3.45 V, the output voltage remains below the limiter level, and the gain stays at 15.5 dB. Once the supply drops

below 3.45 V, the limiter level decreases below 4.78 V, and SpeakerGuard™ decreases the gain.

inflection point, audio gain does not decrease until the Class-D output voltage is above the limiter level.

9.3.1.2 Battery Tracking SpeakerGuard™

The TPA2015D1 monitors the battery voltage and the audio signal, automatically decreasing gain when battery

voltage is low and audio output power is high. It finds the optimal gain to maximize loudness and minimize

battery current, providing louder audio and preventing early shutdown at end-of-charge battery voltages.

SpeakerGuard™ decreases amplifier gain when the audio signal exceeds the limiter level. The limiter level

automatically decreases when the supply voltage (VBAT) is below the inflection point. Figure 9-3 shows a plot of

the limiter level as a function of the supply voltage.

Inflectionpoint

LimiterLevel(VBAT>inflectionpoint)

LimiterLevel(VBAT=inflectionpoint)

SupplyVoltage

Figure 9-3. Limiter Level vs Supply Voltage

The limiter level decreases within 60 µs of the supply voltage dropping below the inflection point. Although this is

slightly slower than the 26 µs/dB SpeakerGuard™ attack time, the difference is audibly imperceptible.

Connect a resistor between the AGC pin and ground to set the inflection point, as shown in Table 9-1. Leave the

AGC pin floating to disable the inflection point, keeping the limiter level constant over all supply voltages.

The maximum limiter level is fixed, as is the slope of the limiter level versus supply voltage. If different values for

maximum limiter level and slope are required, contact your local Texas Instruments representative.

Table 9-1. AGC Function Table

FUNCTION

RESISTOR ON AGC PIN

INFLECTION POINT

Constant limiter level; battery track OFF

Floating or connected to VBAT

disabled

AGC battery track option 1

39 kΩ

3.55 V

AGC battery track option 2

27 kΩ

3.78 V

AGC battery track option 3

18 kΩ

3.96 V

The audio signal is not affected by the SpeakerGuard™ function unless the peak audio output voltage exceeds

the limiter level. Figure 9-7 shows the relationship between the audio signal, the limiter level, the supply voltage,

and the supply current.

When VBAT is greater than the inflection point, the limiter level allows the output signal to slightly clip to roughly

6% THD at 2 W into 8 Ω. This is an acceptable peak distortion level for most small-sized portable speakers,

while ensuring maximum loudness from the speaker.

9.3.2 Fully Differential Class-D Amplifier

The TPA2015D1 uses a fully differential amplifier with differential inputs and outputs. The differential output

voltage equals the differential input multiplied by the amplifier gain. The TPA2015D1 can also be used with

a single-ended input. However, using differential input signals when in a noisy environment, like a wireless

handset, ensures maximum system noise rejection.

www.ti.com

TPA2015D1

SLOS638C – NOVEMBER 2011 – REVISED JUNE 2022

Copyright © 2022 Texas Instruments Incorporated

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