LM49100GREVAL National Semiconductor, LM49100GREVAL Datasheet - Page 17

• Manufacturer Part Number: LM49100GREVAL
• Manufacturer: National Semiconductor

Specifications of LM49100GREVAL

• Lead Free Status / RoHS Status: Supplier Unconfirmed

PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 8Ω 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 ex-
ample, 0.1Ω trace resistance reduces the output power dis-
sipated by an 8Ω 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 de-
creases with increasing load current. Reduced supply voltage
causes decreased headroom, output signal clipping, and re-
duced output power. Even with tightly regulated supplies,
trace resistance creates the same effects as poor supply reg-
ulation. Therefore, making the power supply traces as wide
as possible helps maintain full output voltage swing.
BRIDGE CONFIGURATION EXPLANATION
The LM49100 drives a load, such as a loudspeaker, connect-
ed 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 am-
plifiers 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 sig-
nal is not clipped.
FIGURE 3. I
2
C Timing Diagram
17
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 cou-
pling 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 in-
ternal IC power dissipation and may permanently damage
loads such as loudspeakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful 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 sin-
gle-ended amplifier. From Equation (1), assuming a 5V power
supply and an 8Ω load, the maximum MONO power dissipa-
tion is 634mW.
The LM49100 also has a pair of single-ended amplifiers driv-
ing stereo headphones, HPR and HPL. The maximum inter-
nal power dissipation for HPR and HPL is given by equation
(2). Assuming a 2.8V power supply and a 32Ω load, the max-
imum power dissipation for L
total.
P
The maximum internal power dissipation of the LM49100 oc-
curs when all three amplifiers pairs are simultaneously on;
and is given by Equation (3).
The maximum power dissipation point given by Equation (3)
must not exceed the power dissipation given by Equation (4):
DMAX-HPL
P
DMAX-LS
P
= 4(V
DMAX-LS
= 4(V
DD
HP)
+ P
P
DD
DMAX-TOTAL
2
)
DMAX-HPL
2
/ (2π
/ (2π
OUT
2
2
R
R
L
and R
+ P
): Single-ended Mode
=
L
): Bridge Mode
DMAX-HPR
OUT
is 49mW, or 99mW
300015d6
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(1)
(2)
(3)