LM4947, LM4947TLEVAL

www.ti.com

SNAS349D – JUNE 2006 – REVISED MAY 2013

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value input coupling capacitor (Ci in Figure 1 and

Figure 2). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In

many cases, however, the speakers used in portable systems, whether internal or external, have little ability to

reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little

improvement by using large input capacitor.

The internal input resistor (Ri), nominal 20kΩ, and the input capacitor (Ci) produce a high pass filter cutoff

frequency that is found using Equation 13.

fc = 1 / (2πRiCi)

(13)

As an example when using a speaker with a low frequency limit of 150Hz, Ci, using Equation 13 is 0.053µF. The

0.22µF Ci shown in Figure 1 allows the LM4947 to drive high efficiency, full range speaker whose response

extends below 40Hz.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor

connected to the BYPASS bump. Since CB determines how fast the LM4947 settles to quiescent operation, its

value is critical when minimizing turn-on pops. The slower the LM4947's outputs ramp to their quiescent DC

voltage (nominally VDD/2), the smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of Ci

(in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As discussed above,

choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops. CB's value

should be in the range of 5 times to 7 times the value of Ci. This ensures that output transients are eliminated

when power is first applied or the LM4947 resumes operation after shutdown.

Copyright © 2006–2013, Texas Instruments Incorporated

Submit Documentation Feedback

29

Product Folder Links: LM4947 LM4947TLEVAL