LM48555TLX/NOPB National Semiconductor, LM48555TLX/NOPB Datasheet - Page 8

IC AMP AUDIO PWR MONO AB 12USMD

LM48555TLX/NOPB

Manufacturer Part Number
LM48555TLX/NOPB
Description
IC AMP AUDIO PWR MONO AB 12USMD
Manufacturer
National Semiconductor
Series
Boomer®r
Type
Class ABr
Datasheet

Specifications of LM48555TLX/NOPB

Output Type
1-Channel (Mono)
Voltage - Supply
2.7 V ~ 9 V
Features
Depop, Differential Inputs, Shutdown
Mounting Type
Surface Mount
Package / Case
12-MicroSMD
For Use With
LM48555TLBD - BOARD EVALUATION LM48555TL
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Max Output Power X Channels @ Load
-
Other names
LM48555TLX
www.national.com
Application Information
CHARACTERISTICS OF CERAMIC SPEAKERS
Because of their ultra-thin profile piezoelectric ceramic speak-
ers are ideal for portable applications. Piezoelectric materials
have high dielectric constants and their component electrical
property is like a capacitor. Therefore, piezoelectric ceramic
speakers essentially represent capacitive loads over frequen-
cy. Because these speakers are capacitive rather than resis-
tive, they require less current than traditional moving coil
speakers. However, ceramic speakers require high driving
voltages (approximately 15V
put voltages in battery operated applications, the LM48555
integrates a boost converter with an audio amplifier. High
quality piezoelectric ceramic speakers are manufactured by
TayioYuden
(www.murata.com). Tayio Yuden's MLS-A Series Ceramic
Speaker and Murata's piezoelectric speaker VSL series are
recommended.
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM48555 includes a fully differential audio amplifier that
features differential input and output stages. Internally this is
accomplished by two circuits: a differential amplifier and a
common mode feedback amplifier that adjusts the output volt-
ages so that the average value remains V
the differential gain, the amplifier can be considered to have
"halves". Each half uses an input and feedback resistor (RIN_
and RF_) to set its respective closed-loop gain (see Figure 1).
With RIN+ = RIN- and RF+ = RF-, the gain is set at -RF/RIN
for each half. This results in a differential gain of
It is extremely important to match the input resistors, as well
as the feedback resistors to each other for best amplifier per-
formance. A differential amplifier works in a manner where the
difference between the two input signals is amplified. In most
applications, this would require input signals that are 180° out
of phase with each other. The LM48555 can be used, how-
ever, as a single-ended input amplifier while still retaining its
fully differential benefits. In fact, completely unrelated signals
may be placed at the input pins. The LM48555 simply ampli-
fies the difference between them.
The LM48555 provides what is known as a "bridged mode"
output (bridge-tied-load, BTL). This results in output signals
at OUT+ and OUT- that are 180° out of phase with respect to
each other. Bridged mode operation is different from the tra-
ditional single-ended amplifier configuration that connects the
load between the amplifier output and ground. A bridged am-
plifier design has advantages over the single-ended configu-
ration: it provides differential drive to the load, thus doubling
maximum possible output swing for a specific supply voltage.
Up to four times the output power is possible compared with
a single-ended amplifier under the same conditions.
A bridged configuration, such as the one used in the
LM48555, also creates a second advantage over single-end-
ed amplifiers. Since the differential outputs, OUT+ and OUT-,
are biased at half-supply, no net DC voltage exists across the
load. This assumes that the input resistor pair and the feed-
back resistor pair are properly matched. BTL configuration
eliminates the output coupling capacitor required in single
supply, single-ended amplifier configurations. If an output
coupling capacitor is not used in a single-ended output con-
figuration, the half-supply bias across the load would result in
(www.t-yuden.com)
A
VD
= -RF/RIN
P-P
). To achieve these high out-
DD
and
/2. When setting
muRata
(1)
8
both increased internal IC power dissipation as well as per-
manent loudspeaker damage.
BOOST CONVERTER POWER DISSIPATION
At higher duty cycles, the increased ON-time of the switch
FET means the maximum output current will be determined
by power dissipation within the LM48555 FET switch. The
switch power dissipation from ON-time conduction is calcu-
lated by Equation 2.
where DC is the duty cycle.
There will be some switching losses in addition to the power
loss calculated in Eqaution 3, so some derating needs to be
applied when calculating IC power dissipation. See “Maxi-
mum Power Dissipation” section.
MAXIMUM AMPLIFIER POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful amplifier, whether the amplifier is bridged or single-
ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the amplifier portion of the
LM48555 has two operational amplifiers, the maximum inter-
nal power dissipation is 4 times that of a single-ended ampli-
fier. The maximum power dissipation for a given BTL
application can be derived from Equation 3.
MAXIMUM TOTAL POWER DISSIPATION
The total power dissipation for the LM48555 can be calculated
by adding Equation 2 and Equation 3 together to establish
Equation 4:
where
EFF = Efficiency of boost converter
The result from Equation 4 must not be greater than the power
dissipation that results from Equation 5:
For the TLA12Z1A, θ
LM48555. Depending on the ambient temperature, T
system surroundings, Equation 5 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 4 is greater than that of
Equation 5, then either the supply voltage must be decreased,
the load impedance increased or T
plications, power dissipation is not an issue. Power dissipa-
tion is a function of output power and thus, if typical operation
is not around the maximum power dissipation point, the am-
bient temperature may be increased accordingly.
STARTUP SEQUENCE
Correct startup sequencing is important for optimal device
performance. Using the correct startup sequence will improve
click and pop performance as well as avoid transients that
could reduce battery life. The device should be in Shutdown
mode when the supply voltage is applied. Once the supply
voltage has been supplied the device can be released from
Shutdown mode.
SHUTDOWN FUNCTION
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
P
D(SWITCH)
P
DMAX(TOTAL)
P
DMAX(AMP)
P
= DC x (I
DMAX
= (T
= (2V
= (2V
JA
INDUCTOR(AVE)
JMAX
= 114°C/W. T
DD
DD
2
- T
) / (π
2
) / (π
A
) / θ
2
EFF
A
2
)
RO) (W)
JA
2
reduced. For typical ap-
x R
2
(W)
JMAX
RO) (W)
DS(ON)
= 150°C for the
(W)
A
, of the
(2)
(3)
(4)
(5)

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