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LM3875 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM3875
Description  OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM3875 Datasheet(HTML) 6 Page - National Semiconductor (TI)

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External Components Description
(Figure 1 and Figure 2)
Components
Functional Description
1.
R
IN
Acts as a volume control by setting the voltage level allowed to the amplifier’s input terminals.
2.
R
A
Provides DC voltage biasing for the single supply operation and bias current for the positive input terminal.
3.
C
A
Provides bias filtering.
4.
C
Provides AC coupling at the input and output of the amplifier for single supply operation.
5.
R
B
Prevents currents from entering the amplifier’s non-inverting input which may be passed through to the load
upon power-down of the system due to the low input impedance of the circuitry when the under-voltage
circuitry is off. This phenomenon occurs when the supply voltages are below 1.5V.
6.
*C
C
Reduces the gain (bandwidth of the amplifier) at high frequencies to avoid quasi-saturation oscillations of the
output transistor. The capacitor also suppresses external electromagnetic switching noise created from
fluorescent lamps.
7.
Ri
Inverting input resistance to provide AC Gain in conjunction with R
f1.
8.
*Ci
Feedback capacitor. Ensures unity gain at DC. Also a low frequency pole (highpass roll-off) at:
f
c = 1/(2
π Ri Ci).
9.
R
f1
Feedback resistance to provide AC Gain in conjunction with Ri.
10.
*R
f2
At higher frequencies feedback resistance works with C
f to provide lower AC Gain in conjunction with Rf1 and
Ri. A high frequency pole (lowpass roll-off) exists at:
f
c =[Rf1 Rf2](s+1/Rf2 Cf]/[(Rf1 +Rf2)(s+1/Cf (Rf1 +Rf2))].
11.
*C
f
Compensation capacitor that works with R
f1 and Rf2 to reduce the AC Gain at higher frequencies.
12.
*R
SN
Works with C
SN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.
13.
*C
SN
Works with R
SN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.
f
c
= 1/(2
πR
SN CSN).
14.
*L
Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce the
Q of the series resonant circuit due to capacitive load. Also provides a low impedance at low frequencies to
short out R and pass audio signals to the load.
15.
*R
16.
C
S
Provides power supply filtering and bypassing.
*Optional components dependent upon specific design requirements. Refer to the Application Information section for more information.
OPTIONAL EXTERNAL COMPONENT INTERACTION
Although the optional external components have specific
desired functions that are designed to reduce the bandwidth
and eliminate unwanted high frequency oscillations they may
cause certain undesirable effects when they interact. Inter-
action may occur for components whose reactances are in
close proximity to one another. One example would be the
coupling capacitor, C
C, and the compensation capacitor, Cf.
These two components act as low impedances to certain
frequencies which will couple signals from the input to the
output. Please take careful note of basic amplifier compo-
nent functionality when designing in these components.
The optional external components shown in Figure 2 and
described above are applicable in both single and split volt-
age supply configurations.
www.national.com
6


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