LM4831VF National Semiconductor, LM4831VF Datasheet - Page 11

• Manufacturer Part Number: LM4831VF
• Description: IC AMP AUDIO PWR 1.5W AB 44LQFP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4831VF

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 1.5W x 2 @ 4 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Features: Depop, Input Multiplexer, Microphone, Shutdown
• Mounting Type: Surface Mount
• Package / Case: 44-LQFP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM4831VF

Application Information
sible. For low frequency considerations, a 10µF or greater
tantalum or electrolytic capacitor should be paralleled with
the high frequency bypass capacitor.
If power supply bypass capacitors are not sufficiently large,
the current in the power supply leads, which is a rectified ver-
sion of the output current, may be fed back into internal cir-
cuitry. This internal feedback signal can cause high fre-
quency distortion and oscillation.
If power supply lines to the chip are long, larger bypass ca-
pacitors could be required. Long power supply leads have in-
ductance and resistance associated with them, which could
prevent peak low frequency current demands from being
met. The extra bypass capacitance will reduce the peak cur-
rent requirements from the power supply lines.
Under certain conditions, the LM4831 may refuse to come
out of shutdown. A 1M
supply to the bypass pin, as shown in the Typical Applica-
tion section circuit, Figure 3 , will guarantee startup.
CLICK & POP CIRCUITRY AND THE BYPASS
CAPACITOR
The LM4831 contains circuitry to minimize turn-on tran-
sients. In this case, turn-on refers to either power supply
turn-on or the device coming out of shutdown mode. During
turn-on, an internal current source charges the bypass ca-
pacitor on the bypass pin. Both the inputs and outputs track
the voltage at the bypass pin. As soon as the bypass node is
stable at 1/2 V
Although the bypass pin current source cannot be modified,
the size of the bypass capacitor, Cb, can be changed to alter
the device turn-on time and the amount of “click and pop”.
The relationship between the size of Cb and the turn-on time
is linear. By increasing Cb, the amount of turn-on pop can be
reduced. However, the trade-off for using a larger bypass ca-
pacitor is an increase in the turn-on time for the device. Re-
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, C
down, this capacitor is discharged through an internal 20k
resistor. Depending on the size of C
constant can be quite large. To reduce the time constant, an
external 1k -5k
internal 20k
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.
DD
resistor. The tradeoff for using this resistor is
, the amplifier will become fully operational.
resistor can be placed in parallel with the
o
, is of particular concern. In shut-
resistor connected from the power
o
, the discharging time
(Continued)
11
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 2.5V
20k input impedance, so the AC-coupling capacitor will cre-
ate a high-pass filter with f
−3dB point at 20Hz, C
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 point at 20Hz, C
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 20k . 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
8 or 32 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 (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 (R
rectly connected to microphone networks. The microphone
amplifier has enough output capability to drive a 1k
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
-3dB
= 1/(2 *R
in
out
should be 0.39µF
should be 1000µF.
V
load
DC
<
-3dB
5), the LM4831 may require a
*C
bias. All audio inputs have a
out
= 1/(2 *20k *C
in
). If R
>
1M ) and can be di-
L
=8 , then for a
www.national.com
in
). For a
load.