TPA6141A2YFFR Datasheet, PDF (11/23 Pages) Texas Instruments – CLASS-G DIRECTPATH STEREO HEADPHONE AMPLIFIER

TPA6141A2
www.ti.com.................................................................................................................................................... SLOS634A – MARCH 2009 – REVISED AUGUST 2009
CLASS-G HEADPHONE AMPLIFIER
Class-G amplifiers use adaptive supply rails. The TPA6141A2 includes a built-in step-down converter to create
the headphone amplifier positive supply voltage, HPVDD. A charge pump inverts HPVDD and creates the
amplifier negative supply voltage, HPVSS. This allows the headphone amplifier output to be centered at 0 V.
When audio signal amplitude is low, the step-down converter generates a low HPVDD voltage. This minimizes
TPA6141A2 power consumption while playing low noise, high fidelity audio. If audio amplitude increases, either
due to louder music or a transient peak, then the step-down converter generates a higher HPVDD voltage. The
HPVDD rise rate is faster than the audio peak rise time. This prevents audio distortion or clipping. Audio quality
and noise floor are not affected by HPVDD.
This adaptive HPVDD minimizes TPA6141A2 supply current while avoiding clipping and distortion. Because
normal listening levels are below 200 mV_RMS, HPVDD is most often at its lowest voltage. Thus, the TPA6141A2
has higher efficiency than traditional Class-AB headphone amplifiers.
The following equations compare a Class-AB amplifier to a Class-G amplifier. Both operate with identical battery
voltage, load impedance, and output voltage swing. For this study case, we assume a normal listening level of
200 mV_RMS with no DirectPath™ in order to simplify the calculations.
• P_SUP: Supplied power
• V_SUP: Supply voltage
• I_SUP: Supply current
• V_REG: DC/DC converter output voltage
• P_REG: DC/DC converter output power
• V_LOAD: Voltage across the load
• R_LOAD: Load impedance
• P_LOAD: Power dissipated at the load
• I_LOAD: Current supplied to the load
Given an amplifier driving 200 mV_RMS into a 32 Ω load, the output current to the load is:
I_LOAD
=
V_LOAD
R_LOAD
=
200 mV_RMS
32 W
= 6.25
mA
(1)
Assuming a quiescent current of 1 mA (I_DDQ) the total current supplied to the amplifier is:
I_SUP = I_LOAD + I_DDQ = 7.25 mA
(2)
The total power supplied to a Class-AB amplifier is then calculated as:
P_SUP = V_SUP ´ I_SUP = 4.2 V ´ 7.25 mA = 30.45 mW
(3)
For a Class-G amplifier where the voltage rails are generated by a switching DC/DC converter, the supplied
power will depend on the DC/DC converter output voltage and efficiency. Assuming the DC/DC converter output
voltage is 1.3 V:
P_REG = V_REG ´ I_SUP = 1.3 V ´ 7.25 mA = 9.425 mW
(4)
The total supplied power will be the DC/DC converter output power divided by the efficiency of the DC/DC
converter. Assuming 90% step-down efficiency, total power supplied to the Class-G amplifier is:
P_SUP
=
P_REG
90%
= 11.09
mW
(5)
Class-G headphone amplifiers achieve much higher efficiency than equivalent Class-AB amplifiers.
Copyright © 2009, Texas Instruments Incorporated
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