LM4910MA National Semiconductor, LM4910MA Datasheet - Page 10

• Manufacturer Part Number: LM4910MA
• Description: Output Capacitor-less Stereo 35mW Headphone Amplifier
• Manufacturer: National Semiconductor

www.national.com
Application Information
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that drive single-
ended (SE) headphones use a coupling capacitor on each
SE output. This output coupling capacitor blocks the half-
supply voltage to which the output amplifiers are typically
biased and couples the audio signal to the headphones. The
signal return to circuit ground is through the headphone
jack’s sleeve.
The LM4910 eliminates these output coupling capacitors.
Amp3 is internally configured to apply a bandgap referenced
voltage (V
This voltage matches the quiescent voltage present on the
Amp1 and Amp2 outputs that drive the headphones. The
headphones operate in a manner similar to a bridge-tied-
load (BTL). The same DC voltage is applied to both head-
phone speaker terminals. This results in no net DC current
flow through the speaker. AC current flows through a head-
phone speaker as an audio signal’s output amplitude in-
creases on the speaker’s terminal.
The headphone jack’s sleeve is not connected to circuit
ground. Using the headphone output jack as a line-level
output will place the LM4910’s bandgap referenced voltage
on a plug’s sleeve connection. This presents no difficulty
when the external equipment uses capacitively coupled in-
puts. For the very small minority of equipment that is DC-
coupled, the LM4910 monitors the current supplied by the
amplifier that drives the headphone jack’s sleeve. If this
current exceeds 500mA
tecting the LM4910 and the external equipment.
ELIMINATING THE HALF-SUPPLY BYPASS CAPACITOR
Typical single-supply audio amplifers are normally biased to
1/2V
signal. This is usually achieved with a simple resistor divider
network from V
voltage to the amplifier. However, this scheme requires the
use of a half-supply bypass capacitor to improve the bias
voltage’s stability and the amplifier’s PSRR performance.
The LM4910 utilizes an internally generated, buffered band-
gap reference voltage as the amplifier’s bias voltage. This
bandgap reference voltage is not a direct function of V
and therefore is less susceptible to noise or ripple on the
power supply line. This allows for the LM4910 to have a
stable bias voltage and excellent PSRR performance even
without a half-supply bypass capacitor.
OUTPUT TRANSIENT (’CLICK AND POPS’)
ELIMINATED
The LM4910 contains advanced circuitry that virtually elimi-
nates output transients (’clicks and pops’). This circuitry
prevents all traces of transients when the supply voltage is
first applied or when the part resumes operation after coming
out of shutdown mode. The LM4910 remains in a muted
condition until there is sufficient input signal magnitude
(
occur. Figure 2 shows the LM4910’s lack of transients in the
differential signal (Trace B) across a 320 load. The LM4910’s
active-low SHUTDOWN pin is driven by the logic signal
shown in Trace A. Trace C is the V
D is the V
To ensure optimal click and pop performance under low gain
configurations (less than 0dB), it is critical to minimize the
RC combination of the feedback resistor R
>
5mV
DD
RMS
in order to maximize the output swing of the audio
REF
O3
, typ) to mask any remaining transient that may
output signal.
= 1.58V) to a stereo headphone jack’s sleeve.
DD
to ground that provides the proper bias
PK
, the amplifier is shutdown, pro-
O1
output signal and Trace
F
and stray input
DD
10
capacitance at the amplifier inputs. A more reliable way to
lower gain or reduce power delivered to the load is to place
a current limiting resistor in series with the load as explained
in the Minimizing Output Noise / Reducing Output Power
section.
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4910 has three operational
amplifiers internally. Two of the amplifier’s have externally
configurable gain while the other amplifier is internally fixed
at the bias point acting as a unity-gain buffer. The closed-
loop gain of the two configurable amplifiers is set by select-
ing the ratio of R
channel of the IC is
By driving the loads through outputs V
acting as a buffered bias voltage the LM4910 does not
require output coupling capacitors. The typical single-ended
amplifier configuration where one side of the load is con-
nected to ground requires large, expensive output coupling
capacitors.
A configuration such as the one used in the LM4910 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs V
= 1.58V, no net DC voltage exists across each load. This
eliminates the need for output coupling capacitors that are
required in a single-supply, single-ended amplifier configura-
tion. Without output coupling capacitors in a typical single-
supply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier. A direct consequence of the increased
power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. The maximum power
dissipation for a given application can be derived from the
power dissipation graphs or from Equation 1.
It is critical that the maximum junction temperature T
150˚C is not exceeded. Since the typical application is for
P
f
DMAX
to R
O1
, V
A
= 4(V
FIGURE 2.
i
. Consequently, the gain for each
O2
V
= -(R
, and V
DD
f
)
/R
2
O3
i
)
/ (π
are all biased at V
2
O1
R
L
and V
)
20030592
O2
with V
JMAX
REF
(1)
O3
of