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LM4929 Fiches technique(PDF) 11 Page - Texas Instruments

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No de pièce LM4929
Description  Audio Power Amplifier Series Stereo 40mW Low Noise Headphone Amplifier with OCL Output
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Fabricant  TI1 [Texas Instruments]
Site Internet  http://www.ti.com
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LM4929 Fiches technique(HTML) 11 Page - Texas Instruments

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LM4929
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SNAS293B – DECEMBER 2004 – REVISED APRIL 2013
APPLICATION INFORMATION
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4929 has three operational amplifiers internally. Two of the amplifier's have
externally configurable gain while the other amplifier is internally fixed at the bias point acting as a unity-gain
buffer. The closed-loop gain of the two configurable amplifiers is set by selecting the ratio of Rf to Ri.
Consequently, the gain for each channel of the IC is
AVD = -(Rf / Ri)
(1)
By driving the loads through outputs VoA and VoB with VoC acting as a buffered bias voltage the LM4929 does
not require output coupling capacitors. The classical single-ended amplifier configuration where one side of the
load is connected to ground requires large, expensive output coupling capacitors.
A configuration such as the one used in the LM4929 has a major advantage over single supply, single-ended
amplifiers. Since the outputs VoA, VoB, and VoC are all biased at 1/2 VDD, no net DC voltage exists across each
load. This eliminates the need for output coupling capacitors which are required in a single-supply, single-ended
amplifier configuration. Without output coupling capacitors in a typical single-supply, single-ended amplifier, the
bias voltage is placed across the load resulting in both increased internal IC power dissipation and possible
loudspeaker damage.
The LM4929 eliminates these output coupling capacitors by running in OCL mode. Unless shorted to ground,
VoC is internally configured to apply a 1/2 VDD bias voltage to a stereo headphone jack's sleeve. This voltage
matches the bias voltage present on VoA and VoB outputs that drive the headphones. The headphones operate
in a manner similar to a bridge-tied load (BTL). Because the same DC voltage is applied to both headphone
speaker terminals this results in no net DC current flow through the speaker. AC current flows through a
headphone speaker as an audio signal's output amplitude increases on the speaker's terminal.
The headphone jack's sleeve is not connected to circuit ground when used in OCL mode. Using the headphone
output jack as a line-level output will place the LM4929's 1/2 VDD bias voltage on a plug's sleeve connection. This
presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority
of equipment that is DC coupled, the LM4929 monitors the current supplied by the amplifier that drives the
headphone jack's sleeve. If this current exceeds 500mAPK, the amplifier is shutdown, protecting the LM4929 and
the external equipment.
POWER DISSIPATION
Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to
ensure a successful design. When operating in capacitor-coupled mode, Equation 2 states the maximum power
dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output
load.
PDMAX = (VDD)
2 / (2π2R
L)
(2)
Since the LM4929 has three operational amplifiers in one package, the maximum power dissipation increases
due to the use of the third amplifier as a buffer and is given in Equation 3:
PDMAX = 4(VDD)
2 / (2π2R
L)
(3)
The maximum power dissipation point obtained from Equation 3 must not be greater than the power dissipation
that results from Equation 4:
PDMAX = (TJMAX - TA) / θJA
(4)
For package DGS,
θJA = 190°C/W. TJMAX = 150°C for the LM4929. Depending on the ambient temperature, TA,
of the system surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by
the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then either the supply voltage
must be decreased, the load impedance increased or TA reduced. For the typical application of a 3V power
supply, with a 32
Ω load, the maximum ambient temperature possible without violating the maximum junction
temperature is approximately 144°C provided that device operation is around the maximum power dissipation
point. Thus, for typical applications, power dissipation is not an issue. Power dissipation is a function of output
power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature
may be increased accordingly. Refer to the Typical Performance Characteristics for power dissipation information
for lower output powers.
Copyright © 2004–2013, Texas Instruments Incorporated
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