LM4675TLBD National Semiconductor, LM4675TLBD Datasheet - Page 12

• Manufacturer Part Number: LM4675TLBD
• Description: BOARD EVALUATION LM4675TL
• Manufacturer: National Semiconductor
• Series: Boomer®, PowerWise®r

Specifications of LM4675TLBD

• Amplifier Type: Class D
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 2.7W x 1 @ 4 Ohm
• Voltage - Supply: 2.4 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4675
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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Application Information
GENERAL AMPLIFIER FUNCTION
The LM4675 features a filterless modulation scheme. The
differential outputs of the device switch at 300kHz from V
to GND. When there is no input signal applied, the two outputs
(V
in phase. Because the outputs of the LM4675 are differential,
the two signals cancel each other. This results in no net volt-
age across the speaker, thus there is no load current during
an idle state, conserving power.
With an input signal applied, the duty cycle (pulse width) of
the LM4675 outputs changes. For increasing output voltages,
the duty cycle of V
decreases. For decreasing output voltages, the converse oc-
curs, the duty cycle of V
V
yields the differential output voltage.
SPREAD SPECTRUM MODULATION
The LM4675 features a fitlerless spread spectrum modulation
scheme that eliminates the need for output filters, ferrite
beads or chokes. The switching frequency varies by ±30%
about a 300kHz center frequency, reducing the wideband
spectral contend, improving EMI emissions radiated by the
speaker and associated cables and traces. Where a fixed fre-
quency class D exhibits large amounts of spectral energy at
multiples of the switching frequency, the spread spectrum ar-
chitecture of the LM4675 spreads that energy over a larger
bandwidth. The cycle-to-cycle variation of the switching peri-
od does not affect the audio reproduction of efficiency.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of
useful work output divided by the total energy required to pro-
duce it with the difference being the power dissipated, typi-
cally, in the IC. The key here is “useful” work. For audio
systems, the energy delivered in the audible bands is con-
sidered useful including the distortion products of the input
signal. Sub-sonic (DC) and super-sonic components
(>22kHz) are not useful. The difference between the power
flowing from the power supply and the audio band power be-
ing transduced is dissipated in the LM4675 and in the trans-
ducer load. The amount of power dissipation in the LM4675
is very low. This is because the ON resistance of the switches
used to form the output waveforms is typically less than
0.25Ω. This leaves only the transducer load as a potential
"sink" for the small excess of input power over audio band
output power. The LM4675 dissipates only a fraction of the
excess power requiring no additional PCB area or copper
plane to act as a heat sink.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supply voltages continue to shrink, designers are in-
creasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage swing.
The LM4675 is a fully differential amplifier that features dif-
ferential input and output stages. A differential amplifier am-
plifies the difference between the two input signals. Tradition-
al audio power amplifiers have typically offered only single-
ended inputs resulting in a 6dB reduction in signal to noise
ratio relative to differential inputs. The LM4675 also offers the
possibility of DC input coupling which eliminates the two ex-
ternal AC coupling, DC blocking capacitors. The LM4675 can
O
O
1 decreases. The difference between the two pulse widths
1 and V
O
2) switch with a 50% duty cycle, with both outputs
O
1 increases, while the duty cycle of V
O
2 increases while the duty cycle of
O
DD
2
12
be used, however, as a single ended input amplifier while still
retaining it's fully differential benefits. In fact, completely un-
related signals may be placed on the input pins. The LM4675
simply amplifies the difference between the signals. A major
benefit of a differential amplifier is the improved common
mode rejection ratio (CMRR) over single input amplifiers. The
common-mode rejection characteristic of the differential am-
plifier reduces sensitivity to ground offset related noise injec-
tion, especially important in high noise applications.
PCB LAYOUT CONSIDERATIONS
As output power increases, interconnect resistance (PCB
traces and wires) between the amplifier, load and power sup-
ply create a voltage drop. The voltage loss on the traces
between the LM4675 and the load results is lower output
power and decreased efficiency. Higher trace resistance be-
tween the supply and the LM4675 has the same effect as a
poorly regulated supply, increased ripple on the supply line
also reducing the peak output power. The effects of residual
trace resistance increases as output current increases due to
higher output power, decreased load impedance or both. To
maintain the highest output voltage swing and corresponding
peak output power, the PCB traces that connect the output
pins to the load and the supply pins to the power supply
should be as wide as possible to minimize trace resistance.
The use of power and ground planes will give the best THD
+N performance. While reducing trace resistance, the use of
power planes also creates parasite capacitors that help to fil-
ter the power supply line.
The inductive nature of the transducer load can also result in
overshoot on one or both edges, clamped by the parasitic
diodes to GND and V
point, this is an aggressive waveform that can radiate or
conduct to other components in the system and cause inter-
ference. It is essential to keep the power and output traces
short and well shielded if possible. Use of ground planes,
beads, and micro-strip layout techniques are all useful in pre-
venting unwanted interference.
As the distance from the LM4675 and the speaker increase,
the amount of EMI radiation will increase since the output
wires or traces acting as antenna become more efficient with
length. What is acceptable EMI is highly application specific.
Ferrite chip inductors placed close to the LM4675 may be
needed to reduce EMI radiation. The value of the ferrite chip
is very application specific.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion ratio (PSRR). The capacitor (C
close as possible to the LM4675. Typical applications employ
a voltage regulator with a 10µF and a 0.1µF bypass capacitors
that increase supply stability. These capacitors do not elimi-
nate the need for bypassing on the supply pin of the LM4675.
A 4.7µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4675 contains shutdown circuitry that reduces current
draw to less than 0.01µA. The trigger point for shutdown is
shown as a typical value in the Electrical Characteristics Ta-
bles and in the Shutdown Hysteresis Voltage graphs found in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for minimum current
DD
in each case. From an EMI stand-
S
) location should be as