LM4674ATLX/NOPB National Semiconductor, LM4674ATLX/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM4674ATLX/NOPB
• Description: IC AMP AUDIO PWR 2.5W D 16USMD
• Manufacturer: National Semiconductor
• Series: Boomer®, PowerWise®r
• Type: Class Dr

Specifications of LM4674ATLX/NOPB

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 2.5W x 2 @ 4 Ohm
• Voltage - Supply: 2.4 V ~ 5.5 V
• Features: Depop, Differential Inputs, PWM, Short-Circuit Protection, Shutdown
• Mounting Type: Surface Mount
• Package / Case: 16-MicroSMD
• For Use With: LM4674ATLBD - BOARD EVALUATION LM4674ATL
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4674ATLX

Application Information
GENERAL AMPLIFIER FUNCTION
The LM4674A 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 (OUT_A and OUT_B) 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.
With the input signal applied, the duty cycle (pulse width) of
the LM4674A outputs changes. For increasing output volt-
age, the duty cycle of OUT_A increases, while the duty cycle
of OUT_B decreases. For decreasing output voltages, the
converse occurs. The difference between the two pulse
widths yields the differential output voltage.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supplies continue to shrink, system designers are
increasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage signs.
The LM4674A features two fully differential amplifiers. A
differential amplifier amplifies the difference between the two
input signals. Traditional audio power amplifiers have typi-
cally offered only single-ended inputs resulting in a 6dB
reduction of SNR relative to differential inputs. The LM4674A
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 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 effi-
ciency versus a Class AB. The efficiency of the LM4674A 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 LM4674A features independent left and right channel
shutdown controls, allowing each channel to be disabled
independently. /SDR controls the right channel, while /SDL
controls the left channel. Driving either low disables the
corresponding channel, reducing supply current to 0.03µA.
It is best to switch between ground and V
current consumption while in shutdown. The LM4674A may
be disabled with shutdown voltages in between GND and
V
value. Increased THD+N may also be observed when a
voltage of less than V
between GND and V
The LM4674A shutdown inputs have internal pulldown resis-
tors. The purpose of these resistors is to eliminate any
DD
, the idle current will be greater than the typical 0.03µA
DD
DD
Bypass /SD_ with a 0.1µF capacitor.
is applied to /SD_ for logic levels
DD
for minimum
DD
to GND
13
unwanted 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.
SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION
The LM4674A 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 3 shows the typical single-ended applications
circuit.
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-
tor as close to the device as possible. Typical applications
employ a voltage regulator with 10µF and 0.1µF bypass
capacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing of the LM4674A 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 LM4674A. The input capacitors create
a high-pass filter with the input resistors RI. The -3dB point of
the high pass filter is found using Equation 1 below.
The values for RI can be found in the EC table for each gain
setting.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. Small speakers can-
not reproduce, and may even be damaged by low frequen-
cies. High pass filtering the audio signal helps protect the
speakers. When the LM4674A 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 tolerance of 10% or
better are recommended for impedance matching and im-
proved CMRR and PSRR.
AUDIO AMPLIFIER GAIN SETTING
The LM4674A features four internally configured gain set-
tings. The device gain is selected through the two logic
inputs, G0 and G1. The gain settings are as shown in the
following table.
G1
0
0
1
1
G0
0
1
0
1
f = 1 / 2πR
IN
C
V/V
16
IN
2
4
8
GAIN
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dB
12
18
24
6
(1)