Application Information
(Continued)
to indicate to control inputs that the user has inserted a plug
into a jack and that another mode of operation is desired.
For a system implementation where the headphone amplifier
is designed using a split supply, the output coupling cap, C
C
and resistor R2 of
Figure 5,
can be eliminated. The function-
ality described earlier remains the same, however.
In addition, the HP-SENSE pin, although it may be con-
nected to the SHUTDOWN pin as shown in
Figure 4,
may
still be used as a control flag. It is capable of driving the input
to another logic gate or approximately 2 mA without serious
loading.
ground resulting from the music coming from the headphone
amplifier. The output coupling cap protects the headphones
by blocking the amplifier’s half-supply DC voltage. The ca-
pacitor also protects the headphone amplifier from the low
voltage set up by resistors R1 and R2 when there aren’t any
headphones plugged into the system. The tricky point to this
setup is that the AC output voltage of the headphone ampli-
fier cannot exceed the 2.0V HP-IN1 voltage threshold when
there aren’t any headphones plugged into the system, as-
suming that R1 and R2 are 100k and 1k, respectively. The
LM4860 may not be fully shutdown when this level is ex-
ceeded momentarily, due to the discharging time constant of
the bias-pin voltage. This time constant is established by the
two 50k resistors (in parallel) with the series bypass capaci-
tor value.
When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, en-
abling the headphone function and disabling the LM4860
amplifier. The headphone amplifier then drives the head-
phones, whose impedance is in parallel with resistor R2.
Since the typical impedance of headphones are 32Ω, resis-
tor R2 has negligible effect on the output drive capability.
Also shown in
Figure 5
are the electrical connections for the
headphone jack and plug. A 3-wire plug consists of a Tip,
Ring, and Sleave, where the Tip and Ring are signal carrying
conductors and the Sleave is the common ground return.
One control pin contact for each headphone jack is sufficient
DS011988-7
FIGURE 4. HP-SENSE Pin to
SHUTDOWN Pin Connection
DS011988-8
FIGURE 5. Typical Headphone Control Input Circuitry
7
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