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LM4924 bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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LM4924 bảng dữ liệu(HTML) 9 Page - Texas Instruments |
9 / 20 page LM4924 www.ti.com SNAS272B – OCTOBER 2004 – REVISED APRIL 2013 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. VoC 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 VoA 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 1/2VDD 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 500mAPK, the amplifier is shutdown, protecting the LM4924 and the external equipment. BYPASS CAPACITOR VALUE SELECTION Besides minimizing the input capacitor size, careful consideration should be paid to value of CBYPASS, the capacitor connected to the BYPASS pin. Since CBYPASS 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 (nominally VDD/2), the smaller the turn-on pop. Choosing CB equal to 4.7µF along with a small value of Ci (in the range of 0.1µF to 0.47µF), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci 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 operation 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 VDD/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 VDD/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 CBYPASS alters the device's turn-on time. There is a linear relationship between the size of CBYPASS and the turn-on time. Here are some typical turn-on times for various values of CBYPASS. 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 selecting the ratio of Rf to Ri. Consequently, the gain for each channel of the IC is AV = -(Rf/Ri) (1) By driving the loads through outputs VO1 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 connected to ground requires large, expensive output coupling capacitors. Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM4924 |
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