LM4854 Datasheet, PDF (21/29 Pages) National Semiconductor (TI) – 1.9W Monaural, 85mW Stereo Headphone Audio Amplifier

Application Information (Continued)
power, higher ambient temperatures are allowed as output
power or duty cycle decreases. If the result of Equation (3) is
greater than that of Equation (4), then decrease the supply
voltage, increase the load impedance, or reduce the ambient
temperature. If these measures are insufficient, a heat sink
can be added to reduce θ_JA. The heat sink can be created
using additional copper area around the package, with con-
nections to the ground pin(s), supply pin and amplifier output
pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the θ_JA is the sum of θ_JC, θ_CS, and
θ_SA. (θ_JC is the junction-to-case thermal impedance, θ_CS is
the case-to-sink thermal impedance, and θ_SA is the sink-to-
ambient thermal impedance.) Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion at lower output power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10µF in parallel with a 0.1µF filter capacitors to stabi-
lize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4854’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
capacitors between the LM4854’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
CB, between the BYPASS pin and ground improves the
internal bias voltage’s stability and improves the amplifier’s
PSRR. The PSRR improvements increase as the bypass pin
capacitor value increases. Too large, however, increases
turn-on time and can compromise the amplifier’s click and
pop performance. The selection of bypass capacitor values,
especially CB, depends on desired PSRR requirements,
click and pop performance (as explained in the section,
Proper Selection of External Components), system cost, and
size constraints.
STANDBY
The LM4854 features a low-power, fast turn-on standby
mode. Applying a logic-low to the STANDBY pin act actives
the standby mode. When this mode is active, the power
supply current decreases to a nominal value of 30µA and the
amplifier outputs are muted. Fast turn-on is assured be-
cause all bias points remain at the same voltage as when the
part is in fully active operation. The LM4854 returns to fully
active operation in 100µs (typ) after the input voltage on the
STANDBY pin switches from a logic low to a logic high.
MICRO-POWER SHUTDOWN
The LM4854 features an active-low micro-power shutdown
mode. When active, the LM4854’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, reducing the
supply current. The logic threshold is typically V_DD/2. The
low 0.1µA typical shutdown current is achieved by applying a
voltage to the SHUTDOWN pin that is as near to GND as
possible. A voltage that is greater than GND may increase
the shutdown current.
CONTROLLING STANDBY AND MICROPOWER SHUT-
DOWN
There are a few methods to control standby or micro-power
shutdown. These include using a single-pole, single-throw
switch (SPST), a microprocessor, or a microcontroller. When
using a switch, connect a 100kΩ pull-up resistor between the
STANDBY or SHUTDOWN pin and V_DD and the SPST
switch between the STANDBY or SHUTDOWN pin and
GND. Select normal amplifier operation by opening the
switch. Closing the switch applies GND to the STANDBY or
SHUTDOWN pins, activating micro-power shutdown. The
switch and resistor guarantee that the STANDBY or SHUT-
DOWN pins will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon-
troller, use a digital output to apply the active-state voltage to
the STANDBY or SHUTDOWN pin.
HEADPHONE (SINGLE-ENDED) AMPLIFIER OPERATION
Previous single-supply amplifiers that were designed to drive
both BTL and SE loads used a SE (or headphone) "sense"
input. This input typically required two external resistors to
bias the sense input to a preset voltage that selected BTL
operation.
The LM4854 has a unique headphone sense circuit that
eliminates the external resistors. The amplifier has an inter-
nal comparator that monitors the voltage present on the
R-OUT pin. It compares this voltage against the voltage on
the HP-SENSE pin. When these voltages are equal, BTL
mode is selected and AMP3 is shutdown and its output has
a very high impedance. When the comparator’s input signals
are different, (a typical ∆V of 200mV), the comparator’s
output switches and activates the SE (headphone) mode.
AMP3 changes from shutdown state to an active state and,
along with AMP1, drives a stereo load. AMP2 drives the
headphone jack sleeve.
Figure 3 shows the suggested headphone jack electrical
connections. The jack is designed to mate with a three-wire
plug. The plug’s tip should carry a stereo signal’s left-
channel information. The ring adjacent to the tip should each
carry the right-channel signal and the ring furthest from the
tip provides the return to AMP2. A switch can replace the
headphone jack contact pin. When the switch shorts the
HP-SENSE pin to R-OUT, the bridge-connected speaker is
driven by AMP1 and AMP2. AMP3 is shutdown, its output in
a high-impedance state. When the switch opens, the
LM4854 operates in SE stereo mode. If headphone drive is
not needed, short the HP-SENSE pin to the R-OUT pin.
The LM4854’s unique headphone sense circuit requires a
dual switch headphone jack. A five-terminal headphone jack,
such as the Switchcraft 35RAPC4BH3, is shown in Figure 2.
For applications that require an SPDIF interface in the stereo
headphone jack, use a Foxconn 2F1138-TJ-TR.
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