LM4906, LM4906LDBD, LM4906MMBD

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SNAS191E – APRIL 2003 – REVISED MAY 2013

TURNING ON THE LM4906

The power supply must first be applied before the application of an input signal to the device and the ramp time

to VDD must be less than 4ms, otherwise the wake-up time of the device will be affected. After applying VDD, the

LM4906 will turn-on after an initial minimum threshold input signal of 7mVRMS, resulting in a generated output

differential signal. An input signal of less than 7mVRMS will result in a negligible output voltage. Once the device

is turned on, the input signal can go below the 7mVRMS without shutting the device off. If, however, SHUTDOWN

or VDD is cycled, the minimum threshold requirement for the input signal must first be met again, with VDD

ramping first.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4906 contains shutdown circuitry that is used to

turn off the amplifier's bias circuitry. The device is placed into shutdown mode by toggling the Shutdown pin

Low/ground. The trigger point for shutdown low is shown as a typical value in the Supply Current vs Shutdown

Voltage graphs in the Typical Performance Characteristics section. It is best to switch between ground and

supply for maximum performance. While the device may be disabled with shutdown voltages in between ground

and supply, the idle current may be greater than the typical value of 0.1µA. In either case, the shutdown pin

should be tied to a definite voltage to avoid unwanted state changes.

In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry, which

provides a quick, smooth transition to shutdown. Another solution is to use a single-throw switch in conjunction

with an external pull-up resistor (or pull-down, depending on shutdown high or low application). This scheme

ensures that the shutdown pin will not float, thus preventing unwanted state changes.

SELECTION OF INPUT CAPACITOR SIZE

Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized

capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers

used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to

150Hz. Thus, using a large input capacitor may not increase actual system performance.

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

capacitor, Ci. 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.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω Audio Amplifier

Power Output

1 Wrms

Load Impedance

8

Ω

Given:

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.

Extra supply voltage creates headroom that allows the LM4906 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.

The gain of the LM4906 is internally set at either 6dB or 12dB.

Copyright © 2003–2013, Texas Instruments Incorporated

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Product Folder Links: LM4906 LM4906LDBD LM4906MMBD