LM48511SQBD National Semiconductor, LM48511SQBD Datasheet - Page 15

• Manufacturer Part Number: LM48511SQBD
• Description: BOARD EVAL BOOST C LM48511
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM48511SQBD

• Amplifier Type: Class D
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 6.7W x 1 @ 4 Ohm
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM48511
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

FB_SEL can be susceptible to noise interference. To prevent
an accidental state change, either bypass FB_SEL with a
0.1µF capacitor to GND, or connect the higher voltage feed-
back network to FB_GND0, and the lower voltage feedback
network to FB_GND1. Because the higher output voltage
configuration typically generates more noise on V
figuration minimizes the V
FB_SEL = GND for FB_GND0 (high voltage output) and
FB_SEL = V
The selectable feedback networks maximize efficiency in two
ways. In applications where the system power supply voltage
changes, such as a mobile GPS receiver, that transitions from
battery power, to AC line, to a car power adapter, the
LM48511 can be configured to generate a lower voltage when
the system power supply voltages is lower, and conversely,
generate a higher voltage when the system power supply is
higher. See the Setting the Regulator Output Voltage (PV1)
section.
In applications where the same speaker/amplifier combina-
tion is used for different purposes with different audio power
requirements, such as a cell phone ear piece/speaker phone
speaker, the ability to quickly switch between two different
voltages allows for optimization of the amplifier power supply,
increasing overall system efficiency. When audio power de-
mands are low (ear piece mode) the regulator output voltage
can be set lower, reducing quiescent current consumption.
When audio power demands increase (speaker phone
mode), a higher voltage increases the amplifier headroom,
increasing the audio power delivered to the speaker.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using
integrated power amplifiers, and switching DC-DC convert-
ers, is critical for optimizing device and system performance.
Consideration to component values must be used to maxi-
mize overall system quality. The best capacitors for use with
the switching converter portion of the LM48511 are multi-layer
ceramic capacitors. They have the lowest ESR (equivalent
series resistance) and highest resonance frequency, which
makes them optimum for high frequency switching convert-
ers. When selecting a ceramic capacitor, only X5R and X7R
dielectric types should be used. Other types such as Z5U and
Y5F have such severe loss of capacitance due to effects of
temperature variation and applied voltage, they may provide
as little as 20% of rated capacitance in many typical applica-
tions. Always consult capacitor manufacturer’s data curves
before selecting a capacitor. High-quality ceramic capacitors
can be obtained from Taiyo-Yuden and Murata.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both PV1, V1 and V
as close to the device as possible.
AUDIO AMPLIFIER GAIN SETTING RESISTOR
SELECTION
The amplifier gain of the LM48511 is set by four external re-
sistors, the input resistors, R5 and R7, and the feed back
resistors R6 and R8.. The amplifier gain is given by:
Where R
Careful matching of the resistor pairs, R6 and R8, and R5 and
R7, is required for optimum performance. Any mismatch be-
IN
is the input resistor and R
DD
for FB_GND1 (low voltage output).
A
VD
= 2 X R
DD
noise exposure of FB_SEL, as
F
/ R
IN
F
is the feedback resistor.
DD
pins should be
DD
, this con-
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15
tween the resistors results in a differential gain error that leads
to an increase in THD+N, decrease in PSRR and CMRR, as
well as an increase in output offset voltage. Resistors with a
tolerance of 1% or better are recommended.
The gain setting resistors should be placed as close to the
device as possible. Keeping the input traces close together
and of the same length increases noise rejection in noisy en-
vironments. Noise coupled onto the input traces which are
physically close to each other will be common mode and eas-
ily rejected.
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 LM48511. The input capacitors create a high-
pass filter with the input resistors R
high pass filter is found by:
In single-ended configurations, the input capacitor value af-
fects click-and-pop performance. The LM48511 features a
50mg turn-on delaly. Choose the input capacitor / input re-
sistor values such that the capacitor is charged before the
50ms turn-on delay expires. A capacitor value of 0.18μF and
a 20kΩ input resistor are recommended. In differential appli-
cations, the charging of the input capacitor does not affect
click-and-pop significantly.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. High pass filtering the
audio signal helps protect speakers that can not reproduce or
may be damaged by low frequencies. When the LM48511 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.
SELECTING REGULATOR OUTPUT CAPACITOR
A single 100µF low ESR tantalum capacitor provides suffi-
cient output capacitance for most applications. Higher capac-
itor values improve line regulation and transient response.
Typical electrolytic capacitors are not suitable for switching
converters that operate above 500kHz because of significant
ringing and temperature rise due to self-heating from ripple
current. An output capacitor with excessive ESR reduces
phase margin and causes instability.
SELECTING REGULATING BYPASS CAPACITOR
A supply bypass capacitor is required to serve as an energy
reservoir for the current which must flow into the coil each time
the switch turns on. This capacitor must have extremely low
ESR, so ceramic capacitors are the best choice. A nominal
value of 10μF is recommended, but larger values can be
used. Since this capacitor reduces the amount of voltage rip-
ple seen at the input pin, it also reduces the amount of EMI
passed back along that line to other circuitry.
SELECTING THE SOFTSTART (C
The soft-start function charges the boost converter reference
voltage slowly. This allows the output of the boost converter
to ramp up slowly thus limiting the transient current at startup.
f = 1 / 2
π
R
IN
C
IN
SS
IN
. The -3dB point of the
) CAPACITOR
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