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LM4929 Fiches technique(PDF) 11 Page - Texas Instruments |
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LM4929 Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 20 page LM4929 www.ti.com 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 Submit Documentation Feedback 11 Product Folder Links: LM4929 |
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