LM4883SQ datasheet(18/23 Pages) NSC

18 / 23 page

Application Information (Continued)

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 amplifiers 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.

Although the BYPASS pin current cannot be modified,

changing the size of C

5 alters the device’s turn-on time and

the magnitude of “clicks and pops”. Increasing the value of

5 reduces the magnitude of turn-on pops. However, this

presents a tradeoff: as the size of C

5 increases, the turn-on

time increases. There is a linear relationship between the

size of C

5 and the turn-on time. Here are some typical

turn-on times for various values of C

0.01µF

30ms

0.1µF

40ms

0.22µF

60ms

0.47µF

80ms

1.0µF

140 ms

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”. In a single-ended configuration, the output

is coupled to the load by C7,8. These capacitors usually

have a high value. C7,8 discharges through internal 20k

Ω

resistors. Depending on the size of C7,8, the discharge time

constant can be relatively large. To reduce transients in

single-ended mode, an external 1k

Ω–5kΩ resistor can be

placed in parallel with the internal 20k

Ω resistor. The tradeoff

for using this resistor is increased quiescent current.

NO LOAD STABILITY

The LM4883 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:

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 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 in

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

LM4883 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

maximum power dissipation as explained above in the

Power Dissipation section.

After satisfying the LM4883’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 LM4883’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

1) and

feedback (R

3) resistors. With the desired input impedance

set at 20k

Ω, the feedback resistor is found using Equation

(11).

3/R1 =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

L = 100Hz/5 = 20Hz

and an

H = 20kHz*5 = 100kHz.

As mentioned in the External Components section, R

1 and

1 create a highpass filter that sets the amplifier’s lower

bandpass frequency limit. Find the coupling capacitor’s

value using Equation (12).

≥ 1/(2πR

1fL)

(12)

The result is

1/(2

π*20kΩ*20Hz) = 0.398µF.

Use a 0.39µF capacitor, the closest standard value.

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

this example) and the differential gain, AVD, determines the

upper passband response limit. With A

VD = 3 and fH =

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

300kHz. This is less than the LM4883’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.

LM4883SQ Datasheet(PDF) 18 Page - National Semiconductor (TI)

LM4883SQ Datasheet(HTML) 18 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL