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LM4924 Folha de dados(PDF) 10 Page - National Semiconductor (TI) |
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LM4924 Folha de dados(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Application Information ELIMINATING OUTPUT COUPLING CAPACITORS Typical single-supply audio amplifiers that drive single- ended (SE) headphones use a coupling capacitor on each SE output. This output coupling capacitor blocks the half- supply voltage to which the output amplifiers are typically biased and couples the audio signal to the headphones. The signal return to circuit ground is through the headphone jack’s sleeve. The LM4924 eliminates these output coupling capacitors. V oC is internally configured to apply a 1/2VDD bias voltage to a stereo headphone jack’s sleeve. This voltage matches the quiescent voltage present on the V oA and VoB outputs that drive the headphones. The headphones operate in a manner similar to a bridge-tied-load (BTL). 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. Using the headphone output jack as a line-level output will place the LM4924’s bandgap 1/2V DD bias 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 LM4924 monitors the current supplied by the amplifier that drives the headphone jack’s sleeve. If this current exceeds 500mA PK, the amplifier is shutdown, protecting the LM4924 and the external equipment. BYPASS CAPACITOR VALUE SELECTION Besides minimizing the input capacitor size, careful consid- eration should be paid to value of C BYPASS, the capacitor connected to the BYPASS pin. Since C BYPASS determines how fast the LM4924 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4924’s outputs ramp to their quiescent DC voltage (nomi- nally V DD/2), the smaller the turn-on pop. Choosing CB equal to 4.7µF along with a small value of C i (in the range of 0.1µF to 0.47µF), produces a click-less and pop-less shutdown function. As discussed above, choosing C i no larger than necessary for the desired bandwidth helps minimize clicks and pops. This ensures that output transients are eliminated when power is first applied or the LM4924 resumes opera- tion after shutdown. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4924 contains circuitry that eliminates turn-on and shutdown transients ("clicks and pops"). For this discussion, turn-on refers to either applying the power supply voltage or when the micro-power shutdown mode is deactivated. As the V DD/2 voltage present at the BYPASS pin ramps to its final value, the LM4924’s internal amplifiers are configured as unity gain buffers. An internal current source charges the capacitor connected between the BYPASS pin and GND in a controlled, linear manner. Ideally, the input and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches V DD/2. As soon as the voltage on the bypass pin is stable, the device becomes fully operational and the amplifier outputs are reconnected to their respective output pins. Although the BYPASS pin current cannot be modified, changing the size of C BYPASS alters the device’s turn-on time. There is a linear relationship between the size of C BYPASS and the turn-on time. Here are some typical turn-on times for various values of C BYPASS. AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 1, the LM4924 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 V = -(Rf/Ri) By driving the loads through outputs V O1 and VO2 with VO3 acting as a buffered bias voltage the LM4924 does not require output coupling capacitors. The typical single-ended amplifier configuration where one side of the load is con- nected to ground requires large, expensive output coupling capacitors. A configuration such as the one used in the LM4924 has a major advantage over single supply, single-ended amplifiers. Since the outputs V O1,VO2, and VO3 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 that are re- quired 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. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 1. P DMAX = 4(VDD) 2 /( π2R L) (1) It is critical that the maximum junction temperature T JMAX of 150˚C is not exceeded. Since the typical application is for headphone operation (16 Ω impedance) using a 3.3V supply the maximum power dissipation is only 138mW. Therefore, power dissipation is not a major concern. 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 3.0V regulator with 10µF tan- talum or electrolytic capacitor and a ceramic bypass capaci- tor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4924. 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 LM4924’s shutdown function. Activate micro-power shut- down by applying a logic-low voltage to the SHUTDOWN www.national.com 10 |
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