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PDF LM4802BLQ Data sheet ( Hoja de datos )
| Número de pieza | LM4802BLQ | |
| Descripción | Audio Power Amplifier with Boost Converter to Drive Ceramic Speakers | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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November 2004
LM4802B
Audio Power Amplifier with Boost Converter to Drive
Ceramic Speakers
General Description
The LM4802B integrates a Boost Converter with an Audio
Power Amplifier to drive Ceramic Speakers in portable ap-
plications. When powered by a 3V supply, it is capable of
forcing 12Vp-p across a 2µF + 30Ω bridge-tied-load (BTL)
with less than 1% THD+N.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4802B does not require boot-
strap capacitors, or snubber circuits. Therefore it is ideally
suited for portable applications requiring high output voltage
and minimal size.
The LM4802B features a low-power consumption shutdown
mode. Additionally, the LM4802B features an internal ther-
mal shutdown protection mechanism.
The LM4802B contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.
The LM4802B is unity-gain stable. Its gain is set by external
resistors.
Key Specifications
j Quiescent Power Supply Current
j BTL Voltage Swing
(2µF + 30Ω load, 1% THD+N,
VDD = 3V)
j Shutdown Current
12mA (typ)
12Vp-p (typ)
2µA (max)
Features
n Pop & click circuitry eliminates noise during turn-on and
turn-off transitions
n Low, 2µA (max) shutdown current
n Low, 12mA (typ) quiescent current
n 12Vp-p mono BTL output, load = 2µF + 30Ω, VDD = 3V
n Short circuit protection
n Unity-gain stable
n External gain configuration capability
Applications
n Cellphone
n PDA
Connection Diagram
LM4802BLQ (5x5)
Top View
Order Number LM4802BLQ
See NS Package Number 2LQAQ028
20094184
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation DS200941
www.national.com
1 page  
Typical Performance Characteristics
THD+N vs Frequency
VDD = 3V, ZL = 2µF + 30Ω, VOUT = 10VP-P
THD+N vs Frequency
VDD = 4.2V, ZL = 2µF + 30Ω, VOUT = 10VP-P
20094187
THD+N vs Output Voltage
VDD = 3V, ZL = 2µF + 30Ω, V1 = 5.0V
20094196
THD+N vs Output Voltage
VDD = 4.2V, ZL = 2µF + 30Ω, V1 = 5.0V
20094192
THD+N vs Output Voltage
VDD = 3V, ZL = 2µF + 30Ω, V1 = 5.8V
20094195
THD+N vs Output Voltage
VDD = 4.2V, ZL = 2µF + 30Ω, V1 = 5.8V
20094191
5
20094193
www.national.com
5 Page 
Application Information (Continued)
SHUTDOWN FUNCTION
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch, and a pull-up resistor.
One terminal of the switch is connected to GND. The other
side is connected to the two shutdown pins and the terminal
of the pull-up resistor. The remaining resistance terminal is
connected to VDD. If the switch is open, then the external
pull-up resistor connected to VDD will enable the LM4802.
This scheme guarantees that the shutdown pins will not float
thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers, and switching DC-DC con-
verters, is critical for optimizing device and system perfor-
mance. Consideration to component values must be used to
maximize overall system quality.
The best capacitors for use with the switching converter
portion of the LM4802 are multi-layer ceramic capacitors.
They have the lowest ESR (equivalent series resistance)
and highest resonance frequency, which makes them opti-
mum for high frequency switching converters.
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 pro-
vide as little as 20% of rated capacitance in many typical
applications. Always consult capacitor manufacturer’s data
curves before selecting a capacitor. High-quality ceramic
capacitors can be obtained from Taiyo-Yuden, AVX, 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 V1 and VDD pins 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
distinct 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 at-
tenuation. 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 perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is affected by the value of the input coupling capaci-
tor, 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 capacitor value based on desired low fre-
quency response, turn-on pops can be minimized.
SELECTING BYPASS CAPACITOR FOR AUDIO
AMPLIFIER
Besides minimizing the input capacitor value, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, CB, is the most critical component to minimize
turn-on pops since it determines how fast the amplifer turns
on. The slower the amplifier’s outputs ramp to their quies-
cent DC voltage (nominally 1/2 VDD), the smaller the turn-on
pop. Choosing CB equal to 1.0µF along with a small value of
Ci (in the range of 0.039µF to 0.39µF), should produce a
virtually clickless and popless shutdown function. Although
the device will function properly, (no oscillations or motor-
boating), with CB equal to 0.1µF, the device will be much
more susceptible to turn-on clicks and pops. Thus, a value of
CB equal to 1.0µF is recommended in all but the most cost
sensitive designs.
SELECTING FEEDBACK CAPACITOR FOR AUDIO
AMPLIFIER
The LM4802B is unity-gain stable which gives the designer
maximum system flexability. However, to drive ceramic
speakers, a typical application requires a closed-loop differ-
ential gain of 10. In this case a feedback capacitor (Cf2) will
be needed as shown in Figure 2 to bandwidth limit the
amplifier.
This feedback capacitor creates a low pass filter that elimi-
nates possible high frequency oscillations. Care should be
taken when calculating the -3dB frequency because an in-
correct combination of Rf and Cf2 will cause rolloff before the
desired frequency
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 sup-
port and transient response, tantalum capacitors can be
used. Aluminum electrolytics with ultra low ESR such as
Sanyo Oscon can be used, but are usually prohibitively
expensive. Typical AI electrolytic capacitors are not suitable
for switching frequencies above 500 kHz because of signifi-
cant 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, we 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. We recommend a
nominal value of 4.7µF, 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.
11 www.national.com
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