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參數(shù)資料
| 型號(hào)： | LM4865MWC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻控制 |
| 英文描述： | 1 CHANNEL(S), VOLUME CONTROL CIRCUIT, UUC |
| 封裝： | WAFER |
| 文件頁數(shù)： | 16/18頁 |
| 文件大?。?/td> | 520K |
| 代理商： | LM4865MWC |
Application Information (Continued)
applying less than 300mV
DC to the DC Vol/SD pin. When
shutdown is active, they supply current is reduced to 0.7A
(typ). A degree of uncertainty exists when the voltage applied
to the DC Vol/SD pin is in the range of 300mV to 500mV. The
LM4865 can be in mute, still fully powered, or in micropower
shutdown and fully muted. In mute mode, the LM4865 draws
the typical quiescent supply current. The DC Vol/SD pin
should be tied to GND for best shutdown mode performance.
As the DC Vol/SD is increased above 0.5V the amplifier will
follow the attenuation curve in Typical Performance Char-
acteristics.
HP-Sense FUNCTION
Applying a voltage between 4V and V
CC to the LM4865’s
HP-Sense headphone control pin turns off Amp2 and mutes
a bridged-connected load. Quiescent current consumption is
reduced when the IC is in this single-ended mode.
Figure 3 shows the implementation of the LM4865’s head-
phone control function. With no headphones connected to
the headphone jack, the R1-R2 voltage divider sets the
voltage applied to the HP-Sense pin (pin 3) at approximately
50mV. This 50mV enables the LM4865 and places it in
bridged mode operation.
While the LM4865 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Since the HP-Sense
threshold is set at 4V, even in an ideal situation, the output
swing cannot cause a false single-ended trigger. Connecting
headphones to the headphone jack disconnects the head-
phone jack contact pin from V
O1 and allows R1 to pull the
HP Sense pin up to V
CC. This enables the headphone func-
tion, turns off Amp2, and mutes the bridged speaker. The
amplifier then drives the headphones, whose impedance is
in parallel with resistor R2. Resistor R2 has negligible effect
on output drive capability since the typical impedance of
headphones is 32
. The output coupling capacitor blocks
the amplifier’s half supply DC voltage, protecting the head-
phones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP Sense pin, a bridge-
connected speaker is muted and Amp1 drives the head-
phones.
PROPERLY SELECTING EXTERNAL COMPONENTS
Optimizing the LM4865’s performance requires properly se-
lecting external components. Though the LM4865 operates
well when using external components having wide toler-
ances, the best performance is achieved by optimizing com-
ponent values.
Input Capacitor Value Selection
Amplification of the lowest audio frequencies requires high
value input coupling capacitors. These high value capacitors
can be expensive and may compromise space efficiency in
portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. In application
5 using speakers with this limited frequency response, a
large input capacitor will offer little improvement in system
performance.
Figure 1 shows that the nominal input impedance (R
IN)is
10k
at maximum volume and 110k at minimum volume.
Together, the input capacitor, C
i, and RIN, produce a -3dB
high pass filter cutoff frequency that is found using Equation
(4).
(4)
As the volume changes from minimum to maximum, R
IN
decrease from 110k
to 10k. Equation (4) reveals that the
-3dB frequency will increase as the volume increases. The
nominal value of C
i for lowest desired frequency response
should be calculated with R
IN = 10k . As an example when
using a speaker with a low frequency limit of 150Hz, C
i,
using Equation (4) is 0.1F. The 0.22F C
i shown in Figure 1
is optimized for a speaker whose response extends down to
75Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of the bypass capacitor C
B.
Since C
B determines how fast the LM4865 turns on, its value
is the most critical when minimizing turn-on pops. The slower
the LM4865’s outputs ramp to their quiescent DC voltage
(nominally V
DD/2), the smaller the turn-on pop. Choosing CB
equal to 1.0F, along with a small value of C
i (in the range of
0.1F to 0.39F), produces a clickless and popless shut-
down function. Choosing C
i as small as possible helps mini-
mize clicks and pops.
CLICK AND POP CIRCUITRY
The LM4865 contains circuitry that minimizes turn-on and
shutdown transients or
″clicks and pops″. For this discus-
sion, turn-on refers to either applying the power supply volt-
age or when the shutdown mode is deactivated. While the
power supply is ramping to its final value, the LM4865’s
internal amplifiers are configured as unity gain buffers. An
internal current source changes the voltage of the bypass
pin in a controlled, linear manner. Ideally, the input and
DS101025-34
FIGURE 3. Headphone Circuit
LM4865
www.national.com
7
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