LM4868MT National Semiconductor, LM4868MT Datasheet - Page 16

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
pop performance. The selection of bypass capacitor values,
especially C
click and pop performance (as explained in the section,
Proper Selection of External Components), system cost,
and size constraints.
MICRO−POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4868’s shutdown function. Activate micro−power shut-
down by applying V
the LM4868’s micro−power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
shutdown current is achieved by applying a voltage that is as
near as V
that is less than V
Table 1 shows the logic signal levels that activate and deac-
tivate micro−power shutdown and headphone amplifier op-
HEADPHONE (SINGLE-ENDED) AMPLIFIER
OPERATION
An internal pull−up circuit is connected to the HP−IN (pin 20)
headphone amplifier control pin. When this pin is left uncon-
nected, V
and switches Amp2A’s input signal from an audio signal to
the V
bridge-connected loads. Quiescent current consumption is
reduced when the IC is in this single−ended mode.
Figure 8 shows the implementation of the LM4868’s head-
phone control function. An internal comparator with a nomi-
nal 400mV offset monitors the signal present at the −OUTB
output. It compares this signal against the signal applied to
the HP−IN pin. When these signals are equal, as is the case
when a BTL is connected to the amplifier, the comparator
forces the LM4868 to maintain bridged−amplifier operation.
When the HP−IN pin is externally floated, such as when
headphones are connected to the jack shown in Figure 8,
and internal pull−up forces V
HP−IN inputs. This changes the comparator’s output state
and enables the headphone function: it turns off Amp2B,
switches Amp2A’s input signal from an audio signal to the
V
bridge-connected loads. Amp1A and Amp1B drive the head-
phones.
Figure 8 also shows the suggested headphone jack electri-
cal connections. The jack is designed to mate with a
three−wire plug. The plug’s tip and ring should each carry
one of the two stereo output signals, whereas the sleeve
DD
/2 voltage present on pin 14, and mutes the
DD
/2 voltage present on pin 14. The result is muted
DD
DD
B
is applied to the HP−IN. This turns off Amp2B
as possible to the SHUTDOWN pin. A voltage
, depends on desired PSRR requirements,
SHUTDOWN
Logic High
Logic Low
Logic Low
Logic Low
Logic Low
DD
DD
PIN
to the SHUTDOWN pin. When active,
may increase the shutdown current.
DD
DD
on the internal comparator’s
/2. The low 0.7µA typical
= −OUTB signal
= −OUTB signal
≠ −OUTB signal
≠ −OUTB signal
(Continued)
HP-IN
PIN
X
Truth Table for Logic Inputs
MUX CHANNEL
INPUT SELECT
16
Logic High
Logic High
Logic Low
Logic Low
eration. To ensure that the output signal remains
transient−free, do not cycle the shutdown function
faster than 1Hz.
There are a few ways to control the micro−power shutdown.
These include using a single−pole, single, throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull−up resistor between the
SHUTDOWN pin and V
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
vating micro−power shutdown. The switch and resistor guar-
antee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the pull up resistor.
provides the return to Amp2A. A headphone jack with one
control pin contact is sufficient to drive the HP−IN pin when
connecting headphones.
A switch can replace the headphone jack contact pin. When
a switch shorts the HP−IN pin to V
speakers are muted and Amp1A and Amp2A drive a pair of
headphones. When a switch shorts the HP−IN pin to GND,
the LM4868 operates in bridge mode. If headphone drive is
not needed, short the HP−IN pin to the −OUTB pin.
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 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).
ESD Protection
As stated in the Absolute Maximum Ratings, pin 28 on the
MT and MH packages have a maximum ESD susceptibility
rating of 8000V. For higher ESD voltages, the addition of a
PCDN042 dual transil (from California Micro Devices), as
shown in Figure 7, will provide additional protection.
X
OPERATIONAL MODE (MUX
Single-ended amplifiers (1)
Single-ended amplifiers (2)
Micro−power shutdown
Bridged amplifiers (1)
Bridged amplifiers (2)
INPUTCHANNEL #)
DD
DD
through the pull−up resistor, acti-
. Connect the switch between the
DD
, bridge−connected