C

T

0.01

2

0.1

20

0.22

42

0.47

90

1.0

200

2.2

420

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

discharged before turn-on. Rapidly switching VDD 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

Given:

Power Output

1 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

Ω

Bandwidth

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)

V

(9)

The Output Power vs. Supply Voltage graph for an 8W 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

LM4913 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 LM4913’s power dissipation require-

ments, the minimum differential gain needed to achieve 1W

dissipation in an 8W load is found using Equation (10).

(RESUME HERE- All that is left is to discuss the BTL low

frequency phase shift.)

(10)

Thus, a minimum gain of 2.83 allows the LM4913’s to reach

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

mance. For this example, let AV(BTL) = 3. The amplifier’s

overall gain is set using the input (Ri), the first stage internal

feedback resistor, and the second stage’s fixed gain of 1.25.

With the desired input impedance set at 20kW, the feedback

resistor is found using Equation (11).

R

Ω /A

(11)

The value of Ri is 44.2kW. The nominal output power is

1.13W.

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

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

desired limit. The results are an

f

(12)

and an

f

(13)

As mentioned in the SELECTING EXTERNAL COMPO-

NENTS section, Ri 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).

C

πR

(14)

The result is

1/2

π x 44.2kΩ x 20Hz = 0.180µF

(15)

Use a 180nF capacitor, the closest standard value.

The product of the desired high frequency cutoff (100kHz in

this example) and the differential gain AV(BTL), determines

the upper passband response limit. With AV(BTL) = 3 and fH

= 100kHz, the closed-loop gain bandwidth product (GBWP)

is 300kHz. This is less than the LM4913’s 3.5MHz GBWP.

With this margin, the amplifier can be used in designs that

require more differential gain while avoiding performance

restricting bandwidth limitations.

RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT

Figures 9 through 13 show the recommended four-layer PC

board layout that is optimized for the micro SMD-packaged

LM4913 and associated external components. This circuit is

designed for use with an external 5V supply and 8?(min)

speakers. These circuit boards are easy to use. Apply 5V

and ground to the board’s VDD and GND pads, respectively.

Connect a speaker between the board’s RO/MO+ and LO/

MO- or headphones to the headphone jack (RO/MO+ and

LO/MO- outputs).

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