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PDF LM48510 Data sheet ( Hoja de datos )
| Número de pieza | LM48510 | |
| Descripción | boosted class D audio power amplifier | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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November 2006
LM48510
Boosted Class D Audio Power Amplifier
General Description
The LM48510 integrates a boost converter with a high effi-
ciency mono, Class D audio power amplifier to provide 1.2W
continuous power into an 8Ω speaker when operating on a
3.3V power supply with boost voltage (PV1) of 5.0V. When
operating on a 3.3V power supply, the LM48510 is capable of
driving a 4Ω speaker load at a continuous average output of
1.7W with less than 1% THD+N. The Class D amplifier is a
low noise, filterless PWM architecture that eliminates the out-
put filter, reducing external component count, board area
consumption, system cost, and simplifying design.
The LM48510's switching regulator is a current-mode boost
converter operating at a fixed frequency of 0.6MHz.
The LM48510 is designed for use in mobile phones and other
portable communication devices. The high (76%) efficiency
extends battery life when compared to Boosted Class AB am-
plifiers. The LM48510 features a low-power consumption
shutdown mode. Shutdown may be enabled by driving the
Shutdown pin to a logic low (GND).
The gain of the Class D is externally configurable which allows
independent gain control from multiple sources by summing
the signals. Output short circuit and Thermal shutdown pro-
tection prevent the device from damage during fault condi-
tions. Superior click and pop suppression eliminates audible
transients during power-up and shutdown.
Key Specifications
■ Quiescent Power Supply Current
■ Output Power
(RL = 8Ω, THD+N ≤ 1%,
VDD = 3.3V, PV1 = 5.0V)
■ Shutdown Current
6mA (typ)
1.2W (typ)
0.01μA (typ)
Features
■ Click and Pop Suppression
■ Low 0.01μA Shutdown Current
■ 76% Efficiency
■ Filterless Class D
■ 2.7V - 5.0V operation (VDD)
■ Externally configurable gain on Class D
■ Very fast turn on time: 17μs
■ Independent Boost and Amplifier shutdown pins
Applications
■ Mobile Phones
■ PDAs
■ Portable media
■ Cameras
■ Handheld games
Typical Application
20123266
FIGURE 1. Typical LM48510 Audio Amplifier Application Circuit
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2006 National Semiconductor Corporation 201232
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Typical Performance Characteristics
THD+N vs Frequency
VDD = 2.7V, RL = 15μH + 4Ω + 15μH
THD+N vs Frequency
VDD = 2.7V, RL = 15μH + 8Ω + 15μH
20123237
THD+N vs Frequency
VDD = 3.3V, RL = 15μH + 4Ω + 15μH
20123250
THD+N vs Frequency
VDD = 3.3V, RL = 15μH + 8Ω + 15μH
20123239
THD+N vs Output Power
VDD = 2.7V, RL = 15μH + 4Ω + 15μH
20123240
THD+N vs Output Power
VDD = 2.7V, RL = 15μH + 8Ω + 15μH
20123241
5
20123251
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est resonance frequency, which makes them optimum for
high frequency switching converters.
When selecting a ceramic capacitor, only X5R and X7R di-
electric 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, AVX, and Murata.
The gain of the amplifier is set by the external resistors, Ri in
Figure 1. The gain is given by Equation (3) below. Best THD
+N performance is achieved with a gain of 2V/V (6dB).
AV = 2 * 150kΩ / Ri (V/V)
(3)
It is recommended that resistors with 1% tolerance or better
be used to set the gain of the amplifier. The Ri resistors should
be placed close to the input pins of the amplifier. Keeping the
input traces close to each other and of the same length in a
high noise environment will aid in noise rejection due to the
good CMRR of the Class D. Noise coupled onto input traces
which are physically close to each other will be common mode
and easily rejected by the amplifier.
