LM4892 datasheet(12/22 Pages) NSC

12 / 22 page

Application Information (Continued)

create pops upon device enable. Thus, by minimizing the

capacitor size based on necessary low frequency response,

turn-on pops can be minimized.

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

B, is the most critical component to minimize

turn-on pops since it determines how fast the LM4892 turns

on. The slower the LM4892’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

DD), the smaller the turn-on pop.

Choosing C

B equal to 1.0µF along with a small value of Ci (in

the range of 0.1µF to 0.39µF), should produce a virtually

clickless and popless shutdown function. While the device

will function properly, (no oscillations or motorboating), with

B equal to 0.1µF, the device will be much more susceptible

to turn-on clicks and pops. Thus, a value of C

B equal to

1.0µF is recommended in all but the most cost sensitive

designs.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

1 Wrms

Load Impedance

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

100 Hz–20 kHz ± 0.25 dB

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

opeak using Equation 2

and add the output voltage. Using this method, the minimum

supply voltage would be (V

opeak +(VODTOP +VODBOT)), where

ODBOT and VODTOP are extrapolated from the Dropout Volt-

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(2)

5V is a standard voltage in most applications, it is chosen for

the supply rail. Extra supply voltage creates headroom that

allows the LM4892 to reproduce peaks in excess of 1W

without producing audible distortion. At this time, the de-

signer must make sure that the power supply choice along

with the output impedance does not violate the conditions

explained in the Power Dissipation section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 3.

(3)

f/Ri =AVD/2

From Equation 3, the minimum A

VD is 2.83; use AVD =3.

Since the desired input impedance was 20k

Ω, and with a

VD of 3, a ratio of 1.5:1 of Rf to Ri results in an allocation of

i = 20k

Ω and R

f = 30k

Ω. The final design step is to address

the bandwidth requirements which must be stated as a pair

of −3dB frequency points. Five times away from a −3dB point

is 0.17dB down from passband response which is better

than the required ±0.25dB specified.

L = 100Hz/5 = 20Hz

H = 20kHz*5= 100kHz

As stated in the External Components section, R

i in con-

junction with C

i create a highpass filter.

≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF

The high frequency pole is determined by the product of the

desired frequency pole, f

H, and the differential gain, AVD.

With a A

VD = 3 and fH = 100kHz, the resulting GBWP =

150kHz which is much smaller than the LM4892 GBWP of

4 MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4892 can still be used without running into bandwidth

limitations.

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