LM4868 datasheet(18/28 Pages) NSC

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Application Information (Continued)

DD), the smaller the turn-on pop. Choosing CB equal to

1.0µF along with a small value of C

i (in the range of 0.1µF to

0.39µF), produces a click-less and pop-less shutdown func-

tion. As discussed above, choosing C

i no larger than neces-

sary for the desired bandwidth helps minimize clicks and

pops. C

B’s value should be in the range of 5 times to 7 times

the value of C

i. This ensures that output transients are

eliminated when power is first applied or the LM4868 re-

sumes operation after shutdown.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4868 contains circuitry that eliminates turn-on and

shutdown transients (“clicks and pops“) and transients that

could occur when switching between BTL speakers and

single-ended headphones. For this discussion, turn-on re-

fers to either applying the power supply voltage or when the

shutdown mode is deactivated. While the power supply is

ramping to its final value, the LM4868’s internal amplifiers

are configured as unity gain buffers and are disconnected

from the -OUT and +OUT pins. An internal current source

changes the voltage of the BYPASS pin in a controlled,

linear manner. Ideally, the input and outputs track the voltage

applied to the BYPASS pin. The gain of the internal amplifi-

ers remains unity until the voltage on the bypass pin reaches

1/2 V

DD. As soon as the voltage on the bypass pin is stable,

the device becomes fully operational and the amplifier out-

puts are reconnected to the -OUT and +OUT pins. Although

the BYPASS pin current cannot be modified, changing the

size of C

B alters the device’s turn-on time. There is a linear

relationship between the size of C

B and the turn-on time.

Here are some typical turn-on times for various values of C

0.01µF

3ms

0.1µF

30ms

0.22µF

63ms

0.47µF

134ms

1.0µF

300ms

2.2µF

630ms

In order eliminate “clicks and pops“, all capacitors must be

discharged before turn-on. Rapidly switching V

DD may not

allow the capacitors to fully discharge, which may cause

“clicks and pops“.

NO LOAD STABILITY

The LM4868 may exhibit low level oscillation when the load

resistance is greater than 10k

Ω. This oscillation only occurs

as the output signal swings near the supply voltages. Pre-

vent this oscillation by connecting a 5k

Ω between the output

pins and ground.

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1W into an 8

Ω Load

The following are the desired operational parameters:

Power Output:

1 W

RMS

Load Impedance:

Ω

Input Level:

1 V

RMS

Input Impedance:

20 k

Ω

Bandwidth:

100 Hz−20 kHz ± 0.25 dB

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Supply Voltage curve in the Typical Performance Char-

acteristics section. Another way, using Equation (8), is to

calculate the peak output voltage necessary to achieve the

desired output power for a given load impedance. To ac-

count for the amplifier’s dropout voltage, two additional volt-

ages, based on the Dropout Voltage vs Supply Voltage in the

Typical Performance Characteristics curves, must be

added to the result obtained by Equation (8). The result is

Equation (9).

(8)

≥ (V

OUTPEAK+(VODTOP +VODBOT))

(9)

The Output Power vs Supply Voltage graph for an 8

Ω load

indicates a minimum supply voltage of 4.6V. This is easily

met by the commonly used 5V supply voltage. The additional

voltage creates the benefit of headroom, allowing the

LM4868 to produce peak output power in excess of 1W

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

of maximum power dissipation as explained above in the

Power Dissipation section.

After satisfying the LM4868’s power dissipation require-

ments, the minimum differential gain needed to achieve 1W

dissipation in an 8

Ω load is found using Equation (10).

(10)

Thus, a minimum gain of 2.83 allows the LM4868’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

VD =3.

The amplifier’s overall gain is set using the input (R

i) and

feedback (R

i) resistors. With the desired input impedance

set at 20k

Ω, the feedback resistor is found using Equation

(11).

f/Ri =AVD/2

(11)

The value of R

f is 30k

Ω.

The last step in this design example is setting the amplifier’s

−3dB frequency bandwidth. To achieve the desired ±0.25dB

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

both response limits is 0.17dB, well within the ±0.25dB

desired limit. The results are an

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