Datasheet

LM4871
www.ti.com SNAS002F –FEBRUARY 2000–REVISED MAY 2013
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4871 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the
shutdown pin. The trigger point between a logic low and logic high level is typically half- supply. It is best to
switch between ground and supply to provide maximum device performance. By switching the shutdown pin to
V
DD
, the LM4871 supply current draw will be minimized in idle mode. While the device will be disabled with
shutdown pin voltages less then V
DD
, the idle current may be greater than the typical value of 0.6µA. 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 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 enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4871. 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 LM4871 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4871 is unity-gain stable which gives a designer maximum system flexibility. The LM4871 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 Audio Power Amplifier Design. 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 distinct 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 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, 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
LM4871 turns on. The slower the LM4871's outputs ramp to their quiescent DC voltage (nominally 1/2 V
DD
), the
smaller the turn-on pop. Choosing C
B
equal to 1.0µF along with a small value of C
i
(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 C
B
equal to 0.1µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C
B
equal to 1.0µF is recommended in all but the most cost sensitive
designs.
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