lm48410sqx National Semiconductor Corporation, lm48410sqx Datasheet - Page 11

• Manufacturer Part Number: lm48410sqx
• Description: Low Emi, Filterless, 2.3w Stereo Class D Audio Power Amplifier With National 3d Enhancement
• Manufacturer: National Semiconductor Corporation

Application Information
GENERAL AMPLIFIER FUNCTION
The LM48410 stereo Class D audio power amplifier features
a filterless modulation scheme that reduces external compo-
nent count, conserving board space and reducing system
cost. The outputs of the device transition from V
with a 300kHz switching frequency. With no signal applied,
the outputs switch with a 50% duty cycle, in phase, causing
the two outputs to cancel. This cancellation results in no net
voltage across the speaker, thus there is no current to the load
in the idle state.
When an input signal is applied, the duty cycle (pulse width)
of the LM48410 output's change. For increasing output volt-
age, the duty cycle of one side of each output increases, while
the duty cycle of the other side of each output decreases. For
decreasing output voltages, the converse occurs. The differ-
ence between the two pulse widths yields the differential
output voltage.
FIXED FREQUENCY MODE
The LM48410 features two modulations schemes, a fixed fre-
quency mode and a spread spectrum mode. Select the fixed
frequency mode by setting SS/FF = GND. In fixed frequency
mode, the amplifier outputs switch at a constant 300kHz. In
fixed frequency mode, the output spectrum consists of the
fundamental and its associated harmonics (see Typical Per-
formance Characteristics).
SPREAD SPECTRUM
The logic selectable spread spectrum mode eliminates the
need for output filters, ferrite beads or chokes. In spread
spectrum mode, the switching frequency varies randomly by
30% about a 300kHz center frequency, reducing the wide-
band spectral content and improving EMI emissions radiated
by the speaker and associated cables and traces. A fixed fre-
quency class D exhibits large amounts of spectral energy at
multiples of the switching frequency. The spread spectrum
architecture of the LM48410 spreads the same energy over
a larger bandwidth (See Typical Performance Characteris-
tics). The cycle-to-cycle variation of the switching period does
not affect the audio reproduction, efficiency, or PSRR. Set SS/
FF = V
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 swings.
The LM48410 features two fully differential speaker ampli-
fiers. A differential amplifier amplifies the difference between
the two input signals. Traditional audio power amplifiers have
typically offered only single-ended inputs resulting in a 6dB
reduction of SNR relative to differential inputs. The LM48410
also offers the possibility of DC input coupling which elimi-
nates the input coupling capacitors. A major benefit of the fully
differential amplifier is the improved common mode rejection
ratio (CMRR) over single-ended input amplifiers. The in-
creased 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 LM48410 is attributed
to the region of operation of the transistors in the output stage.
The Class D output stage acts as current steering switches,
DD
for spread spectrum mode.
DD
to GND
11
consuming negligible amounts of power compared to a Class
AB amplifier. Most of the power loss associated with the out-
put stage is due to the IR loss of the MOSFET on-resistance,
along with switching losses due to gate charge.
SHUTDOWN FUNCTION
The LM48410 features independent left and right channel
shutdown controls, allowing each channel to be disabled in-
dependently. SDR controls the right channel, while SDL con-
trols the left channel. Driving either low disables the
corresponding channel, reducing supply current to 0.1µA.
It is best to switch between ground and V
rent consumption while in shutdown. The LM48410 may be
disabled with shutdown voltages in between GND and V
the idle current will be greater than the typical 0.1μA value.
Increased THD+N may also be observed when a voltage of
less than V
The LM48410 shutdown inputs have internal pulldown resis-
tors. The purpose of these resistors is to eliminate any un-
wanted 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.
PROPER SELECTION OF EXTERNAL COMPONENTS
Power Supply Bypassing/Filtering
Proper power supply bypassing is important for low noise
performance and high PSRR. Place the supply bypass ca-
pacitor 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 LM48410 supply
pins. A 1µF capacitor is recommended.
Input Capacitor Slection
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 LM48410. The input capacitors create a high-
pass filter with the input resistance R
high-pass filter is found using Equation 1 below.
The values for R
istics table for each gain setting.
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 speak-
ers. When the LM48410 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
frequencies, 217 Hz 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.
National 3D Enhancement
The LM48410 features National’s 3D enhancement effect that
widens the perceived soundstage of a stereo audio signal.
The 3D enhancement increases the apparent stereo channel
separation, improving audio reproduction whenever the left
and right speakers are too close to one another.
DD
is applied to SD.
IN
f = 1 / 2πR
can be found in the Electrical Character-
IN
C
IN
IN
. The -3dB point of the
DD
for minimum cur-
www.national.com
DD
(1)
,