Datasheet

LM4898, LM4898MMBD
www.ti.com SNAS216E –MAY 2003–REVISED APRIL 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 and0.1µF bypass capacitors that increase supply
stability. This, however, does not eliminate the need for bypassing the supply nodes of the LM4898. Although the
LM4898 will operate without the bypass capacitor C
B
, 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 PROPER SELECTION OF EXTERNAL COMPONENTS.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4898 contains shutdown circuitry that is used to
turn off the amplifier’s bias circuitry. In addition, the LM4898 contains a Shutdown Mode pin, allowing the
designer to designate whether the part will be driven into shutdown with a high level logic signal or a low level
logic signal. This allows the designer maximum flexibility in device use, as the Shutdown Mode pin may simply
be tied permanently to either V
DD
or GND to set the LM4898 as either a "shutdown-high" device or a "shutdown-
low" device, respectively. The device may then be placed into shutdown mode by toggling the Shutdown Select
pin to the same state as the Shutdown Mode pin. For simplicity’s sake, this is called "shutdown same", as the
LM4898 enters shutdown mode whenever the two pins are in the same logic state. The trigger point for either
shutdown high or 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 maybe 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.
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 LM4898 is tolerant to a variety of external component
combinations, consideration of component values must be made when maximizing overall system quality.
The LM4898 is unity-gain stable, giving the designer maximum system flexibility. The LM4898 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 LM4898 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
f
1 and R
f
2) to each other as well as
matching the input resistors (R
i
1 and R
i
2) to each other (see Figure 4). 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 4, and assumes that V
DD
= 5V, R
L
= 8Ω, and the system has DC coupled
inputs tied to ground.
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