Datasheet

LM4923, LM4923LQBD
www.ti.com SNAS211E –JULY 2004–REVISED MAY 2013
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the bypass and power supply pins should be as close to
the device as possible. A larger half-supply bypass capacitor improves PSRR because it increases half-supply
stability. Typical applications employ a 5V regulator with 10µF and 0.1µF bypass capacitors that increase supply
stability. This, however, does not eliminate the need for bypassing the supply nodes of the LM4923. The LM4923
will operate without the bypass capacitor C
B
, although the PSRR may decrease. A 1µF capacitor is
recommended for C
B
. This value maximizes PSRR performance. Lesser values may be used, but PSRR
decreases at frequencies below 1kHz. The issue of C
B
selection is thus dependant upon desired PSRR and click
and pop performance as explained in the section Proper Selection of External Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4923 contains shutdown circuitry that is used to
turn off the amplifier's bias circuitry. The device may then be placed into shutdown mode by toggling the
Shutdown Select pin to logic low. The trigger point for shutdown 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. 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 when
optimizing device and system performance. Although the LM4923 is tolerant to a variety of external component
combinations, consideration of component values must be made when maximizing overall system quality.
The LM4923 is unity-gain stable, giving the designer maximum system flexibility. The LM4923 should be used in
low closed-loop gain configurations to minimize THD+N values and maximize signal to noise ratio. Low gain
configurations require large input signals to obtain a given output power. Input signals equal to or greater than
1Vrms are available from sources such as audio codecs. Please refer to the AUDIO POWER AMPLIFIER
DESIGN section for a more complete explanation of proper gain selection. When used in its typical application as
a fully differential power amplifier the LM4923 does not require input coupling capacitors for input sources with
DC common-mode voltages of less than V
DD
. Exact allowable input common-mode voltage levels are actually a
function of V
DD
, R
i
, and R
f
and may be determined by Equation 5:
V
CMi
< (V
DD
-1.2)*((R
f
+(R
i
)/(R
f
)-V
DD
*(R
i
/ 2R
f
) (5)
-R
F
/ R
I
= A
VD
(6)
Special care must be taken to match the values of the feedback resistors (R
F1
and R
F2
) to each other as well as
matching the input resistors (R
i1
and R
i2
) to each other (see Figure 1) more in front. Because of the balanced
nature of differential amplifiers, resistor matching differences can result in net DC currents across the load. This
DC current can increase power consumption, internal IC power dissipation, reduce PSRR, and possibly
damaging the loudspeaker. The chart below demonstrates this problem by showing the effects of differing values
between the feedback resistors while assuming that the input resistors are perfectly matched. The results below
apply to the application circuit shown in Figure 1, and assumes that V
DD
= 5V, R
L
= 8Ω, and the system has DC
coupled inputs tied to ground.
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM4923 LM4923LQBD