LM4927SDBD National Semiconductor, LM4927SDBD Datasheet - Page 10

• Manufacturer Part Number: LM4927SDBD
• Description: BOARD EVALUATION LM4927SD
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4927SDBD

• Amplifier Type: Class AB
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 2.5W x 1 @ 4 Ohm
• Voltage - Supply: 2.4 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4927
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4927 is a fully differential audio amplifier that fea-
tures differential input and output stages. Internally this is
accomplished by two circuits: a differential amplifier and a
common mode feedback amplifier that adjusts the output
voltages so that the average value remains V
setting the differential gain, the amplifier can be considered
to have "halves". Each half uses an input and feedback
resistor (R
(see Figure 1). With R
at -R
It is extremely important to match the input resistors to each
other, as well as the feedback resistors to each other for best
amplifier performance. See the Proper Selection of Exter-
nal Components section for more information. A differential
amplifier works in a manner where the difference between
the two input signals is amplified. In most applications, this
would require input signals that are 180˚ out of phase with
each other. The LM4927 can be used, however, as a single
ended input amplifier while still retaining its fully differential
benefits. In fact, completely unrelated signals may be placed
on the input pins. The LM4927 simply amplifies the differ-
ence between them.
All of these applications provide what is known as a "bridged
mode" output (bridge-tied-load, BTL). This results in output
signals at V
respect to each other. Bridged mode operation is different
from the single-ended amplifier configuration that connects
the load between the amplifier output and ground. A bridged
amplifier design has distinct advantages over the single-
ended configuration: it provides differential drive to the load,
thus doubling maximum possible output swing for a specific
supply voltage. Four times the output power is possible
compared with a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or clipped. In
order to choose an amplifier’s closed-loop gain without caus-
ing excess clipping, please refer to the Audio Power Am-
plifier Design section.
A bridged configuration, such as the one used in the
LM4927, also creates a second advantage over single-
ended amplifiers. Since the differential outputs, V
are biased at half-supply, no net DC voltage exists across
the load. This assumes that the input resistor pair and the
feedback resistor pair are properly matched (see Proper
Selection of External Components). BTL configuration
eliminates the output coupling capacitor required in single-
supply, single-ended amplifier configurations. If an output
coupling capacitor is not used in a single-ended output con-
figuration, the half-supply bias across the load would result
in both increased internal IC power dissipation as well as
permanent loudspeaker damage. Further advantages of
bridged mode operation specific to fully differential amplifiers
like the LM4927 include increased power supply rejection
ratio, common-mode noise reduction, and click and pop
reduction.
F
/ R
i
i1
for each half. This results in a differential gain of
and R
o1
and V
F1
) to set its respective closed-loop gain
i1
o2
A
= R
VD
that are 180˚ out of phase with
i2
= -R
and R
F
/R
i
F1
= R
F2
, the gain is set
DD
o1
/ 2. When
and V
(1)
o2
,
10
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4927’s exposed-DAP (die attach paddle) package
(LLP) provide a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane and, finally, surrounding
air. Failing to optimize thermal design may compromise the
LM4927’s high power performance and activate unwanted,
though necessary, thermal shutdown protection. The LLP
package must have its DAP soldered to a copper pad on the
PCB. The DAP’s PCB copper pad is connected to a large
plane of continuous unbroken copper. This plane forms a
thermal mass and heat sink and radiation area. Place the
heat sink area on either outside plane in the case of a
two-sided PCB, or on an inner layer of a board with more
than two layers. Connect the DAP copper pad to the inner
layer or backside copper heat sink area with a thermal via.
The via diameter should be 0.012in - 0.013in. Ensure effi-
cient thermal conductivity by plating-through and solder-
filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. In all circumstances and condi-
tions, the junction temperature must be held below 150˚C to
prevent activating the LM4927’s thermal shutdown protec-
tion. The LM4927’s power de-rating curve in the Typical
Performance Characteristics shows the maximum power
dissipation versus temperature. Example PCB layouts are
shown in the Demonstration Board Layout section. Further
detailed and specific information concerning PCB layout,
fabrication, and mounting an LLP package is available from
National Semiconductor’s package Engineering Group un-
der application note AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. This problem of decreased load dissipation
is exacerbated as load impedance decreases. Therefore, to
maintain the highest load dissipation and widest output volt-
age swing, PCB traces that connect the output pins to a load
must be as wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifer, whether the amplifier is bridged or
single-ended. Equation 2 states the maximum power dissi-
pation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.