lm4940tsx National Semiconductor Corporation, lm4940tsx Datasheet - Page 10

• Manufacturer Part Number: lm4940tsx
• Description: 6w Streo Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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. Further,
ensure that speakers rated at a nominal 4Ω do not fall below
3Ω. If these measures are insufficient, a heat sink can be
added to reduce θ
additional copper area around the package, with connec-
tions to the ground pins, supply pin and amplifier output pins.
Refer to the Typical Performance Characteristics curves
for power dissipation information at lower output power lev-
els.
POWER SUPPLY VOLTAGE LIMITS
Continuous proper operation is ensured by never exceeding
the voltage applied to any pin, with respect to ground, as
listed in the Absolute Maximum Ratings section.
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 voltage regulator typi-
cally use a 10µF in parallel with a 0.1µF filter capacitors to
stabilize 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
LM4940’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 LM4940’s power supply pin and
ground as short as possible. Connecting a 10µF capacitor,
C
the internal bias voltage’s stability and improves the amplifi-
er’s PSRR. The PSRR improvements increase as the by-
pass 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 capaci-
tor values, especially C
requirements, click and pop performance (as explained in
the section, SELECTING EXTERNAL COMPONENTS),
system cost, and size constraints.
MICRO-POWER SHUTDOWN
The LM4940 features an active-low micro-power shutdown
mode. When active, the LM4940’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, reducing the
supply current. The low 40µA 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.
There are a few methods to control the micro-power shut-
down. 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
SHUTDOWN pin and V
SHUTDOWN pin and GND. Select normal amplifier opera-
tion by opening the switch. Closing the switch applies GND
to the SHUTDOWN pin, activating micro-power shutdown.
The switch and resistor guarantee 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 active-state voltage to the SHUT-
DOWN pin.
BYPASS
, between the BYPASS pin and ground improves
JA
. The heat sink can be created using
DD
BYPASS
and the SPST switch between the
, depends on desired PSRR
(Continued)
10
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Two quantities determine the value of the input coupling
capacitor: the lowest audio frequency that requires amplifi-
cation and desired output transient suppression.
As shown in Figure 3, the input resistor (R
capacitor (C
that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 50Hz, C
C
efficiency, full range speaker whose response extends below
30Hz.
Output Coupling Capacitor Value Selection
The capacitors C
put DC bias voltage and couple the output AC signal to the
amplifier loads also determine low frequency response.
These capacitors, combined with their respective loads cre-
ate a highpass filter cutoff frequency. The frequency is also
given by Equation (6).
Using the same conditions as above, with a 4Ω speaker,
C
Bypass Capacitor Value
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 LM4940 settles to quiescent operation, its value
is critical when minimizing turn-on pops. The slower the
LM4940’s outputs ramp to their quiescent DC voltage (nomi-
nally V
equal to 10µF along with a small value of C
0.1µF to 0.39µF), produces a click-less and pop-less shut-
down function. As discussed above, choosing C
than necessary for the desired bandwidth helps minimize
clicks and pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4940 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 LM4940’s internal amplifiers are configured
as unity gain buffers and are disconnected from the AMP
and AMP
pacitor connected between the BYPASS pin and GND in a
controlled manner. Ideally, the input and outputs track the
voltage applied to the BYPASS pin. The gain of the internal
amplifiers remains unity until the voltage applied to the BY-
PASS pin.
The gain of the internal amplifiers remains unity until the
voltage on the bypass pin reaches V
voltage on the bypass pin is stable, the device becomes fully
operational and the amplifier outputs are reconnected to
their respective output pins. Although the BYPASS pin cur-
rent cannot be modified, changing the size of C
the device’s turn-on time. Here are some typical turn-on
times for various values of C
INA
OUT
shown in Figure 3allows the LM4940 to drive high
is 820µF (nearest common valve).
DD
DD
/2), the smaller the turn-on pop. Choosing C
B
/2 voltage present at the BYPASS pin ramps to its
pins. An internal current source charges the ca-
IN
) produce a high pass filter cutoff frequency
i
, using Equation (7) is 0.159µF. The 0.39µF
OUTA
and C
f
c
= 1/2πR
BYPASS
OUTB
that block the V
i
C
:
i
BYPASS
DD
/2. As soon as the
BYPASS
IN
IN
(in the range of
) and the input
, the capacitor
BYPASS
IN
determines
DD
no larger
BYPASS
/2 out-
alters
(5)
A