Datasheet

LM4910
SNAS151G –MAY 2004–REVISED MARCH 2007 www.ti.com
1/(2π*20kΩ*20Hz) = 0.397µF (7)
Use a 0.39µF capacitor, the closest standard value.
The high frequency pole is determined by the product of the desired frequency pole, f
H
, and the differential gain,
A
V
. With an A
V
= 1 and f
H
= 100kHz, the resulting GBWP = 100kHz 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.
MINIMIZING OUTPUT NOISE / REDUCING OUTPUT POWER
Figure 6.
Output noise delivered to the load can be minimized with the use of an external resistor, R
SERIES
, placed in series
with each load as shown in Figure 6. R
SERIES
forms a voltage divider with the impedance of the headphone driver
R
L
. As a result, output noise is attenuated by the factor R
L
/ (R
L
+ R
SERIES
). Figure 7 illustrates the relationship
between output noise and R
SERIES
for different loads. R
SERIES
also decreases output power delivered to the load
by the factor R
L
/ (R
L
+ R
SERIES
)
2
. However, this may not pose a problem since most headphone applications
require less than 10mW of output power. Figure 9 illustrates output power (@1% THD+N) vs R
SERIES
for different
loads.
Figure 7 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 connected 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).
ESD PROTECTION
As stated in the Absolute Maximum Ratings, pin 6 (V
o
3) on the LM4910 has a maximum ESD susceptibility rating
of 10kV. For higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as
shown in Figure 7, will provide additional protection.
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