Input capacitors may be needed for some applications or
when the source is single-ended (see Figure1). Input capac-
itors are needed to block any DC voltage at the source so that
the DC voltage seen between the input terminals of the Class
D is 0V. Input capacitors create a high-pass filter with the input
resistors, Ri. The –3dB point of the high-pass filter is found
using Equation (4) below.
fC = 1 / (2πRi Ci ) (Hz)
(4)
The input capacitors may also be used to remove low audio
frequencies. Small speakers cannot reproduce low bass fre-
quencies so filtering may be desired . When the Class D is
using a single-ended source, power supply noise on the
ground is seen as an input signal by the +IN input pin that is
capacitor coupled to ground. Setting the high-pass filter point
above the power supply noise frequencies, 217Hz in a GSM
phone, for example, will filter out this noise so it is not ampli-
fied and heard on the output. Capacitors with a tolerance of
10% or better are recommended for impedance matching.
POWER SUPPLY BYPASSING FOR AMPLIFIER
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor (Cs2, see Figure 1) location on both PV1 and V1 pin
should be as close to the device as possible.
SELECTING INPUT CAPACITOR FOR AUDIO AMPLIFIER
One of the major considerations is the closedloop bandwidth
of the amplifier. To a large extent, the bandwidth is dictated
by the choice of external components shown in Figure 1. The
input coupling capacitor, Ci, forms a first order high pass filter
which limits low frequency response. This value should be
chosen based on needed frequency response for a few dis-
tinct reasons.
High value input capacitors are both expensive and space
hungry in portable designs. Clearly, a certain value capacitor
is needed to couple in low frequencies without severe atten-
uation. But ceramic speakers used in portable systems,
whether internal or external, have little ability to reproduce
signals below 100Hz to 150Hz. Thus, using a high value input
capacitor may not increase actual system performance.
In addition to system cost and size, click and pop performance
is affected by the value of the input coupling capacitor, Ci. A
high value input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 VDD). This
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the ca-
pacitor value based on desired low frequency response, turn-
on pops can be minimized.
SELECTING OUTPUT CAPACITOR (CO) FOR BOOST
CONVERTER
A single 4.7µF to 10µF ceramic capacitor will provide suffi-
cient output capacitance for most applications. If larger
amounts of capacitance are desired for improved line support
and transient response, tantalum capacitors can be used.
Aluminum electrolytics with ultra low ESR such as Sanyo Os-
con can be used, but are usually prohibitively expensive.
Typical electrolytic capacitors are not suitable for switching
frequencies above 500 kHz because of significant ringing and
temperature rise due to self-heating from ripple current. An
output capacitor with excessive ESR can also reduce phase
margin and cause instability.
In general, if electrolytics are used, it is recommended that
they be paralleled with ceramic capacitors to reduce ringing,
switching losses, and output voltage ripple.
SELECTING INPUT CAPACITOR (Cs1) FOR BOOST
CONVERTER
An input 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 is the best choice. A nominal value of 4.7µF
is recommended, but larger values can be used. Since this
capacitor reduces the amount of voltage ripple seen at the
input pin, it also reduces the amount of EMI passed back
along that line to other circuitry.
SETTING THE OUTPUT VOLTAGE (V1) OF BOOST
CONVERTER
The output voltage is set using the external resistors R1 and
R2 (see Figure 1). A value of approximately 13.3kΩ is rec-
ommended for R2 to establish a divider current of approxi-
mately 92µA. R1 is calculated using the formula:
R1 = R2 X (V1/1.23 − 1)
(5)
FEED-FORWARD COMPENSATION FOR BOOST
CONVERTER
Although the LM48510's internal Boost converter is internally
compensated, the external feed-forward capacitor Cf1 is re-
quired for stability (see Figure 1). Adding this capacitor puts
a zero in the loop response of the converter. The recom-
mended frequency for the zero fz should be approximately
6kHz. Cf1 can be calculated using the formula:
Cf1 = 1 / (2 X R1 X fz)
(6)
SELECTING DIODES FOR BOOST
The external diode used in Figure 1 should be a Schottky
diode. A 20V diode such as the MBR0520 is recommended.
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| PDF Descargar | [ Datasheet LM48510.PDF ] | |
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