LM4906LD National Semiconductor, LM4906LD Datasheet - Page 9

• Manufacturer Part Number: LM4906LD
• Description: 1W/ Bypass-Capacitor-less Audio Amplifier with Internal Selectable Gain
• Manufacturer: National Semiconductor

Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4906 has two internal opera-
tional amplifiers. The first amplifier’s gain is either 6dB or
12dB depending on the gain select input (Low = 6dB, High =
12dB). The second amplifier’s gain is fixed by the two inter-
nal 20kΩ resistors. Figure 2 shows that the output of ampli-
fier one serves as the input to amplifier two which results in
both amplifiers producing signals identical in magnitude, but
out of phase by 180˚. Consequently, the differential gain for
the IC is
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4906,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4906 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 1.
It is critical that the maximum junction temperature T
150˚C is not exceeded. T
power derating curves by using P
area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of θ
resulting in higher P
protection circuitry being activated. Additional copper foil can
be added to any of the leads connected to the LM4906. It is
especially effective when connected to V
output pins. Refer to the application information on the
LM4906 reference design board for an example of good heat
A
VD
P
DMAX
= 2 * (20k / 20k) or 2 * (40k / 20k)
= 4 * (V
DMAX
JMAX
values without thermal shutdown
DD
)
can be determined from the
2
DMAX
/ (2π
2
and the PC board foil
R
L
)
DD
, GND, and the
JMAX
(1)
JA
of
,
9
sinking. If T
changes must be made. These changes can include re-
duced supply voltage, higher load impedance, or reduced
ambient temperature. Internal power dissipation is a function
of output power. Refer to the Typical Performance Charac-
teristics curves for power dissipation information for differ-
ent output powers and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on the power supply pin should be as
close to the device as possible. Typical applications employ
a 5V regulator with 10µF tantalum or electrolytic capacitor
and a ceramic bypass capacitor which aid in supply stability.
This does not eliminate the need for bypassing the supply
nodes of the LM4906.
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 V
must be less than 4ms, otherwise the wake-up time of the
device will be affected. After applying V
turn-on after an initial minimum threshold input signal of
7mV
An input signal of less than 7mV
output voltage. Once the device is turned on, the input signal
can go below the 7mV
however, SHUTDOWN or V
threshold requirement for the input signal must first be met
again, with V
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 shut-
down 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 Typi-
cal Performance Characteristics section. It is best to
switch between ground and supply for maximum perfor-
mance. 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 pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut-
down high or low application). This scheme guarantees 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 attenu-
ation. 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 perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
RMS
, resulting in a generated output differential signal.
DD
JMAX
ramping first.
still exceeds 150˚C, then additional
RMS
without shutting the device off. If,
DD
RMS
is cycled, the minimum
will result in a negligible
DD
, the LM4906 will
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DD