LM48411TL NSC [National Semiconductor], LM48411TL Datasheet - Page 14

• Manufacturer Part Number: LM48411TL
• Description: Ultra-Low EMI, Filterless, 2.5W, Stereo, Class D Audio Power Amplifier with E2S
• Manufacturer: NSC [National Semiconductor]

www.national.com
capacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing on the supply pin of the
LM48411. A 4.7µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM48411 contains shutdown circuitry that reduces current
draw to less than 0.01µA. The trigger point for shutdown is
shown as a typical value in the Electrical Characteristics Ta-
bles and in the Shutdown Hysteresis Voltage graphs found in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for minimum current
usage while in the shutdown state. While the LM48411 may
be disabled with shutdown voltages in between ground and
supply, the idle current will be greater than the typical 0.01µA
value. Increased THD may also be observed with voltages
less than V
The LM48411 has an internal resistor connected between
GND and Shutdown pins. The purpose of this resistor is to
eliminate any unwanted state changes when the Shutdown
pin is floating. The LM48411 will enter the shutdown state
when the Shutdown pin is left floating or if not floating, when
the shutdown voltage has crossed the threshold. To minimize
the supply current while in the shutdown state, the Shutdown
pin should be driven to GND or left floating. If the Shutdown
pin is not driven to GND, the amount of additional resistor
current due to the internal shutdown resistor can be found by
Equation (1) below.
With only a 0.5V difference, an additional 1.7µA of current will
be drawn while in the shutdown state.
PROPER SELECTION OF EXTERNAL COMPONENTS
The gain of the LM48411 is set by the external resistors, Ri
in Figure 1, The Gain is given by Equation (2) below. Best
THD+N performance is achieved with a gain of 2V/V (6dB).
It is recommended that resistors with 1% tolerance or better
be used to set the gain of the LM48411. The Ri resistors
should be placed close to the input pins of the LM48411.
Keeping the input traces close to each other and of the same
length in a high noise environment will aid in noise rejection
due to the good CMRR of the LM48411. Noise coupled onto
input traces which are physically close to each other will be
common mode and easily rejected by the LM48411.
Input capacitors may be needed for some applications or
when the source is single-ended (see Figures 3, 5). Input ca-
pacitors are needed to block any DC voltage at the source so
that the DC voltage seen between the input terminals of the
LM48411 is 0V. Input capacitors create a high-pass filter with
the input resistors, R
is found using Equation (3) below.
DD
on the Shutdown pin when in PLAY mode.
A
V
f
(V
C
= 2 * 150 kΩ / R
= 1 / (2πR
SD
i
. The –3dB point of the high-pass filter
- GND) / 300kΩ
i
C
i
) (Hz)
i
(V/V)
(1)
(2)
(3)
14
The input capacitors may also be used to remove low audio
frequencies. Small speakers cannot reproduce low bass fre-
quencies so filtering may be desired . When the LM48411 is
using a single-ended source, power supply noise on the
ground is seen as an input signal by the +IN input pin that is
capacitor coupled to ground (See Figures 5 – 7). Setting the
high-pass filter point above the power supply noise frequen-
cies, 217Hz in a GSM phone, for example, will filter out this
noise so it is not amplified and heard on the output. Capacitors
with a tolerance of 10% or better are recommended for
impedance matching.
DIFFERENTIAL CIRCUIT CONFIGURATIONS
The LM48411 can be used in many different circuit configu-
rations. The simplest and best performing is the DC coupled,
differential input configuration shown in Figure 2. Equation (2)
above is used to determine the value of the R
desired gain.
Input capacitors can be used in a differential configuration as
shown in Figure 3. Equation (3) above is used to determine
the value of the C
sponse due to the high-pass filter created by C
Equation (2) above is used to determine the value of the R
resistors for a desired gain.
The LM48411 can be used to amplify more than one audio
source. Figure 4 shows a dual differential input configuration.
The gain for each input can be independently set for maxi-
mum design flexibility using the R
Equation (2). Input capacitors can be used with one or more
sources as well to have different frequency responses de-
pending on the source or if a DC voltage needs to be blocked
from a source.
SINGLE-ENDED CIRCUIT CONFIGURATIONS
The LM48411 can also be used with single-ended sources
but input capacitors will be needed to block any DC at the
input terminals. Figure 5 shows the typical single-ended ap-
plication configuration. The equations for Gain, Equation (2),
and frequency response, Equation (3), hold for the single-
ended configuration as shown in Figure 5.
When using more than one single-ended source as shown in
Figure 6, the impedance seen from each input terminal should
be equal. To find the correct values for C
to the +IN input pin the equivalent impedance of all the single-
ended sources are calculated. The single-ended sources are
in parallel to each other. The equivalent capacitor and resis-
tor, C
nation of all C
(5) below are for any number of single-ended sources.
The LM48411 may also use a combination of single-ended
and differential sources. A typical application with one single-
ended source and one differential source is shown in Figure
7. Using the principle of superposition, the external compo-
nent values can be determined with the above equations
corresponding to the configuration.
i3
and R
R
i3
= 1 / (1/R
i
values and then all R
i3
C
, are found by calculating the parallel combi-
i3
= C
i
capacitors for a desired frequency re-
i1
i1
+ C
+ 1/R
i2
+ C
i2
+ 1/R
in
i
resistors for each input and
i
... (F)
values. Equations (4) and
in
...) (Ω)
i3
and R
i
resistors for a
i3
connected
i
and R
(4)
(5)
i
.
i