LM4867MT National Semiconductor, LM4867MT Datasheet - Page 15

• Manufacturer Part Number: LM4867MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

Application Information
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.
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 4, the LM4867 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.) External resistors
R
internal 20kΩ resistors set Amp2A’s gain at -1. The LM4867
drives a load, such as a speaker, connected between the two
amplifier outputs, -OUTA and +OUTA.
Figure 4 shows that Amp1A’s output serves as Amp2A’s
input. This results in both amplifiers producing signals iden-
tical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between -OUTA
and +OUTA and driven differentially ("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 as-
sumes 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.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling the output swing for a speci-
fied supply voltage. Four times the output power is possible
as compared to 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
f
and R
i
set the closed-loop gain of Amp1A, whereas two
A
VD
= 2 * (R
f
/R
i
)
(Continued)
(1)
15
order to choose an amplifier’s closed-loop gain without caus-
ing excessive clipping, please 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.
The LM4867 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and an
4Ω load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
The LM4867’s power dissipation is twice that given by Equa-
tion (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not ex-
ceed the power dissipation given by Equation (4):
The LM4867’s TJMAX = 150˚C. In the LQ package soldered
to a DAP pad that expands to a copper area of 5in
PCB, the LM4867’s θ
soldered to a DAP pad that expands to a copper area of 2in
on a PCB, the LM4867’s θ
temperature T
nal power dissipation supported by the IC packaging. Rear-
ranging Equation (4) and substituting P
sults in Equation (5). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4867’s maximum junction
temperature.
For a typical application with a 5V power supply and an 4Ω
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 99˚C for the LQ
package and 45˚C for the MTE package.
P
P
DMAX
DMAX
A
, use Equation (4) to find the maximum inter-
T
= 4 * (V
= (V
P
A
DMAX
= T
JA
DD
JMAX
’ = (T
)
DD
JA
is 20˚C/W. In the MTE package
2
/(2π
)
is 41˚C/W. At any given ambient
2
− 2 X P
/(2π
JMAX
2
R
2
L
):
R
− T
L
): Bridge Mode
DMAX
Single-Ended
A
DMAX
)/θ
JA
θ
JA
for P
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DMAX
2
on a
’ re-
(2)
(3)
(4)
(5)
2