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PDF LM4834 Data sheet ( Hoja de datos )
| Número de pieza | LM4834 | |
| Descripción | 1.75W Audio Power Amplifier with DC Volume Control and Microphone Preamp | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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November 1997
LM4834
1.75W Audio Power Amplifier with DC Volume Control
and Microphone Preamp
General Description
The LM4834 is a monolithic integrated circuit that provides
DC volume control, and a bridged audio power amplifier ca-
pable of producing 1.75W into 4Ω with less than 1.0%
(THD). In addition, the headphone/lineout amplifier is ca-
pable of driving 70 mW into 32Ω with less than 0.1%(THD).
The LM4834 incorporates a volume control and an input mi-
crophone preamp stage capable of driving a 1 kΩ load im-
pedance.
Boomer® audio integrated circuits were designed specifically
to provide high quality audio while requiring a minimum
amount of external components in surface mount packaging.
The LM4834 incorporates a DC volume control, a bridged
audio power amplifier and a microphone preamp stage,
making it optimally suited for multimedia monitors and desk-
top computer applications.
The LM4834 features an externally controlled, low-power
consumption shutdown mode, and both a power amplifier
and headphone mute for maximum system flexibility and
performance.
Key Specifications
n THD at 1.1W continuous average
output power into 8Ω at 1 kHz
n Output Power into 4Ω at 1.0%
THD+N
n THD at 70mW continuous average
output power into 32Ω at 1 kHz
n Shutdown Current
n Supply Current
0.5%(max)
1.75W(typ)
0.1%(typ)
1.0µA(max)
17.5mA(typ)
Features
n PC98 Compliant
n “Click and Pop” suppression circuitry
n Stereo line level outputs with mono input capability for
system beeps
n Microphone preamp with buffered power supply
n DC Volume Control Interface
n Thermal shutdown protection circuitry
Applications
n Multimedia Monitors
n Desktop and Portable Computers
Block Diagram
Connection Diagram
SSOP Package
DS100015-1
FIGURE 1. LM4834 Block Diagram
Boomer® is a registered trademark of NationalSemiconductor Corporation.
© 1997 National Semiconductor Corporation DS100015
DS100015-2
Top View
Order Number LM4834MS
See NS Package Number MSA028CB for SSOP
www.national.com
1 page  
External Components Description Figure 2 (Continued)
Components.
4. CO
5. RS
6. RLFE
7. RF
8. RM1
9. RM2
Functional Description
Output coupling capacitor which blocks the DC voltage at the amplifiers output. Forms a high pass filter
with RL at fo = 1/(2πRLCO).
Summing resistor that combines the right and left line level outputs into the mono input of the bridged
amplifier. The two summing resistors in parallel determine the value of the input resistance of the bridged
amplifier.
Resistor for the bridged power amplifier in series with RF at high frequencies. Used in conjunction with
CLFE to increase closed-loop gain at low frequencies.
Feedback resistor which sets the closed-loop gain in conjunction with the equivalent RS for the bridged
power amplifier.
Resistor in series with Microphone supply pin and the microphone for biasing differential input
microphones.
Resistor in series with reference ground and the microphone used for biasing differential input
microphones.
Typical Performance Characteristics
THD+N vs Frequency
Bridged Power Amp
THD+N vs Frequency
Bridged Power Amp
THD+N vs Frequency
Bridge Power Amp
DS100015-5
THD+N vs Frequency
Line Out/HP Amplifiers
DS100015-4
THD+N vs Frequency
Line Out/HP Amplifiers
DS100015-6
THD+N vs Frequency
Line Out/HP Amplifiers
DS100015-7
DS100015-8
DS100015-9
5 www.national.com
5 Page 
Application Information (Continued)
Ci. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 VDD.) This
charge comes from the output through the feedback and is
apt to create pops once the device is enabled. By minimizing
the capacitor size based on necessary low frequency re-
sponse, turn-on pops can be minimized.
CLICK AND POP CIRCUITRY
The LM4834 contains circuitry to minimize turn-on transients
or “click and pops”. In this case, turn-on refers to either
power supply turn-on or the device coming out of shutdown
mode. When the device is turning on, the amplifiers are inter-
nally configured as unity gain buffers. An internal current
source ramps up the voltage of the bypass pin. Both the in-
puts and outputs ideally track the voltage at the bypass pin.
The device will remain in buffer mode until the bypass pin
has reached its half supply voltage, 1/2 VDD. As soon as the
bypass node is stable, the device will become fully opera-
tional.
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”. By
increasing CB, the amount of turn-on pop can be reduced.
However, the trade-off for using a larger bypass capacitor is
an increase in the turn-on time for the device. Reducing CB
will decrease turn-on time and increase “click and pop”.
There is a linear relationship between the size of CB and the
turn-on time. Here are some typical turn-on times for differ-
ent values of CB:
CB
0.01 µF
TON
20 ms
0.1 µF
200 ms
0.22 µF 420 ms
0.47 µF 840 ms
1.0 µF
2 sec
In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the de-
vice or shutdown function may cause the “click and pop” cir-
cuitry to not operate fully, resulting in increased “click and
pop” noise.
In systems where the line out and headphone jack are the
same, the output coupling cap, CO, is of particular concern.
COis chosen for a desired cutoff frequency with a headphone
load. This desired cutoff frequency will change when the
headphone load is replaced by a high impedance line out
load(powered speakers). The input impedance of head-
phones are typically between 32Ω and 64Ω. Whereas, the
input impedance of powered speakers can vary from 1kΩ
top 100kΩ. As the RC time constant of the load and the out-
put coupling capacitor increases, the turn off transients are
increased.
To improve click and pop performance in this situation, exter-
nal resistors R6 and R7 should be added. The recom-
mended value for R6 is between 150Ω to 1kΩ. The recom-
mended value for R7 is between 100Ω to 500Ω. To achieve
virtually clickless and popless performance R6 = 150Ω, R7 =
100Ω, CO = 220µF, and CB = 0.47µF should be used. Lower
values of R6 will result in better click and pop performance.
However, it should be understood that lower resistance val-
ues of R6 will increase quiescent current.
DS100015-33
FIGURE 4. Resistors for Varying Output Loads
LOW FREQUENCY ENHANCEMENT
In some cases a designer may want to improve the low fre-
quency response of the bridged amplifier. This low frequency
boost can be useful in systems where speakers are housed
in small enclosures. A resistor, RLFE, and a capacitor, CLFE,
in parallel, can be placed in series with the feedback resistor
of the bridged amplifier as seen in Figure 5.
DS100015-32
FIGURE 5. Low Frequency Enhancement
At low frequencies the capacitor will be virtually an open cir-
cuit. At high frequencies the capacitor will be virtually a short
circuit. As a result of this, the gain of the bridge amplifier is
increased at low frequencies. A first order pole is formed with
a corner frequency at:
fc = 1/(2πRLFECLFE)
The resulting low frequency differential gain of this bridged
amplifier becomes:
2(Rf + RLFE) / Ri = Avd
With RF = 20kΩ, RLFE = 20kΩ, and CLFE = 0.068 µF, a first
order pole is formed with a corner frequency of 120 Hz. At
low frequencies the differential gain will be 4, assuming RS =
20k. The low frequency boost formulas assume that CO, Ci,
fIC, fOC allow the appropriate low frequency response.
11 www.national.com
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LM4834.PDF ] | |
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