TAS5121 datasheet(13/17 Pages) TI

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TAS5121

SLES086A − NOVEMBER 2003 − REVISED MARCH 2004

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Table 3. Output Mode Selection

OUTPUT MODE

Bridge-tied load output stage (BTL)

Reserved

APPLICATION INFORMATION

DEMODULATION FILTER DESIGN

The TAS5121 amplifier outputs are driven by high-current

DMOS transistors in an H-bridge configuration. These

transistors are either off or fully on.

The result is a square-wave output signal with a duty cycle

that is proportional to the amplitude of the audio signal. It

is recommended that a second-order LC filter be used to

recover the audio signal.

Output A

TAS5121

Output B

R(Load)

Figure 10. Demodulation Filter

The main purpose of the demodulation filter is to attenuate

the high-frequency components of the output signals that

are out of the audio band.

Design of the demodulation filter affects the audio

performance of the power amplifier significantly. As a

result, to ensure proper operation of the overcurrent (OC)

protection

circuit

and

meet

the

device

THD+N

specifications, the selection of the inductors used in the

output filter must be considered according to the following.

The rule is that the inductance should remain stable within

the range of peak current seen at maximum output power

and deliver approximately 5

µH of inductance at 15 A.

If this rule is observed, the TAS5121 should not have

distortion issues due to the output inductors. This prevents

device damage due to overcurrent conditions because of

inductor saturation in the output filter.

Another parameter to be considered is the idle current loss

in the inductor. This can be measured or specified as

inductor dissipation (D). The target specification for

dissipation is less than 0.05. If this specification is not met,

idle current increases.

In general, 10-

µH inductors suffice for most applications.

The frequency response of the amplifier is slightly altered

by the change in output load resistance; however, unless

tight control of frequency response is necessary (better

than 0.5 dB), it is not necessary to deviate from 10

µH.

The graphs in Figure 11 display the inductance vs current

characteristics of two inductors that are suggested for use

with the TAS5121.

Figure 11. Inductance Saturation

I − Current − A

INDUCTANCE

CURRENT

DBF1310A

DASL983XX−1023

The selection of the capacitors that are placed from the

output of each inductor to ground is simple. To complete

the output filter, use a 1-

µF capacitor with a voltage rating

at least twice the voltage applied to the output stage

(PVDD_x).

This capacitor should be a good quality polyester

dielectric.

THERMAL INFORMATION

The following is provided as an example.

The thermally enhanced package provided with the

TAS5121 are designed to be interfaced directly to

heatsinks using a thermal interface compound (for

example, Wakefield Engineering type 126 thermal

grease.) The heatsink then absorbs heat from the ICs and

transfers it to the ambient air. If the heatsink is carefully

designed, this process can reach equilibrium and heat can

be continually removed from the ICs without device

overtemperature shutdown. Because of the efficiency of

the TAS5121, heatsinks are smaller than those required

for linear amplifiers of equivalent performance.

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