LM4890MSLASHNOPB datasheet(21/36 Pages) TI

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LM4890

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SNAS138L – SEPTEMBER 2001 – REVISED MAY 2013

In addition to system cost and size, click and pop performance is effected by the size of the input coupling

capacitor, CIN. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage

(nominally 1/2 VDD). This charge comes from the output via the feedback and is apt to 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 consideration should be paid to the bypass capacitor value.

Bypass capacitor, CBYPASS, is the most critical component to minimize turn-on pops since it determines how fast

the LM4890 turns on. The slower the LM4890's outputs ramp to their quiescent DC voltage (nominally 1/2VDD),

the smaller the turn-on pop. Choosing CBYPASS equal to 1.0µF along with a small value of CIN, (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 CBYPASS equal to 0.1µF, the device will be much more

susceptible to turn-on clicks and pops. Thus, a value of CBYPASS 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 Performance Characteristics section, the supply

rail can be easily found. A second way to determine the minimum supply rail is to calculate the required Vopeak

using Equation 2 and add the output voltage. Using this method, the minimum supply voltage would be (Vopeak +

(VODTOP + VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs Supply Voltage curve in

theTypical Performance Characteristics.

(2)

5V is a standard voltage which in most applications is chosen for the supply rail. Extra supply voltage creates

headroom that allows the LM4890 to reproduce peaks in excess of 1W 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 differential gain can be determined

from Equation 3.

(3)

Rf/RIN = AVD/2

(4)

From Equation 3, the minimum AVD is 2.83; use AVD = 3.

Since the desired input impedance is 20 k

Ω, and with an AVD gain of 3, a ratio of 1.5:1 of Rf to RIN results in an

allocation of RIN = 20 kΩ and Rf = 30 kΩ. The final design step is to address the bandwidth requirements which

must be stated as a pair of

−3 dB frequency points. Five times away from a −3 dB point is 0.17 dB down from

passband response which is better than the required ±0.25 dB specified.

fL = 100Hz/5 = 20Hz

fH = 20kHz * 5 = 100kHz

As stated in the External Components Description section, RIN in conjunction with CIN create a highpass filter.

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

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