LM4868MT National Semiconductor, LM4868MT Datasheet - Page 17

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

Application Information
Single-Ended Output Power Performance and
Measurement Considerations
The LM4868 delivers clean, low distortion SE output power
into loads that are greater than 10Ω. As an example, output
power for 16Ω and 32Ω loads are shown in the Typical
Performance Characteristic curves. For loads less than
10Ω, the LM4876 can typically supply 180mW of low distor-
tion power. However, when higher dissipation is desired in
loads less than 10Ω, a dramatic increase in THD+N may
occur. This is normal operation and does not indicate that
proper functionality has ceased. When a jump from moder-
ate to excessively high distortion is seen, simply reducing
the output voltage swing will restore the clean, low distortion
SE operation.
The dramatic jump in distortion for loads less than 10Ω
occurs when current limiting circuitry activates. During SE
operation, AMP2A (refer to Figure 4) drives the headphone
sleeve. An on-board circuit monitors this amplifier’s output
current. The sudden increase in THD+N is caused by the
current limit circuitry forcing AMP2A into a high−impedance
output mode. When this occurs, the output waveform has
discontinuities that produce large amounts of distortion. It
has been observed that as the output power is steadily
increased, the distortion may jump from 5% to greater than
35%. Indeed, 10% THD+N may not actually be achievable.
Using the Single−Ended Output for Line Level
Applications
Some samples of the LM4868 may exhibit small amplitude,
high frequency oscillation when the SE output is connected
to a line-level input. This oscillation can be eliminated by
connecting a 5%, 300Ω resistor between Amp2A’s output pin
and each amplifier, AMP1A and AMP1B, output.
protection beyond the 8000V shown in the Absolute
FIGURE 7. The PCDN042 provides additional ESD
Maximum Ratings for the AMP2A output
(Continued)
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17
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.
Besides effecting system cost and size, C
the LM4868’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
capacitor value that is no higher than necessary to meet the
desired −3dB frequency and is between 0.14C
A shown in Figure 4, the input resistor (R
capacitor, C
that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
ciency, full range speaker whose response extends below
30Hz.
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 LM4868 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4868’s
outputs ramp to their quiescent DC voltage (nominally 1/2
f
i
. Thus, pops can be minimized by selecting an input
(Pin numbers in ( ) are for the 20-pin MTE and MT
shown in Figure 4 allows the LM4868 to drive high effi-
I
produce a −3dB high pass filter cutoff frequency
FIGURE 8. Headphone Circuit
i
, using Equation (4) is 0.063µF. The 1.0µF
f
−3dB
packages.)
= 1/(2πR
i
in Figure 4). A high value capaci-
IN
C
B
I
B
)
determines how fast
, the capacitor con-
i
has an affect on
I
) and the input
B
and 0.20C
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20026724
DD
(7)
/2)
B
.