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PDF LM4831 Data sheet ( Hoja de datos )
| Número de pieza | LM4831 | |
| Descripción | Multimedia Computer Audio Chip | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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November 1998
LM4831
Multimedia Computer Audio Chip
General Description
The LM4831 is a monolithic integrated circuit that provides a
stereo three input mixer, two stereo input analog multiplexer,
a stereo line out and a dual 1W bridged audio power ampli-
fier. In addition, a low noise microphone preamp is included
on-chip.
The LM4831 is ideal for multimedia computers since it incor-
porates an input mixer, analog multiplexer, and configurable
stereo audio power amplifier, as well as a microphone
preamp stage. This combination allows for all of the analog
audio processing to be enclosed in a 44-pin TQFP package.
The LM4831 features an externally controlled, low-power
consumption shutdown mode, as well as both headphone
and docking station modes. To temporarily override the shut-
down mode and allow audio signals to be amplified, the
LM4831 provides four “beep” pins.
Key Specifications
n THD+N at 1W into 8Ω
n Microphone Input Referred Noise
n Supply Current - Bridged Mode
n Shutdown Current
0.6% (typ)
10µV (typ)
16mA (typ)
2µA (typ)
Features
n Stereo 1W audio power amplifier
n “Click and pop” suppression circuitry
n Stereo three input mixer
n Shutdown mode
n Multiple operating modes — bridged, single-ended and
docking station modes
n Internal mux for switching in/out external filter
n Beep circuitry for “wake-up” while in shutdown
n 44 Pin TQFP Packaging
Applications
n Portable and Desktop Computers
Block Diagram
Connection Diagram
DS100057-1
FIGURE 1. LM4831 Block Diagram
Boomer® is a registered trademark of National Semiconductor Corporation.
TRI-STATE® is a registered trademark of National Semiconductor Corporation.
© 1998 National Semiconductor Corporation DS100057
Top View
Order Number LM4831VF
See NS Package Number VEJ44A
DS100057-3
www.national.com
1 page  
Pin Descriptions
VDDA
This is the analog power supply pin
which powers all internal circuitry, with
the exceptions of the output amplifiers
and the digital logic in the Beep and
Switching circuit sections. This pin
should be connected to the same
supply voltage as the two VDDD pins
(typically 5V), but have a separate
ground return path to the supply
ground to minimize interaction with the
high current amplifier returns and
digital switching noise. In addition, this
pin should be bypassed with a 0.01
µF–0.1 µF capacitor.
VDDD
These pins are the “digital” and high
current power supply pins which
power the stereo bridged output
amplifier and the digital logic in the
Beep and Switching circuit sections.
These pins should be connected to
the same supply voltage as the VDDA
pin (typically 5V), but have a separate
return path to the supply to avoid
interferring with low level signals. In
addition, this pin should be bypassed
with a 0.01 µF–0.1 µF capacitor. At
the power supply connection, a bulk
storage capacitor of at least 10 µF will
reduce the instantaneous current
demanded from the power supply.
GNDA,
GNDD,
GNDM
These are the power supply ground
pins. GNDA is the ground pin for the
low current analog circuitry. The two
GNDD pins are for the digital logic
and bridged output amplifiers. GNDM
is the ground for the microphone
amplifier. Make sure that the high
current GNDD paths are not returned
through the low current GNDM or
GNDA paths. These four ground pins
should be star-grounded at a stable,
low-impedance, noise-free system
ground.
BYPASS
HP-IN
DS-IN
This voltage at this pin is nominally
1/2 VDDD and is created by an
internal 50 kΩ resistor divider. This
node should be bypassed with a
capacitor value from 0.1 µF-1.0 µF.
Increasing the capacitor value will
increase the ramp time of the
amplifiers, thereby improving turn-on
pop performance. 0.1µF is typical for
the bypass capacitor. In addition, a 1
MΩ resistor from the bypass pin to the
positive supply is shown in Figure 3.
This resistor guarantees that the
LM4831 will turn-on if the device is
powered up with both the PWRDWN
and DS-IN pins high. If the the
LM4831 will never enter that state,
then the 1MΩ resistor can be
removed.
This pin places the output power
amplifier in “headphone” mode. If
HP-IN is low, the amplifier is in
bridged mode and the 2:1 mux passes
the input on the EQ_IN pin. If HP-IN is
high, the amplifier is in single-ended
mode and the 2:1 mux passes the
output of the mixing stage.
Single-ended mode places the
non-inverting amplifier in the output
amplifiers into a high impedance state.
HP-IN also has priority over the DS-IN
pin, so if HP-IN and DS-IN are both
high, the device is in single-ended
mode and the stereo line out amplifier
is in a high-impedance state.
This pin is used to put the LM4831
into “docking-station” mode and
control the line out drivers and the
state of the internal 2:1 analog
multiplexer. If DS-IN is high, the
stereo line out amplifier is on and the
stereo bridged amplifier is in a high
impedance state. Asserting the DS-IN
pin also changes the 2:1 analog
multiplexer output from the stereo
signal on the L_EQIN and R_EQIN
pins to the internal path from the
stereo input mixer.
5 www.national.com
5 Page 
Application Information (Continued)
ducing Cb will decrease turn-on time and increase “click and
pop”. If Cb is too small, the LM4831 can develop a
low-frequency oscillation (“motorboat”) when used at high
gains.
