lm48312tlx National Semiconductor Corporation, lm48312tlx Datasheet - Page 13

• Manufacturer Part Number: lm48312tlx
• Description: 2.6w, Ultra-low Emi, Filterless, Mono Class D Audio Power Amplifier With E2s
• Manufacturer: National Semiconductor Corporation

Application Information
GENERAL AMPLIFIER FUNCTION
The LM48312 mono 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 (V
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.
With the input signal applied, the duty cycle (pulse width) of
the LM48312 outputs changes. For increasing output voltage,
the duty cycle of V
V
verse occurs. The difference between the two pulse widths
yields the differential output voltage.
ENHANCED EMISSIONS SUPPRESSION SYSTEM (E
The LM48312 features National’s patented E
reduces EMI, while maintaining high quality audio reproduc-
tion and efficiency. The E
and advanced edge rate control (ERC). The LM48312 ERC
greatly reduces the high frequency components of the output
square waves by controlling the output rise and fall times,
slowing the transitions to reduce RF emissions, while maxi-
mizing THD+N and efficiency performance. The overall result
of the E
FCC Class B radiated emissions standards with 20in of twist-
ed pair cable, with excellent 0.03% THD+N and high 88%
efficiency.
SPREAD SPECTRUM
The spread spectrum modulation reduces the need for output
filters, ferrite beads or chokes. The switching frequency varies
randomly by 30% about a 300kHz center frequency, reducing
the wideband spectral contend, improving EMI emissions ra-
diated by the speaker and associated cables and traces.
Where a fixed frequency class D exhibits large amounts of
spectral energy at multiples of the switching frequency, the
spread spectrum architecture of the LM48312 spreads that
energy over a larger bandwidth (See Typical Performance
Characteristics). The cycle-to-cycle variation of the switching
period does not affect the audio reproduction, efficiency, or
PSRR.
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 LM48312 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 LM48312 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 LM48312 is attributed
OUTB
decreases. For decreasing output voltages, the con-
2
S system is a filterless Class D amplifier that passes
OUTA
and V
OUTA
2
OUTB
increases, while the duty cycle of
S system features spread spectrum
) switch with a 50% duty cycle,
2
S system that
DD
to GND
2
S)
13
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.
GAIN SETTING
The LM48312 features five internally configured gain settings,
0, 3, 6, 9, and 12dB. The device gain is selected through a
single pin (GAIN). The gain settings are shown in Table 2. The
gain of the LM48312 is determined at startup. When the
LM48312 is powered up or brought out of shutdown, the de-
vice checks the state of GAIN, and sets the amplifier gain
accordingly. Once the gain is set, the state of GAIN is ignored
and the device gain cannot be changed until the device is
either shutdown or powered down.
For proper gain selection:
9dB and 12dB gain settings.
for the 3dB and 6dB gain settings.
SHUTDOWN FUNCTION
The LM48312 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 LM48312 may be disabled with
shutdown voltages in between GND and V
will be greater than the typical 0.1µA value. Increased THD
+N may also be observed when a voltage of less than V
applied to SD.
The LM48312 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.
PROPER SELECTION OF EXTERNAL COMPONENTS
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 LM48312 supply
pins. A 1µF capacitor is recommended.
1. Use 20kΩ resistors with 10% tolerance or better for the
2. Short GAIN to either V
3. FLOAT = 20MΩ or more for the 0dB gain setting.
20kΩ to GND
20kΩ to V
FLOAT
GAIN
GND
V
DD
DD
TABLE 2. Gain Setting
DD
or GND through 100Ω or less
DD
for minimum current con-
GAIN SETTING
12dB
DD
0dB
3dB
6dB
9dB
, the idle current
www.national.com
DD
is