LM4856LQ/NOPB National Semiconductor, LM4856LQ/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM4856LQ/NOPB
• Description: IC AMP AUDIO PWR 1.5W MONO 24LLP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4856LQ/NOPB

• Output Type: 1-Channel (Mono) with Stereo Headphones
• Max Output Power X Channels @ Load: 1.5W x 1 @ 4 Ohm; 60mW x 2 @ 32 Ohm
• Voltage - Supply: 2.6 V ~ 5 V
• Features: Depop, I²C, Mute, Shutdown, Thermal Protection, Volume Control
• Mounting Type: Surface Mount
• Package / Case: 24-LLP
• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 1-Channel Mono/2-Channel Stereo
• Audio Amplifier Function: Headphone/Speaker
• Total Harmonic Distortion: 0.5@8Ohm@400mW%
• Single Supply Voltage (typ): 3V
• Dual Supply Voltage (typ): Not RequiredV
• Supply Current (max): 12@5VmA
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 68dB
• Single Supply Voltage (min): 2.6V
• Single Supply Voltage (max): 5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 24
• Package Type: LLP EP
• For Use With: LM4856LQBD - BOARD EVALUATION LM4856LQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4856LQ; LM4856LQTR

Application Information
not placed on the same PCB layer as the LM4856 should be
5in
The last two area recommendations apply for 25˚C ambient
temperature. Increase 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 prevent activating the LM4856’s thermal shutdown
protection. Further detailed and specific information con-
cerning PCB layout and fabrication and mounting an LD
(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 1.7W to 1.6W.
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
As shown in Figure 1, the LM4856 consists of three pairs of
output amplifier blocks (A4-A6). Amplifier block A6 consists
of a bridged-tied amplifier pair that drives SPKROUT. The
LM4856 drives a load, such as a speaker, connected be-
tween outputs, SPKROUT+ and SPKROUT-. In the amplifier
block A6, the output of the amplifier that drives SPKROUT-
serves as the input to the unity gain inverting amplifier that
drives SPKROUT+.
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 SPKROUT- and
SPKROUT+ and driven differentially (commonly referred to
as ’bridge mode’). This results in a differential or BTL 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. Theoretically, this produces four times the
output power when compared to a single-ended amplifier
2
(min) for the same supply voltage and load resistance.
A
VD
= 2(R
f
/ R
i
) = 2
(Continued)
(1)
17
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
SPKROUT- and SPKROUT+ outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor 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 LM4856 has a pair of bridged-tied amplifiers
driving a handsfree speaker, SPKROUT. The maximum in-
ternal power dissipation operating in the bridge mode is
twice that of a single-ended amplifier. From Equation (2),
assuming a 5V power supply and an 8Ω load, the maximum
SPKROUT power dissipation is 634mW.
The LM4856 also has a pair of single-ended amplifiers driv-
ing stereo headphones, ROUT and LOUT. The maximum
internal power dissipation for ROUT and LOUT is given by
equation (3) and (4). From Equations (3) and (4), assuming
a 5V power supply and a 32Ω load, the maximum power
dissipation for LOUT and ROUT is 40mW, or 80mW total.
The maximum internal power dissipation of the LM4856
occurs when all 3 amplifiers pairs are simultaneously on; and
is given by Equation (5).
The maximum power dissipation point given by Equation (5)
must not exceed the power dissipation given by Equation
(6):
The LM4856’s T
LM4856’s θ
DAP pad that expands to a copper area of 2.5in
the LM4856’s θ
ture T
dissipation supported by the IC packaging. Rearranging
Equation (6) and substituting P
in Equation (7). This equation gives the maximum ambient
P
P
P
DMAX-ROUT
DMAX-LOUT
A
DMAX-SPKROUT
, use Equation (6) to find the maximum internal power
P
DMAX-SPKROUT
JA
is 48˚C/W. In the LD package soldered to a
JA
= (V
P
= (V
DMAX
JMAX
is 42˚C/W. At any given ambient tempera-
DD
DD
= 4(V
P
’ = (T
)
)
= 150˚C. In the ITL package, the
+ P
2
2
DMAX-TOTAL
/ (2π
/ (2π
DD
DMAX-LOUT
JMAX
)
2
2
2
DMAX-TOTAL
/ (2π
R
R
L
L
- T
): Single-ended Mode (3)
): Single-ended Mode (4)
=
2
A
) / θ
R
+ P
L
): Bridge Mode (2)
JA
DMAX-ROUT
for P
DMAX
2
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on a PCB,
’ results
(5)
(6)