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
5秒后页面跳转
深入解析CS8416引脚说明、参数、技术特点
TPA3116D2功放芯片参数详解、引脚说明
移位寄存器74HC165引脚说明、驱动程序示例解读
深入解析AD9833：DDS频率合成器的卓越性能与广泛应用
微信公众号
抖音公众号
浙公网安备 33010502006866号 浙ICP备10014259号-119
营业执照ICP证
技术参数
数据手册
如果您发现信息不准确，
