LM4855IBLX National Semiconductor, LM4855IBLX Datasheet - Page 18

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LM4855IBLX

Manufacturer Part Number
LM4855IBLX
Description
Manufacturer
National Semiconductor
Datasheet

Specifications of LM4855IBLX

Operational Class
Class-AB
Audio Amplifier Function
Headphone/Speaker
Total Harmonic Distortion
0.5@8Ohm@400mW%
Single Supply Voltage (typ)
3V
Dual Supply Voltage (typ)
Not RequiredV
Supply Current (max)
12@5VmA
Power Supply Requirement
Single
Rail/rail I/o Type
No
Power Supply Rejection Ratio
62dB
Single Supply Voltage (min)
2.6V
Single Supply Voltage (max)
5V
Dual Supply Voltage (min)
Not RequiredV
Dual Supply Voltage (max)
Not RequiredV
Operating Temp Range
-40C to 85C
Operating Temperature Classification
Industrial
Mounting
Surface Mount
Pin Count
18
Package Type
uSMD
Lead Free Status / RoHS Status
Not Compliant

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APPLICATION INFORMATION
voltage, increase the load impedance, or reduce the ambient
temperature. If these measures are insufficient, a heat sink
can be added to reduce θ
using additional copper area around the package, with con-
nections to the ground pin(s), supply pin and amplifier output
pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the θ
θ
the case-to-sink thermal impedance, and θ
ambient thermal impedance.) Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion at lower output power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10µF in parallel with a 0.1µF filter capacitors to stabi-
lize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4855’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4855’s
power supply pin and ground as short as possible. Connect-
ing a 1µF capacitor, C
ground improves the 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, especially C
PSRR requirements, click and pop performance (as ex-
plained in the section, Proper Selection of External Compo-
nents), system cost, and size constraints.
(Continued)
SA
. (θ
JC
is the junction-to-case thermal impedance, θ
B
, between the BYPASS pin and
JA
. The heat sink can be created
JA
is the sum of θ
B
, depends on desired
SA
is the sink-to-
JC
, θ
CS
, and
CS
is
18
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
The internal input resistor (R
produce a high pass filter cutoff frequency that is found using
Equation (9).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
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
nected to the BYPASS pin. Since C
the LM4855 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4855’s
outputs ramp to their quiescent DC voltage (nominally V
2), the smaller the turn-on pop. Choosing C
along with a small value of C
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
sary for the desired bandwidth helps minimize clicks and
pops. C
the value of C
eliminated when power is first applied or the LM4855 re-
sumes operation after shutdown.
i
shown in Figure 1 allows the LM4855 to drive high effi-
B
’s value should be in the range of 5 times to 7 times
i
. This ensures that output transients are
i
, using Equation (9) is 0.063µF. The 0.22µF
f
c
= 1 / (2πR
i
in Figure 2). A high value capaci-
i
) and the input capacitor (C
i
(in the range of 0.1µF to
i
C
i
)
B
i
B
no larger than neces-
determines how fast
, the capacitor con-
B
equal to 1.0µF
DD
(9)
i
)
/

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