LM1876 datasheet(18/20 Pages) NSC

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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 R

INP

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.

As shown in Figure 3, the resistors labeled R

BI 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 R

BI, namely 10 k

Ω and 200 kΩ. These

resistors bring up the inputs at the same rate resulting in a

popless turn-on. Adjusting these resistors values slightly

may reduce pops resulting from power supplies that ramp

extremely quick or exhibit overshoot during system turn-on.

AUDIO POWER AMPLlFIER DESIGN

Design a 15W/8

Ω Audio Amplifier

Given:

Power Output

15 Wrms

Load Impedance

Ω

Input Level

1 Vrms(max)

Input Impedance

47 k

Ω

Bandwidth

20 Hz−20 kHz

±0.25 dB

A designer must first determine the power supply require-

ments in terms of both voltage and current needed to obtain

the specified output power. V

OPEAK can be determined from

equation (4) and I

OPEAK from equation (5).

(4)

(5)

To determine the maximum supply voltage the following

conditions must be considered. Add the dropout voltage to

the peak output swing V

OPEAK, to get the supply rail at a

current of I

OPEAK. The regulation of the supply determines

the unloaded voltage which is usually about 15% higher. The

supply voltage will also rise 10% during high line conditions.

Therefore the maximum supply voltage is obtained from the

following equation.

Max supplies

≈ ± (V

OPEAK +VOD) (1 + regulation) (1.1)

For 15W of output power into an 8

Ω load, the required

OPEAK is 15.49V. A minimum supply rail of 20.5V results

from adding V

OPEAK and VOD. With regulation, the maximum

supplies are ±26V and the required I

OPEAK is 1.94A from

equation (5). It should be noted that for a dual 15W amplifier

into an 8

Ω load the I

OPEAK 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

required 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

SA is feasible

given system cost and size constraints. Once the heatsink

issues have been addressed, the required gain can be de-

termined from Equation (6).

(6)

From equation 6, the minimum A

V is:

≥ 11.

By selecting a gain of 21, and with a feedback resistor, R

f =

20 k

Ω, the value of R

i follows from equation (7).

i =Rf (AV − 1)

(7)

Thus with R

i =1 k

Ω a non-inverting gain of 21 will result.

Since the desired input impedance was 47 k

Ω, a value of 47

Ω was selected for R

IN. 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

better than the required ±0.25 dB specified. This fact results

in a low and high frequency pole of 4 Hz and 100 kHz

respectively. As stated in the External Components sec-

tion, R

i in conjunction with Ci create a high-pass filter.

≥ 1/(2π * 1kΩ * 4 Hz) = 39.8 µF;

use 39 µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the gain, AV. With a

V = 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.

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