In order to eliminate “click and pop”, all coupling capacitors
must be discharged before turn-on. Rapid on/off switching of
the device or shutdown function may cause the “click and
pop” circuitry to not operate fully, resulting in increased “click
and pop” noise. For single-ended (headphone) circuitry, the
output coupling cap, Co, is of particular concern. In shut-
down, this capacitor is discharged through an internal 20kΩ
resistor. Depending on the size of Co, the discharging time
constant can be quite large. To reduce the time constant, an
external 1kΩ-5kΩ resistor can be placed in parallel with the
internal 20kΩ resistor. The tradeoff for using this resistor is
an increase in quiescent current and an increase in turn-off
“click and pop”.
Changing the bypass capacitor size also affects the amount
of time that the beep circuitry turns on the LM4831. Increas-
ing the bypass capacitor size increases the turn-on time,
which reduces the amount of time that the LM4831 is fully on
for during the RC-timed beep period.
The bypass capacitor also helps determine the power supply
rejection ratio. The smaller the bypass capacitor, the more
the power supply ripples couple onto the half supply and
then to all circuitry which uses the half supply for biasing.
COUPLING CAPACITORS
Since the LM4831 is a single supply circuit, all audio signals
(excepting the bridged outputs) must be capacitor coupled to
the chip to remove the 2.5VDC bias. All audio inputs have a
20kΩ input impedance, so the AC-coupling capacitor will cre-
ate a high-pass filter with f-3dB = 1/(2π*20kΩ*Cin). For a
−3dB point at 20Hz, Cin should be 0.39µF
Single-ended and line-out loads need to be AC-coupled back
to the LM4831 amplifiers. This high-pass filter is comprised
of the output load and the coupling capacitor, where the filter
cutoff is at f-3dB = 1/(2π*Rload*Cout). If RL=8Ω, then for a
−3dB point at 20Hz, Cout should be 1000µF.
EQUALIZER INPUT/OUTPUT
In some systems, the internal speakers require filtering to
improve their frequency response. The LM4831 provides the
system designer with external access to the signal using the
equalizer output and equalizer inputpins. When the DS_IN
and HP_IN pins are low (ie. the system is not in the docking
station and no headphone are plugged in), an internal mux
routes the audio signal to the equalizer output pin. After the
signal is filtered, it is returned to the LM4831 audio path
through the equalizer inputpin.
The input impedance to the equalizer input pin is 20kΩ. If the
external filter’s bias voltage is not derived from the half sup-
ply pin on the LM4831, AC-coupling capacitors must be used
on the equalizer input and output pins. If no equalization is
required, the equalizer out pin can be connected directly to
the equalizer in pin without any coupling capacitors.
LINE OUT
The line out pins are designed for use with a docking station
system. When the computer is plugged into the docking sta-
tion, the DS_IN pin should be forced high, thereby turning off
the power amplifier outputs and turning on the line out ampli-
fiers. All audio amplification and filtering is then done by the
docking station. The line out pins must be AC-coupled to the
docking station audio inputs.
POWER AMPLIFIERS
The power amplifiers in the LM4831 are designed to drive
8Ω or 32Ω loads at 1W (continuous) or 250mW(continuous),
respectively, with 1% THD+N. If the power amplifiers are
used to drive single-ended loads, such as headphones, the
amplifier inverting outputs should be AC-coupled to the out-
put load. When the LM4831 is in headphone (single-ended)
mode, the amplifier non-inverting inputs are in a high-
impedance state.
In low gain applications (AV<5), the LM4831 may require a
small feedback capacitance to prevent oscillation. Typically,
5-10pF will prevent oscillation.
MICROPHONE AMPLIFIER
The microphone amplifier is an uncommitted op-amp which
is intended to amplify low-level signals. The microphone in-
puts are very high impedance (Rin>1MΩ) and can be di-
rectly connected to microphone networks. The microphone
amplifier has enough output capability to drive a 1kΩ load.
All microphone inputs and outputs must be AC-coupled.
As shown in Figure 1, the microphone amplifier is typically
configured as an inverting amplifier. The positive terminal is
connected to the half-supply bypass pin to properly bias the
amplifier output to interface with the other inputs on the
LM4831. The microphone input pin is connected to the in-
verting node of a CMOS op amp, so the input impedance is
very high (>10MΩ)
BEEP CIRCUITRY
The beep circuitry is designed to allow a “sleeping” system to
temporarily power-up the LM4831 and output an audio alert
(“beep”). This feature might be used in a computer which is
“sleeping”, but needs to notify the user that the computer
batteries are low or that the user has new e-mail.
The beep circuitry is activated by any edge which occurs on
the BEEP A-D pins. With a resistor, Rbeep, and a capacitor,
Cbeep, in parallel at the RC pin of the LM4831, the LM4831
will be activated for Rbeep Cbeep seconds. Typical values for
Rbeep and Rbeep are 1-10ΩM and 0.1µF.
The BEEP OUT pin is designed to signal other audio circuitry
that the LM4831 is powering up. Generally a CODEC will re-
ceive this signal and begin sending audio information to the
LM4831. Logically, the BEEP OUT signal is the result of an
XOR of the BEEP A-D pins.
11 www.national.com
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LM4831.PDF ] | |
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