LM4854 datasheet(23/29 Pages) NSC

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Application Information (Continued)

OPTIMIZING CLICK AND POP REDUCTION PERFOR-

MANCE

The LM4854 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

micro-power shutdown mode is deactivated.

As the V

DD/2 voltage present at the BYPASS pin ramps to its

final value, the LM4854’s internal amplifiers are configured

as unity gain buffers and are disconnected from the L-OUT,

BTL-OUT, and R-OUT pins. An internal current source

charges the capacitor connected between the BYPASS pin

and GND in a controlled, linear manner. Ideally, the input and

outputs track the voltage applied to the BYPASS pin. The

gain of the internal amplifiers remains unity until the voltage

on the bypass pin reaches V

DD/2. Once the voltage on the

bypass pin is stable and after a fixed nominal delay of

120ms, the device becomes fully operational and the ampli-

fier outputs are reconnected to their respective output pins.

Although the BYPASS pin current cannot be modified,

changing the size of CB alters the device’s turn-on time.

There is a linear relationship between the size of CB and the

turn-on time. Here are some typical turn-on times for various

values of CB:

B (µF)

ON (ms)

0.01

120

0.1

130

0.22

140

0.47

160

1.0

200

2.2

300

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".

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1W into an 8

Ω Load

The following are the desired operational parameters:

•

Power Output:

RMS

•

Load Impedance

Ω

•

Input Level:

RMS

•

Input Impedance:

20k

Ω

•

Bandwidth:

100Hz - 20kHz ± 0.25dB

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 Charac-

teristics section. Another way, using Equation (8), is to cal-

culate 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)

DD =VOUTPEAK +VODTOP +VODBOT

(9)

The Output Power vs. Supply Voltage graph for an 8

Ω load

indicates a minimum supply voltage of 4.6V. The commonly

used 5V supply voltage easily meets this. The additional

voltage creates the benefit of headroom, allowing the

LM4854 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 LM4854’s

power dissipation requirements, 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 LM4854’s to reach

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

mance. For this example, let AVD = 3. The amplifier’s overall

gain is set using the input (R

i) and feedback (Rf) 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 nominal output power is 1.13W.

TThe last step in this design example is setting the amplifi-

er’s -3dB frequency bandwidth. To achieve the desired

±0.25dB pass band magnitude variation limit, the low fre-

quency response must extend to at least one-fifth the lower

bandwidth limit and the high frequency response must ex-

tend 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

L = 100Hz / 5 = 20Hz

(12)

and an

L = 20kHz x 5 = 100kHz

(13)

As mentioned in the SELECTING EXTERNAL COMPO-

NENTS section, R

i and Ci create a highpass filter that sets

the amplifier’s lower bandpass frequency limit. Find the cou-

pling capacitor’s value using Equation (14).

i =1/(2

πR

ifL)

(14)

The result is

1/(2

π x 20kΩ x 20Hz) = 0.397µF

(15)

Use a 0.39µF capacitor, the closest standard value.

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