LM3875 Datasheet, PDF (7/29 Pages) National Semiconductor (TI) – OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier

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Single Supply Application Circuit
LM3875
SNAS083D – JUNE 1999 – REVISED APRIL 2013
*Optional components dependent upon specific design requirements. Refer to the External Components Description
section for a component function description.
Figure 5. Typical Single Supply Audio Amplifier Application Circuit
External Components Description
(Figure 1 and Figure 5)
Components
Functional Description
1. R_IN
2. R_A
3. C_A
4. C
Acts as a volume control by setting the voltage level allowed to the amplifier's input terminals.
Provides DC voltage biasing for the single supply operation and bias current for the positive input terminal.
Provides bias filtering.
Provides AC coupling at the input and output of the amplifier for single supply operation.
5. R_B
6. C_C ⁽¹⁾
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.
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
8. Ci⁽¹⁾
Inverting input resistance to provide AC Gain in conjunction with R_f1.
Feedback capacitor. Ensures unity gain at DC. Also a low frequency pole (highpass roll-off) at:
9. R_f1
10. R_f2 ⁽¹⁾
f_c = 1/(2π Ri Ci).
Feedback resistance to provide AC Gain in conjunction with Ri.
At higher frequencies feedback resistance works with C_f to provide lower AC Gain in conjunction with R_f1 and Ri. A high
frequency pole (lowpass roll-off) exists at:
11. C_f ⁽¹⁾
12. R_SN ⁽¹⁾
13. C_SN ⁽¹⁾
14. L⁽¹⁾
15. R⁽¹⁾
f_c = [R_f1 R_f2] (s + 1/R_f2 C_f]/[(R_f1 + R_f2) (s + 1/C_f (R_f1 +R_f2))].
Compensation capacitor that works with R_f1 and R_f2 to reduce the AC Gain at higher frequencies.
Works with C_SN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.
Works with R_SN to stabilize the output stage by creating a pole that eliminates high frequency oscillations. f_c = 1/(2πR_SN
C_SN).
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.
16. C_S
Provides power supply filtering and bypassing.
(1) Optional components dependent upon specific design requirements. Refer to the Application Information section for more information.
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