LM4947TLEVAL National Semiconductor, LM4947TLEVAL Datasheet - Page 29

• Manufacturer Part Number: LM4947TLEVAL
• Description: BOARD EVALUATION LM4947TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4947TLEVAL

• Amplifier Type: Class D
• Output Type: 1-Channel (Mono) with Stereo Headphones
• Max Output Power X Channels @ Load: 1.19W x 1 @ 8 Ohm; 87mW x 2 @ 32 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4947
• Lead Free Status / RoHS Status: Not applicable / Not applicable

ΔAV (change in AC gain) = 1 / 1 + M, where M represents
some ratio of the nominal internal resistor, 20kΩ (see exam-
ple below).
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.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of
useful work output divided by the total energy required to pro-
duce it with the difference being the power dissipated, typi-
cally, in the IC. The key here is “useful” work. For audio
systems, the energy delivered in the audible bands is con-
sidered useful including the distortion products of the input
signal. Sub-sonic (DC) and super-sonic components
(>22kHz) are not useful. The difference between the power
flowing from the power supply and the audio band power be-
ing transduced is dissipated in the LM4947 and in the trans-
ducer load. The amount of power dissipation in the LM4947
is very low. This is because the ON resistance of the switches
used to form the output waveforms is typically less than
0.25Ω. This leaves only the transducer load as a potential
"sink" for the small excess of input power over audio band
output power. The LM4947 dissipates only a fraction of the
excess power requiring no additional PCB area or copper
plane to act as a heat sink.
The LM4947 also has a pair of single-ended amplifiers driving
stereo headphones, R
(optional)
R
3D
10
20
f
0
1
5
3dB
(kΩ)
(3D) = 1 / 2π (1 + M)(20kΩ * C
HP
C
3D
68
68
68
68
68
and L
(nF)
HP
. The maximum internal
3D
0.05
0.25
0.50
1.00
)
M
0
TABLE 7. Pole Locations
(5)
ΔAV (dB)
–0.4
–1.9
–3.5
–6.0
29
0
power dissipation for R
(10). From Equations (9) and (10), assuming a 5V power sup-
ply and a 32Ω load, the maximum power dissipation for L
and R
The maximum internal power dissipation of the LM4947 oc-
curs when all 3 amplifiers pairs are simultaneously on; and is
given by Equation (11).
The maximum power dissipation point given by Equation (11)
must not exceed the power dissipation given by Equation
(12):
The LM4947's T
LM4947's θ
T
dissipation supported by the IC packaging. Rearranging
Equation (12) and substituting P
in Equation (13). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4947's maximum junction tem-
perature.
For a typical application with a 5V power supply and an 8Ω
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 104°C for the ITL
package.
Equation (14) gives the maximum junction temperature
T
maximum junction temperature by reducing the power supply
A
JMAX
P
P
, use Equation (12) to find the maximum internal power
DMAX-RHP
DMAX-LHP
. If the result violates the LM4947's 150°C, reduce the
HP
P
DMAX-SPKROUT
is 40mW, or 80mW total.
f-3dB (3D)
JA
C
(Hz)
T
117
111
= (V
= (V
T
94
78
59
JMAX
Equivalent
P
A
is 65°C/W. At any given ambient temperature
DMAX
= T
DD
DD
JMAX
= P
)
JMAX
)
2
2
P
= (T
/ (2π
DMAX-TOTAL
/ (2π
(new) = C
+ P
DMAX-TOTAL
HP
= 150°C. In the ITL package, the
- P
and L
JMAX
DMAX-LHP
to keep same
pole location
2
2
Value of C
DMAX-TOTAL
R
R
L
L
- T
): Single-ended Mode
64.8
54.4
45.3
34.0
HP
): Single-ended Mode
(nF)
DMAX-TOTAL
3D
=
A
is given by equation (9) and
θ
) / θ
JA
/ 1 + M
+ P
3D
+ T
JA
θ
DMAX-RHP
JA
A
for P
new Pole
Location
DMAX
www.national.com
(Hz)
117
117
117
117
' results
(10)
(11)
(12)
(6)
(7)
(8)
(9)
HP