junction temperature for the TPA0172 is 150°C.

The internal dissipation figures are taken from the power dissipation vs output power graphs.

Max T

Max Θ

150 45

(

0.6 2

)

96°C

(

15-dB crest factor

)

(13)

NOTE:

Internal dissipation of 0.6 W is estimated for a 2-W system with 15-dB crest factor per channel.

Tables 3 and 4 show that for some applications no airflow is required to keep junction temperatures in the

specified range. The TPA0172 is designed with thermal protection that turns the device off when the junction

temperature surpasses 150°C to prevent damage to the IC. Tables 3 and 4 were calculated for maximum

listening volume without distortion. When the output level is reduced, the numbers in the table change

significantly. Also, using 8-Ω speakers significantly increases the thermal performance by increasing amplifier

efficiency.

Part #	TPA0172PWP
Description	STEREO 2W AUDIO AMP
Category	IC
Availability	In Stock
Qty	33

Qty	Price
1 - 6	$8.37375
7 - 13	$6.66094
14 - 20	$6.28031
21 - 27	$5.83625
28 +	$5.20188

TPA0172PWP

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