LM4947TLEVAL National Semiconductor, LM4947TLEVAL Datasheet - Page 30

• 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

www.national.com
voltage or increasing the load resistance. Further allowance
should be made for increased ambient temperatures.
The above examples assume that a device is a surface mount
part operating around the maximum power dissipation point.
Since internal power dissipation is a function of output power,
higher ambient temperatures are allowed as output power or
duty cycle decreases. If the result of Equation (11) is greater
than that of Equation (12), then decrease the supply voltage,
increase the load impedance, or reduce the ambient temper-
ature. If these measures are insufficient, a heat sink can be
added to reduce θ
ditional copper area around the package, with connections to
the ground pin(s), supply pin and amplifier output pins. Ex-
ternal, solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the θ
junction-to-case thermal impedance, θ
thermal impedance, and θ
impedance). Refer to the Typical Performance Characteris-
tics curves for power dissipation information at lower output
power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion. Applications that employ a 5V regulator typically use a
1µF in parallel with a 0.1µF filter capacitors to stabilize the
regulator's output, reduce noise on the supply line, and im-
prove the supply's transient response. However, their pres-
ence does not eliminate the need for a local 1.1µF tantalum
bypass capacitance connected between the LM4947's supply
pins and ground. Keep the length of leads and traces that
connect capacitors between the LM4947's power supply pin
and ground as short as possible. Connecting a 2.2µF capac-
itor, C
internal bias voltage's stability and improves the amplifier's
PSRR. The PSRR improvements increase as the bypass pin
capacitor value increases. Too large, however, increases
turn-on time and can compromise the amplifier's click and pop
performance. The selection of bypass capacitor values, es-
pecially C
and pop performance (as explained in the section, Proper
Selection of External Components), system cost, and size
constraints.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
capacitor can be expensive and may compromise space ef-
ficiency in portable designs. In many cases, however, the
speakers used in portable systems, whether internal or ex-
ternal, have little ability to reproduce signals below 150Hz.
Applications using speakers with this limited frequency re-
sponse reap little improvement by using large input capacitor.
The internal input resistor (R
capacitor (C
is found using Equation (15).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
i
shown in Figure 1 allows the LM4947 to drive high effi-
B
, between the BYPASS pin and ground improves the
B
, depends on desired PSRR requirements, click
i
) produce a high pass filter cutoff frequency that
JA
i
, using Equation (15) is 0.053µF. The 0.22µF
is the sum of θ
JA
f
c
. The heat sink can be created using ad-
= 1 / (2πR
SA
i
in Figures 1 & 2). A high value
i
), nominal 20kΩ, and the input
is the sink-to-ambient thermal
i
C
JC
i
)
, θ
CS
CS
, and θ
is the case-to-sink
SA
. (θ
JC
is the
(13)
30
ciency, full range speaker whose response extends below
40Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
to the BYPASS bump. Since C
LM4947 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4947's out-
puts ramp to their quiescent DC voltage (nominally V
the smaller the turn-on pop. Choosing C
with a small value of C
duces a click-less and pop-less shutdown function. As dis-
cussed above, choosing C
desired bandwidth helps minimize clicks and pops. C
should be in the range of 5 times to 7 times the value of C
This ensures that output transients are eliminated when pow-
er is first applied or the LM4947 resumes operation after
shutdown.
i
(in the range of 0.1µF to 0.39µF), pro-
i
no larger than necessary for the
B
B
determines how fast the
, the capacitor connected
B
equal to 1.0µF along
B
's value
DD
/2),
i
.