loaded voltage which is usually about 15% higher. The sup-
ply voltage will also rise 10% during high line conditions.
Therefore the maximum supply voltage is obtained from the
following equation.
Application Information (Continued)
To achieve a transient free power-up and power-down, the
voltage seen at the input terminals should be ideally the
same. Such a signal will be common-mode in nature, and
will be rejected by the LM1876. In Figure 3, the resistor RINP
serves to keep the inputs at the same potential by limiting the
voltage difference possible between the two nodes. This
should significantly reduce any type of turn-on pop, due to an
uneven charging of the amplifier inputs. This charging is
based on a specific application loading and thus, the system
designer may need to adjust these values for optimal perfor-
mance.
±
Max supplies ≈ (VOPEAK + VOD) (1 + regulation) (1.1)
For 15W of output power into an 8Ω load, the required
VOPEAK is 15.49V. A minimum supply rail of 20.5V results
from adding VOPEAK and VOD. With regulation, the maximum
±
supplies are 26V and the required IOPEAK is 1.94A from
equation (5). It should be noted that for a dual 15W amplifier
into an 8Ω load the IOPEAK drawn from the supplies is twice
1.94 Apk or 3.88 Apk. At this point it is a good idea to check
the Power Output vs Supply Voltage to ensure that the re-
quired output power is obtainable from the device while
maintaining low THD+N. In addition, the designer should
verify that with the required power supply voltage and load
impedance, that the required heatsink value θSA is feasible
given system cost and size constraints. Once the heatsink
issues have been addressed, the required gain can be deter-
mined from Equation (6).
As shown in Figure 3, the resistors labeled RBI help bias up
the LM1876 off the half-supply node at the emitter of the
2N3904. But due to the input and output coupling capacitors
in the circuit, along with the negative feedback, there are two
different values of RBI, namely 10 kΩ and 200 kΩ. These re-
sistors bring up the inputs at the same rate resulting in a pop-
less turn-on. Adjusting these resistors values slightly may re-
duce pops resulting from power supplies that ramp
extremely quick or exhibit overshoot during system turn-on.
(6)
AUDIO POWER AMPLlFIER DESIGN
Design a 15W/8Ω Audio Amplifier
Given:
From equation 6, the minimum AV is: AV ≥ 11.
=
By selecting a gain of 21, and with a feedback resistor, Rf
20 kΩ, the value of Ri follows from equation (7).
=
Ri Rf (AV − 1)
(7)
Power Output
Load Impedance
Input Level
15 Wrms
8Ω
=
Thus with Ri 1 kΩ a non-inverting gain of 21 will result.
Since the desired input impedance was 47 kΩ, a value of 47
kΩ was selected for RIN. The final design step is to address
the bandwidth requirements which must be stated as a pair
of −3 dB frequency points. Five times away from a −3 dB
point is 0.17 dB down from passband response which is bet-
1 Vrms(max)
47 kΩ
Input Impedance
Bandwidth
20 Hz−20 kHz
±
0.25 dB
±
ter than the required 0.25 dB specified. This fact results in
A designer must first determine the power supply require-
ments in terms of both voltage and current needed to obtain
the specified output power. VOPEAK can be determined from
equation (4) and IOPEAK from equation (5).
a low and high frequency pole of 4 Hz and 100 kHz respec-
tively. As stated in the External Components section, Ri in
conjunction with Ci create a high-pass filter.
=
*
*
Ci ≥ 1/(2π 1 kΩ 4 Hz) 39.8 µF;
use 39 µF.
The high frequency pole is determined by the product of the
desired high frequency pole, fH, and the gain, AV. With a
(4)
(5)
=
=
AV 21 and fH 100 kHz, the resulting GBWP is 2.1 MHz,
which is less than the guaranteed minimum GBWP of the
LM1876 of 5 MHz. This will ensure that the high frequency
response of the amplifier will be no worse than 0.17 dB down
at 20 kHz which is well within the bandwidth requirements of
the design.
To determine the maximum supply voltage the following con-
ditions must be considered. Add the dropout voltage to the
peak output swing VOPEAK, to get the supply rail at a current
of IOPEAK. The regulation of the supply determines the un-
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