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LM4929 Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM4929 Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 13 page 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 select- ing the ratio of R f to Ri. Consequently, the gain for each channel of the IC is A VD = -(Rf /Ri) By driving the loads through outputs V oA 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 cou- pling capacitors. A configuration such as the one used in the LM4929 has a major advantage over single supply, single-ended amplifiers. Since the outputs V oA, VoB, and VoC are all biased at 1/2 V DD, no net DC voltage exists across each load. This elimi- nates the need for output coupling capacitors which are required in a single-supply, single-ended amplifier configura- tion. 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 V DD 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 V DD bias voltage on a plug’s sleeve connection. This pre- sents no difficulty when the external equipment uses capaci- tively coupled inputs. For the very small minority of equip- ment 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 equip- ment. 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 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. P DMAX =(VDD) 2 /(2 π2R L) (1) 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 2: P DMAX = 4(VDD) 2 /(2 π2R L) (2) The maximum power dissipation point obtained from Equa- tion 2 must not be greater than the power dissipation that results from Equation 3: P DMAX =(TJMAX -TA)/ θ JA (3) For package MUB10A, θ JA = 190˚C/W. TJMAX = 150˚C for the LM4929. Depending on the ambient temperature, T A,of the system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 2 is greater than that of Equation 3, then either the supply voltage must be de- creased, the load impedance increased or T A 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 Perfor- mance Characteristics curves for power dissipation informa- tion for lower output powers. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is important for low noise performance and high power supply rejection. The capacitor location on the power supply pins should be as close to the device as possible. Typical applications employ a 3V regulator with 10mF tanta- lum or electrolytic capacitor and a ceramic bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4929. A bypass capacitor value in the range of 0.1µF to 1µF is recommended for C S. MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4929’s shutdown function. Activate micro-power shut- down by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4929’s micro-power shutdown fea- ture turns off the amplifier’s bias circuitry, reducing the sup- ply current. The trigger point varies depending on supply voltage and is shown in the Shutdown Hysteresis Voltage graphs in the Typical Performance Characteristics section. The low 0.1µA(typ) shutdown current is achieved by apply- ing a voltage that is as near as ground as possible to the SHUTDOWN pin. A voltage that is higher than ground may increase the shutdown current. There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw switch, a microprocessor, or a mi- crocontroller. When using a switch, connect an external 100k Ω pull-up resistor between the SHUTDOWN pin and V DD. Connect the switch between the SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SHUTDOWN pin to ground, activating micro-power shutdown. www.national.com 10 |
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