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15.6 PCB Layout and Supply Regulation Considerations for Driving 8
Ω
Load
Power dissipated by a load is a function of the voltage swing across the load and the load's impedance.
As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect
(PCB trace and wire) resistance between the amplifier output pins and the load's connections. Residual
trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load
as desired. For example, 0.1
Ω
trace resistance reduces the output power dissipated by an 8
Ω
load from
158.3mW to 156.4mW. 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 supplies, 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.
15.7 Bridge Configuration Explanation
The LM49100 drives a load, such as a loudspeaker, connected between outputs, LS+ and LS-.
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 LS- and LS+ and driven differentially
(commonly referred to as ”bridge mode”).
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a
single amplifier's output and ground. For a given supply voltage, bridge mode has a distinct advantage
over the single-ended configuration: 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 LS- and LS+ 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 loudspeakers.
15.8 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 dissipation. The LM49100 has a pair of bridged-tied amplifiers driving a handsfree loudspeaker, LS.
The maximum internal power dissipation operating in the bridge mode is twice that of a single-ended
amplifier. From
Equation 1
, assuming a 5V power supply and an 8
Ω
load, the maximum MONO power
dissipation is 634mW.
P
DMAX-LS
= 4(V
DD
)
2
/ (2
π
2
R
L
): Bridge Mode
(1)
The LM49100 also has a pair of single-ended amplifiers driving stereo headphones, HPR and HPL. The
maximum internal power dissipation for HPR and HPL is given by
Equation 2
. Assuming a 2.8V power
supply and a 32
Ω
load, the maximum power dissipation for L
OUT
and R
OUT
is 49mW, or 99mW total.
P
DMAX-HPL
= 4(V
DD
HP)
2
/ (2
π
2
R
L
): Single-ended Mode
(2)
The maximum internal power dissipation of the LM49100 occurs when all three amplifiers pairs are
simultaneously on; and is given by:
P
DMAX-TOTAL
= P
DMAX-LS
+ P
DMAX-HPL
+ P
DMAX-HPR
(3)
16
AN-1622 LM49100 Evaluation Board»
SNAA043A – October 2007 – Revised May 2013
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