LM4857SP/NOPB National Semiconductor, LM4857SP/NOPB Datasheet - Page 29

IC AUDIO SUBSYSTEM W/3D AB 28LLP

LM4857SP/NOPB

Manufacturer Part Number
LM4857SP/NOPB
Description
IC AUDIO SUBSYSTEM W/3D AB 28LLP
Manufacturer
National Semiconductor
Series
Boomer®r
Type
Class ABr
Datasheet

Specifications of LM4857SP/NOPB

Output Type
2-Channel (Stereo) with Mono and Stereo Headphones
Max Output Power X Channels @ Load
1.6W x 2 @ 4 Ohm; 75mW x 2 @ 32 Ohm
Voltage - Supply
2.7 V ~ 5.5 V
Features
3D, Depop, I²C, Mute, Shutdown, Thermal Protection, Volume Control
Mounting Type
Surface Mount
Package / Case
28-LLP
Operational Class
Class-AB
Input Offset Voltage
40@5VmV
Single Supply Voltage (typ)
Not RequiredV
Dual Supply Voltage (typ)
3/5V
Supply Current (max)
21@5VmA
Power Supply Requirement
Dual
Rail/rail I/o Type
No
Power Supply Rejection Ratio
86dB
Single Supply Voltage (min)
Not RequiredV
Single Supply Voltage (max)
Not RequiredV
Dual Supply Voltage (min)
2.5/2.7V
Dual Supply Voltage (max)
5.5V
Operating Temp Range
-40C to 85C
Operating Temperature Classification
Industrial
Mounting
Surface Mount
Pin Count
28
For Use With
LM4857SPBD - BOARD EVALUATION LM4857SP
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Other names
LM4857SP
LM4857SPTR
Application Information
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 2in
for 5V operation with a 4Ω load. Heatsink areas not placed
on the same PCB layer as the LM4857 should be 4in
same supply voltage and load resistance. The last two area
recommendations apply for 25˚C ambient temperature. In-
crease the area to compensate for ambient temperatures
above 25˚C. In all circumstances and under all conditions,
the junction temperature must be held below 150˚C to pre-
vent activating the LM4857’s thermal shutdown protection.
An example PCB layout for the exposed-DAP SP package is
shown in the Demonstration Board Layout section. Further
detailed and specific information concerning PCB layout and
fabrication and mounting an SP (LLP) is found 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 1.6W to 1.5W.
The problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
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
The LM4857 consists of three sets of a bridged-tied amplifier
pairs that drive the left loudspeaker (LLS), the right loud-
speaker (RLS), and the mono earpiece (EP). For this discus-
sion, only the LLS bridge-tied amplifier pair will be referred
to. The LM4857 drives a load, such as a speaker, connected
between outputs, LLS+ and LLS-. In the LLS amplifier block,
the output of the amplifier that drives LLS- serves as the
input to the unity gain inverting amplifier that drives LLS+.
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 LLS- and LLS+
and driven differentially (commonly referred to as ’bridge
mode’). This results in a differential or BTL gain of:
Both the feedback resistor, R
internally set.
A
VD
= 2(R
f
, and the input resistor, R
f
/ R
i
) = 2
2
(Continued)
area is necessary
2
for the
i
, are
(2)
29
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. Theoretically, 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 and
that the output signal is not clipped.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
LLS- and LLS+ outputs at half-supply. This eliminates the
coupling capacitor that single supply, single-ended amplifiers
require. Eliminating an output coupling capacitor in a typical
single-ended configuration forces a single-supply amplifier’s
half-supply bias voltage across the load. This increases
internal IC power dissipation and may permanently damage
loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is higher internal power dissi-
pation. The LM4857 has 3 sets of bridged-tied amplifier pairs
driving LLS, RLS, and EP. The maximum internal power
dissipation operating in the bridge mode is twice that of a
single-ended amplifier. From Equation (3) and (4), assuming
a 5V power supply and an 8Ω load, the maximum power
dissipation for LLS and RLS is 634mW per channel. From
equation (5), assuming a 5V power supply and a 32Ω load,
the maximum power dissipation for EP is 158mW.
The LM4857 also has 3 sets of single-ended amplifiers
driving LHP, RHP, and LINEOUT. The maximum internal
power dissipation for ROUT and LOUT is given by equation
(6) and (7). From Equations (6) and (7), assuming a 5V
power supply and a 32Ω load, the maximum power dissipa-
tion for LOUT and ROUT is 40mW per channel. From equa-
tion (8), assuming a 5V power supply and a 5kΩ load, the
maximum power dissipation for LINEOUT is negligible.
The maximum internal power dissipation of the LM4857
occurs during output modes 3, 8, and 13 when both loud-
speaker and headphone amplifiers are simultaneously on;
and is given by Equation (9).
P
DMAX-LLS
P
P
P
DMAX-RHP
DMAX-LINE
DMAX-LHP
P
P
P
DMAX-RLS
DMAX-LLS
DMAX-EP
+ P
= (V
= (V
= (V
DMAX-RLS
= 4(V
= 4(V
= 4(V
P
DD
DD
DD
DMAX-TOTAL
)
)
)
DD
DD
DD
2
2
2
/ (2π
/ (2π
/ (2π
)
)
)
+ P
2
2
2
/ (2π
/ (2π
/ (2π
DMAX-LHP
2
2
2
R
R
R
2
2
=
2
L
L
L
): Single-ended
): Single-ended
R
): Single-ended
R
R
L
L
L
): Bridged
): Bridged
): Bridged
+ P
DMAX-RHP
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(3)
(4)
(5)
(6)
(7)
(8)
(9)

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