LM4910MA National Semiconductor, LM4910MA Datasheet - Page 12

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

www.national.com
Application Information
From Equation 2, the minimum A
the desired input impedance is 20kΩ, and with A
a ratio of 1:1 results from Equation 1 for R
are chosen with R
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the
desired limit. The results are an
and an
MINIMIZING OUTPUT NOISE / REDUCING OUTPUT POWER
Output noise delivered to the load can be minimized with the
use of an external resistor, R
each load as shown in Figure 3. R
divider with the impedance of the headphone driver R
result, output noise is attenuated by the factor R
R
noise and R
creases output power delivered to the load by the factor R
/ (R
since most headphone applications require less than 10mW
of output power. Figure 5 illustrates output power (
THD+N) vs R
SERIES
L
+ R
). Figure 4 illustrates the relationship between output
SERIES
SERIES
SERIES
)
2
f
H
. However, this may not pose a problem
i
= 20kΩ and R
f
= 20kHz x 5 = 100kHz
L
for different loads.
for different loads. R
= 100Hz/5 = 20Hz
SERIES
V
f
is 0.98; use A
= 20kΩ.
SERIES
, placed in series with
f
(Continued)
to R
SERIES
forms a voltage
i
. The values
V
V
equal to 1,
= 1. Since
L
±
±
also de-
0.25dB
0.25dB
/ (R
L
. As a
@
L
1%
(3)
(4)
FIGURE 3.
+
L
12
As mentioned in the Selecting Proper External Compo-
nents section, R
amplifier’s lower bandpass frequency limit. Find the coupling
capacitor’s value using Equation (3).
The result is
Use a 0.39µF capacitor, the closest standard value.
The high frequency pole is determined by the product of the
desired frequency pole, f
an A
which is much smaller than the LM4910 GBWP of 11MHz.
This figure displays that if a designer has a need to design
an amplifier with higher differential gain, the LM4910 can still
be used without running into bandwidth limitations.
Figure 4 shows an optional resistor connected between the
amplifier output that drives the headphone jack sleeve and
ground. This resistor provides a ground path that supressed
power supply hum. This hum may occur in applications such
as notebook computers in a shutdown condition and con-
nected to an external powered speaker. The resistor’s 100Ω
value is a suggested starting point. Its final value must be
determined based on the tradeoff between the amount of
noise suppression that may be needed and minimizing the
additional current drawn by the resistor (25mA for a 100Ω
resistor and a 5V supply).
V
= 1 and f
1/(2π
H
i
and C
= 100kHz, the resulting GBWP = 100kHz
*
20kΩ
C
i
i
create a highpass filter that sets the
H
≥ 1/(2πR
, and the differential gain, A
*
20Hz) = 0.397µF
20030568
i
f
L
)
V
. With
(5)