lm48413tlx National Semiconductor Corporation, lm48413tlx Datasheet - Page 10

• Manufacturer Part Number: lm48413tlx
• Description: Ultra Low Emi, Filterless, 1.2w Stereo Class D Audio Power Amplifier With E2s And National 3d Enhancement
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
GENERAL AMPLIFIER FUNCTION
The LM48413 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 PV
with a 390kHz 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 LM48413 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.
SHUTDOWN FUNCTION
The LM48413 features a low current shutdown mode. Set
SD = GND to disable the amplifier and reduce supply current
to 0.03μA.
Switch SD between GND and V
sumption in shutdown. The LM48413 may be disabled with
shutdown voltages in between GND and V
current will be greater than the typical value. The LM48413
shutdown input has an internal 300kΩ pull-down resistor. The
purpose of this resistor is to eliminate any unwanted state
changes when this pin is floating. To minimize shutdown cur-
rent, it should be driven to GND or left floating. If it is not driven
to GND, or floating, a small increase in shutdown supply cur-
rent will be noticed.
SPREAD SPECTRUM
The LM48413 outputs are modulated in spread spectrum
scheme eliminating the need for output filters, ferrite beads or
chokes. During its operation, the switching frequency varies
randomly by 30% about a 390kHz center frequency, reducing
the wideband spectral content and improving EMI emissions
radiated by the speaker and associated cables and traces. A
fixed frequency class D exhibits large amounts of spectral
energy at multiples of the switching frequency. The spread
spectrum architecture of the LM48413 spreads the same en-
ergy over a larger bandwidth. The cycle-to-cycle variation of
the switching period does not affect the audio reproduction,
efficiency, or PSRR.
ENHANCED EMISSIONS SUPPRESSION SYSTEM (E
The LM48413 features National’s patented E
further reduces EMI, while maintaining high quality audio re-
production and efficiency. The advanced edge rate control
(ERC) embedded within the E2S system works simultane-
ously with the spread spectrum already activated. The
LM48413 ERC greatly reduces the high frequency compo-
nents of the output square waves by controlling the output rise
and fall times, slowing the transitions to reduce RF emissions,
while maximizing THD+N and efficiency performance.
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 LM48413 features two fully differential speaker ampli-
DD
for minimum current con-
DD
2
S system that
, but the idle
DD
to GND
2
S)
10
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 LM48413
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 LM48413 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 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.
PROPER SELECTION OF EXTERNAL COMPONENTS
Power Supply Bypassing/Filtering
Proper power supply bypassing is important for low noise
performance and high PSRR. Place the 1μF supply bypass
capacitor as close to the device as possible. Traditionally, a
pair of bypass capacitors with typical value 0.1μF and 10μF
are applied to the supply rail for increasing stability. Never-
theless, these capacitors do not eliminate the need for by-
passing of the LM48413 supply pins.
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 LM48413. The input capacitors create a high-
pass filter with the input resistance R
high-pass filter is found using Equation 1 below.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. When the LM48413 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, 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 tol-
erance of 10% or better are recommended for impedance
matching and improved CMRR and PSRR.
National 3D Enhancement
The LM48413 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.
An external RC network shown in Figure 1 is required to en-
able the 3D effect. Because the LM48413 is a fully differential
amplifier, there are two separate RC networks, one for each
stereo input pair (INL+ and INR+, and INL- and INR-). Set
3DEN high to enable the 3D effect. Set 3DEN low to disable
the 3D effect.
f = 1 / 2
π
R
IN
C
IN
(Hz)
IN
. The -3dB point of the
(1)