LM4859SP National Semiconductor, LM4859SP Datasheet - Page 23

• Manufacturer Part Number: LM4859SP
• Description: Audio Amp Headphone/Speaker 2-CH Stereo 1.6W Class-AB 28-Pin LLP EP T/R
• Manufacturer: National Semiconductor

Specifications of LM4859SP

• Package: 28LLP EP
• Function: Headphone/Speaker
• Amplifier Type: Class-AB
• Total Harmonic Distortion Noise: 0.04@32Ohm@15mW|0.05@8Ohm@400mW %
• Typical Psrr: 86 dB
• Output Signal Type: Differential|Single
• Output Type: 2-Channel Stereo
• Maximum Input Offset Voltage: 40@5V mV
• Maximum Load Resistance: 32 Ohm

Application Information
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 LM4859 consists of two sets of bridged-tied amplifier
pairs that drive the left loudspeaker (LLS) and the right
loudspeaker (RLS). For this discussion, only the LLS bridge-
tied amplifier pair will be referred to. The LM4859 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.
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
A
VD
= 2(R
f
, and the input resistor, R
f
/ R
i
) = 2
(Continued)
i
, are
(2)
23
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 LM4859 has 2 sets of bridged-tied amplifier pairs
driving LLS and RLS. The maximum internal power dissipa-
tion 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 dissipa-
tion for LLS and RLS is 634mW per channel.
The LM4859 also has a pair of single-ended amplifiers driv-
ing LHP and RHP. The maximum internal power dissipation
for ROUT and LOUT is given by equation (5) and (6). From
Equations (5) and (6), assuming a 5V power supply and a
32Ω load, the maximum power dissipation for LOUT and
ROUT is 40mW per channel.
The maximum internal power dissipation of the LM4859
occurs during output modes 3, 8, and 13 when both loud-
speaker and headphone amplifiers are simultaneously on;
and is given by Equation (7).
The maximum power dissipation point given by Equation (7)
must not exceed the power dissipation given by Equation
(8):
The LM4859’s T
LM4859’s θ
T
dissipation supported by the IC packaging. Rearranging
Equation (8) and substituting P
in Equation (9). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4859’s maximum junction tem-
perature.
A
, use Equation (8) to find the maximum internal power
P
DMAX-LLS
P
P
DMAX-RHP
DMAX-LHP
P
P
DMAX-RLS
DMAX-LLS
JA
is 42˚C/W. At any given ambient temperature
T
+ P
A
P
DMAX
JMAX
= T
= (V
= (V
DMAX-RLS
= 4(V
= 4(V
JMAX
P
’ = (T
DD
DD
= 150˚C. In the SP package, the
DMAX-TOTAL
)
)
DD
DD
2
2
- P
/ (2π
JMAX
/ (2π
)
)
+ P
2
2
DMAX-TOTAL
DMAX-TOTAL
/ (2π
/ (2π
DMAX-LHP
2
2
- T
R
R
2
=
2
L
L
A
): Single-ended
): Single-ended
) / θ
R
R
L
L
): Bridged
): Bridged
JA
θ
for P
+ P
JA
DMAX-RHP
DMAX
www.national.com
’ results
(3)
(4)
(5)
(6)
(7)
(8)
(9)