lm48310sd National Semiconductor Corporation, lm48310sd Datasheet - Page 14

• Manufacturer Part Number: lm48310sd
• Description: Ultra-low Emi, Filterless, 2.6w, Mono, Class D Audio Power Amplifier With E2s
• Manufacturer: National Semiconductor Corporation

www.national.com
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supplies continue to shrink, system designers are in-
creasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage signs.
The LM48310 features a fully differential speaker amplifier. A
differential amplifier amplifies the difference between the two
input signals. Traditional audio power amplifiers have typical-
ly offered only single-ended inputs resulting in a 6dB reduc-
tion of SNR relative to differential inputs. The LM48310 also
offers the possibility of DC input coupling which eliminates the
input coupling capacitors. A major benefit of the fully differ-
ential amplifier is the improved common mode rejection ratio
(CMRR) over single ended input amplifiers. The increased
CMRR of the differential amplifier reduces sensitivity to
ground offset related noise injection, especially important in
noisy systems.
POWER DISSIPATION AND EFFICIENCY
The major benefit of a Class D amplifier is increased efficiency
versus a Class AB. The efficiency of the LM48310 is attributed
to the region of operation of the transistors in the output stage.
The Class D output stage acts as current steering switches,
consuming negligible amounts of power compared to their
Class AB counterparts. Most of the power loss associated
with the output stage is due to the IR loss of the MOSFET on-
resistance, along with switching losses due to gate charge.
SHUTDOWN FUNCTION
The LM48310 features a low current shutdown mode. Set
SD = GND to disable the amplifier and reduce supply current
to 0.01µA.
Switch SD between GND and V
sumption is shutdown. The LM48310 may be disabled with
shutdown voltages in between GND and V
will be greater than the typical 0.1µA value.
The LM48310 shutdown input has and internal pulldown re-
sistor. The purpose of this resistor is to eliminate any unwant-
ed state changes when SD is floating. To minimize shutdown
current, SD should be driven to GND or left floating. If SD is
not driven to GND or floating, an increase in shutdown supply
current will be noticed.
AUDIO AMPLIFIER POWER SUPPLY BYPASSING/
FILTERING
Proper power supply bypassing is critical for low noise per-
formance and high PSRR. Place the supply bypass capaci-
tors as close to the device as possible. Typical applications
employ a voltage regulator with 10µF and 0.1µF bypass ca-
pacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing of the LM48310 supply
pins. A 1µF capacitor is recommended.
AUDIO AMPLIFIER INPUT CAPACITOR SELECTION
Input capacitors may be required for some applications, or
when the audio source is single-ended. Input capacitors block
the DC component of the audio signal, eliminating any conflict
between the DC component of the audio source and the bias
voltage of the LM48310. The input capacitors create a high-
pass filter with the input resistors R
high pass filter is found using Equation (1) below.
f = 1 / 2
π
R
DD
IN
C
for minimum current con-
IN
IN
. The -3dB point of the
DD
, the idle current
14
Where R
trical Characteristics table.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. Small speakers cannot
reproduce, and may even be damaged by low frequencies.
High pass filtering the audio signal helps protect the speakers.
When the LM48310 is using a single-ended source, power
supply noise on the ground is seen as an input signal. Setting
the high-pass filter point above the power supply noise fre-
quencies, 217Hz in a GSM phone, for example, filters out the
noise such that it is not amplified and heard on the output.
Capacitors with a tolerance of 10% or better are recommend-
ed for impedance matching and improved CMRR and PSRR.
AUDIO AMPLIFIER GAIN
The gain of the LM48310 is internally set to 12dB. The gain
can be reduced by adding additional input resistance (Figure
6). In this configuration, the gain of the device is given by:
Where R
additional external resistor.
SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION
The LM48310 is compatible with single-ended sources. When
configured for single-ended inputs, input capacitors must be
used to block and DC component at the input of the device.
Figure 7 shows the typical single-ended applications circuit.
FIGURE 7. Single-Ended Input Configuration
IN
F
FIGURE 6. Reduced Gain Configuration
is 40kΩ, R
is the value of the input resistor given in the Elec-
A
V
= 2 x [R
IN
is 20kΩ, and R
F
/ (R
INEXT
INEXT
+ R
IN
is the value of the
)]
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