LM4924SD National Semiconductor, LM4924SD Datasheet - Page 11

• Manufacturer Part Number: LM4924SD
• Manufacturer: National Semiconductor

Specifications of LM4924SD

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 0.04x2@16OhmW
• Audio Amplifier Function: Headphone
• Total Harmonic Distortion: 0.1@16Ohm@10mW%
• Single Supply Voltage (typ): 3V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 66dB
• Single Supply Voltage (min): 1.5V
• Single Supply Voltage (max): 3.6V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 10
• Package Type: LLP EP
• Lead Free Status / Rohs Status: Not Compliant

Application Information
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that drive single-ended
(SE) headphones use a coupling capacitor on each SE out-
put. This output coupling capacitor blocks the half-supply
voltage to which the output amplifiers are typically biased and
couples the audio signal to the headphones. The signal return
to circuit ground is through the headphone jack's sleeve.
The LM4924 eliminates these output coupling capacitors.
V
a stereo headphone jack's sleeve. This voltage matches the
quiescent voltage present on the V
drive the headphones. The headphones operate in a manner
similar to a bridge-tied-load (BTL). The same DC voltage is
applied to both headphone speaker terminals. This results in
no net DC current flow through the speaker. AC current flows
through a headphone speaker as an audio signal's output
amplitude increases on the speaker's terminal.
The headphone jack's sleeve is not connected to circuit
ground. Using the headphone output jack as a line-level out-
put will place the LM4924's bandgap 1/2V
sleeve connection. This presents no difficulty when the ex-
ternal equipment uses capacitively coupled inputs. For the
very small minority of equipment that is DC-coupled, the
LM4924 monitors the current supplied by the amplifier that
drives the headphone jack's sleeve. If this current exceeds
500mA
and the external equipment.
BYPASS CAPACITOR VALUE SELECTION
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
connected to the BYPASS pin. Since C
how fast the LM4924 settles to quiescent operation, its value
is critical when minimizing turn-on pops. The slower the
LM4924's outputs ramp to their quiescent DC voltage (nomi-
nally V
to 4.7µF along with a small value of C
to 0.47µF), produces a click-less and pop-less shutdown
function. As discussed above, choosing C
necessary for the desired bandwidth helps minimize clicks
and pops. This ensures that output transients are eliminated
when power is first applied or the LM4924 resumes operation
after shutdown.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4924 contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops"). For this discussion,
turn-on refers to either applying the power supply voltage or
when the micro-power shutdown mode is deactivated.
As the V
final value, the LM4924's internal amplifiers are configured as
unity gain buffers. An internal current source charges the ca-
pacitor connected between the BYPASS pin and GND in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the inter-
nal amplifiers remains unity until the voltage on the bypass
pin reaches V
is stable, the device becomes fully operational and the am-
plifier outputs are reconnected to their respective output pins.
Although the BYPASS pin current cannot be modified, chang-
ing the size of C
is a linear relationship between the size of C
o
C is internally configured to apply a 1/2V
DD
PK
DD
/2), the smaller the turn-on pop. Choosing C
, the amplifier is shutdown, protecting the LM4924
/2 voltage present at the BYPASS pin ramps to its
DD
BYPASS
/2. As soon as the voltage on the bypass pin
alters the device's turn-on time. There
o
A and V
i
BYPASS
(in the range of 0.1µF
BYPASS
DD
DD
i
bias on a plug's
o
, the capacitor
BYPASS
no larger than
bias voltage to
B outputs that
determines
and the
B
equal
11
turn-on time. Here are some typical turn-on times for various
values of C
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4924 has three operational am-
plifiers internally. Two of the amplifier's have externally con-
figurable gain while the other amplifier is internally fixed at the
bias point acting as a unity-gain buffer. The closed-loop gain
of the two configurable amplifiers is set by selecting the ratio
of R
is
By driving the loads through outputs V
acting as a buffered bias voltage the LM4924 does not require
output coupling capacitors. The typical single-ended amplifier
configuration where one side of the load is connected to
ground requires large, expensive output coupling capacitors.
A configuration such as the one used in the LM4924 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs V
V
nates the need for output coupling capacitors that are re-
quired in a single-supply, single-ended amplifier configura-
tion. Without output coupling capacitors in a typical single-
supply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful amplifier. A direct consequence of the increased pow-
er delivered to the load by a bridge amplifier is an increase in
internal power dissipation. The maximum power dissipation
for a given application can be derived from the power dissi-
pation graphs or from Equation 1.
It is critical that the maximum junction temperature T
150°C is not exceeded. Since the typical application is for
headphone operation (16Ω impedance) using a 3.3V supply
the maximum power dissipation is only 138mW. Therefore,
power dissipation is not a major concern.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be as
close to the device as possible.
Typical applications employ a 3.0V regulator with 10µF tan-
talum or electrolytic capacitor and a ceramic bypass capacitor
which aid in supply stability. This does not eliminate the need
for bypassing the supply nodes of the LM4924. A bypass ca-
pacitor value in the range of 0.1µF to 1µF is recommended
for C
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4924's shutdown function. Activate micro-power shutdown
by applying a logic-low voltage to the SHUTDOWN pin. When
active, the LM4924's micro-power shutdown feature turns off
DD
, no net DC voltage exists across each load. This elimi-
f
S
to R
.
i
. Consequently, the gain for each channel of the IC
BYPASS
P
DMAX
.
O1
= 4(V
, V
A
V
O2
DD
= -(R
, and V
)
2
/ (
f
/R
π
i
)
2
O3
R
L
are all biased at 1/2
)
O1
and V
www.national.com
O2
with V
JMAX
(1)
O3
of