LM4929 datasheet(13/20 Pages) TI1

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LM4929

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SNAS293B – DECEMBER 2004 – REVISED APRIL 2013

In addition to system cost and size, turn on time is affected by the size of the input coupling capacitor Ci. A larger

input coupling capacitor requires more charge to reach its quiescent DC voltage. This charge comes from the

output via the feedback Thus, by minimizing the capacitor size based on necessary low frequency response,

turn-on time can be minimized. A small value of Ci (in the range of 0.1µF to 0.39µF), is recommended.

AUDIO POWER AMPLIFIER DESIGN

A 25mW/32

Ω AUDIO AMPLIFIER

Given:

Power Output

25mWrms

Load Impedance

Ω

Input Level

1Vrms

Input Impedance

20k

Ω

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 Performance Characteristics section, the supply

rail can be easily found.

3V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates

headroom that allows the LM4929 to reproduce peak in excess of 25mW without producing audible distortion. At

this time, the designer 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 gain can be determined from

Equation 5.

(5)

From Equation 5, the minimum AV is 0.89; use AV = 1. Since the desired input impedance is 20kΩ, and with a AV

gain of 1, a ratio of 1:1 results from Equation 1 for Rf to Ri. The values are chosen with Ri = 20kΩ and Rf = 20kΩ.

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.

fL = 100Hz/5 = 20Hz

fH = 20kHz * 5 = 100kHz

As stated in the External Components section, Ri in conjunction with Ci creates a

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

The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain,

AV. With an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the LM4929

GBWP of 10MHz. This figure displays that is a designer has a need to design an amplifier with higher differential

gain, the LM4929 can still be used without running into bandwidth limitations.

Figure 32 shows an optional resistor connected between the amplifier output that drives the headphone jack

sleeve and ground. This resistor provides a ground path that supressed power supply hum. Thishum may occur

in applications such as notebook computers in a shutdown condition and connected to an external powered

speaker. The resistor's 100

Ω value is a suggested starting point. Its final value must be determined based on the

tradeoff between the amount of noise suppression that may be needed and minimizing the additional current

drawn by the resistor (25mA for a 100

Ω resistor and a 5V supply).

ESD PROTECTION

As stated in the Absolute Maximum Ratings, the LM4929 has a maximum ESD susceptibility rating of 2000V. For

higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as shown in

Figure 32, will provide additional protection.

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