Datasheet

LM4889
www.ti.com SNAS157H –APRIL 2002–REVISED MAY 2013
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4889 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic low is placed on the
shutdown pin. By switching the shutdown pin to ground, the LM4889 supply current draw will be minimized in idle
mode. While the device will be disabled with shutdown pin voltages less than 0.5V
DC
, the idle current may be
greater than the typical value of 0.1µA. (Idle current is measured with the shutdown pin grounded).
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to
provide a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground
and disables the amplifier. If the switch is open, then the external pull-up resistor will enable the LM4889. This
scheme ensures that the shutdown pin will not float thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using integrated power amplifiers is critical to optimize
device and system performance. While the LM4889 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4889 is unity-gain stable which gives the designer maximum system flexibility. The LM4889 should be
used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain
configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1
Vrms are available from sources such as audio codecs. Please refer to the section, AUDIO POWER AMPLIFIER
DESIGN, for a more complete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, C
i
,
forms a first order high pass filter which limits low frequency response. This value should be chosen based on
needed frequency response for a few reasons.
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 100 Hz to
150 Hz. 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, C
i.
A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally
1/2 V
DD
). 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, C
B
, is the most critical component to minimize turn-on pops since it determines how fast the
LM4889 turns on. The slower the LM4889's outputs ramp to their quiescent DC voltage (nominally 1/2 V
DD
), the
smaller the turn-on pop. Choosing C
B
equal to 1.0 µF along with a small value of C
i
(in the range of 0.1 µF to
0.39 µF), should produce a virtually clickless and popless shutdown function. While the device will function
properly, (no oscillations or motorboating), with C
B
equal to 0.1 µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C
B
equal to 1.0 µF is recommended in all but the most cost sensitive
designs.
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