LM4938MHX/NOPB National Semiconductor, LM4938MHX/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM4938MHX/NOPB
• Description: IC AMP AUDIO PWR 2.2W AB 28TSSOP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4938MHX/NOPB

• Output Type: 2-Channel (Stereo) with Stereo Headphones
• Max Output Power X Channels @ Load: 2.2W x 2 @ 3 Ohm; 92mW x 2 @ 32 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Features: Bass Boost, Depop, Mute, Shutdown, Thermal Protection, Volume Control
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Audio Amplifier Function: Headphone/Speaker
• Total Harmonic Distortion: 0.05@8Ohm@400mW%
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 78dB
• Single Supply Voltage (min): 2.7V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -20C to 85C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 28
• Package Type: TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4938MHX

Application Information
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4938’s exposed-DAP (die attach paddle) package
(MH) provides 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. The result is a low voltage audio power amplifier that
produces 2.0W at ≤ 1% THD with a 4Ω load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4938’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MH package must have its exposed DAP soldered to a
grounded copper pad on the PCB. The DAP’s PCB copper
pad is connected to a large grounded plane of continuous
unbroken copper. This plane forms a thermal mass 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
32(4x8)
0.012in–0.013in with a 1.27mm pitch. Ensure efficient ther-
mal conductivity by plating-through and solder-filling the
vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
necessary for 5V operation with a 4Ω load. Heatsink areas
not placed on the same PCB layer as the LM4938 MH
package should be 5in
and load resistance. The last two area recommendations
apply for 25˚C ambient temperature. Increase the area to
compensate for ambient temperatures above 25˚C. In sys-
tems using cooling fans, the LM4938MH can take advantage
of forced air cooling. With an air flow rate of 450 linear-feet
per minute and a 2.5in
copper plane heatsink, the LM4938MH can continuously
drive a 3Ω load to full power. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4938’s thermal shutdown
protection. The LM4938’s power de-rating curve in the Typi-
cal Performance Characteristics shows the maximum
power dissipation versus temperature. Example PCB layouts
for the exposed-DAP TSSOP are shown in the Demonstra-
tion Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mount-
ing a package is available in National Semiconductor’s
AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 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. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
(MH)
vias.
2
2
exposed copper or 5.0in
The
(min) for the same supply voltage
via
diameter
2
(min) area is
2
should
inner layer
be
15
highest load dissipation and widest output voltage 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.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4938 output stage consists of
two pairs of operational amplifiers, forming a two-channel
(channel A and channel B) stereo amplifier. (Though the
following discusses channel A, it applies equally to channel
B.)
Figure 2 shows that the first amplifier’s negative (-) output
serves as the second amplifier’s input. This results in both
amplifiers producing signals identical in magnitude, but 180˚
out of phase. Taking advantage of this phase difference, a
load is placed between −OUTA and +OUTA and driven dif-
ferentially (commonly referred to as “bridge mode”). This
results in a differential gain of
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage
swing across the load. This produces four times the output
power when compared to a single-ended amplifier under the
same conditions. This increase in attainable output power
assumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
P
DMAX
= (V
A
DD
VD
)
2
= 2 * (R
/(2π
2
R
L
) Single-Ended
f
/R
i
)
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(1)
(2)