of CBYPASS and the turn-on time. Here are some typical
turn-on times for various values of CBYPASS
Application Information
.
ELIMINATING OUTPUT COUPLING CAPACITORS
AMPLIFIER CONFIGURATION EXPLANATION
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.
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 Rf to Ri. Consequently, the gain for each
channel of the IC is
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.
AV = -(Rf/Ri)
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 con-
nected to ground requires large, expensive output coupling
capacitors.
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.
A configuration such as the one used in the LM4924 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs VO1, VO2, and VO3 are all biased at 1/2
VDD, 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.
BYPASS CAPACITOR VALUE SELECTION
POWER DISSIPATION
Besides minimizing the input capacitor size, careful consid-
eration 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 (nomi-
nally 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 opera-
tion after shutdown.
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.
2
PDMAX = 4(VDD
)
/ (π2RL)
(1)
It is critical that the maximum junction temperature TJMAX of
150˚C is not exceeded. Since the typical application is for
headphone operation (16Ω impedance) using a 3.3V supply
the maximum power dissipation is only 138mW. Therefore,
power dissipation is not a major concern.
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.
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.
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
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 CS.
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
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，
