LM4674SQBD National Semiconductor, LM4674SQBD Datasheet - Page 13

• Manufacturer Part Number: LM4674SQBD
• Description: BOARD EVALUATION LM4674SQ
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4674SQBD

• Amplifier Type: Class D
• 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
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4674
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
GENERAL AMPLIFIER FUNCTION
The LM4674 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 for each channel 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 LM4674 outputs changes. For increasing output voltage,
the duty cycle of the A output increases, while the duty cycle
of the B output decreases for each channel. 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 in-
creasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage signs.
The LM4674 features two fully differential amplifiers. A differ-
ential amplifier amplifies the difference between the two input
signals. Traditional audio power amplifiers have typically of-
fered only single-ended inputs resulting in a 6dB reduction of
SNR relative to differential inputs. The LM4674 also offers the
possibility of DC input coupling which eliminates the input
coupling capacitors. A major benefit of the fully differential
amplifier is the improved common mode rejection ratio (CM-
RR) 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 amplifier. The efficiency of the LM4674 is
attributed to the region of operation of the transistors in the
output stage. The Class D output stage acts as current steer-
ing switches, consuming negligible amounts of power com-
pared 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 (R
due to gate charge.
SHUTDOWN FUNCTION
The LM4674 features independent left and right channel shut-
down controls, allowing each channel to be disabled inde-
pendently. SDR controls the right channel, while SDL controls
the left channel. Driving either low disables the corresponding
channel.
It is best to switch between ground and V
rent consumption while in shutdown. The LM4674 may be
disabled with shutdown voltages in between GND and V
the idle current will be greater than the typical 0.03µA value.
For logic levels between GND and V
0.1μF capacitor.
The LM4674 shutdown inputs have internal pulldown resis-
tors. The purpose of these resistors is to eliminate any un-
DS(ON)
), along with switching losses
DD
DD
bypass SD_ with a
for minimum cur-
DD
to GND
DD
,
13
wanted state changes when SD_ is floating. To minimize
shutdown current, SD_ should be driven to GND or left float-
ing. If SD_ is not driven to GND or floating, an increase in
shutdown supply current will be noticed.
SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION
The LM4674 is compatible with single-ended sources. When
configured for single-ended inputs, input capacitors must be
used to block any 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 capacitor
as close to the device as possible. Typical applications em-
ploy a voltage regulator with 10µF and 0.1µF bypass capac-
itors that increase supply stability. These capacitors do not
eliminate the need for bypassing of the LM4674 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 LM4674. The input capacitors create a high-
pass filter with the input resistance 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 cannot
reproduce, and may even be damaged by low frequencies.
High pass filtering the audio signal helps protect the speakers.
When the LM4674 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, 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.
AUDIO AMPLIFIER GAIN SETTING
The LM4674 features four internally configured gain settings.
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
LOGIC INPUT
f = 1 / 2
G0
π
0
1
0
1
R
i
C
i
V/V
16
2
4
8
GAIN
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dB
12
18
24
6
(1)