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LM4962 bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
tên linh kiện LM4962
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Application Information
BRIDGE CONFIGURATION EXPLANATION
The Audio Amplifier portion of the LM4962 has two internal
amplifiers allowing different amplifier configurations. The first
amplifier’s gain is externally configurable, whereas the sec-
ond amplifier is internally fixed in a unity-gain, inverting
configuration. The closed-loop gain of the first amplifier is set
by selecting the ratio of Rf to Ri while the second amplifier’s
gain is fixed by the two internal 20k
Ω resistors. Figure 1
shows that the output of amplifier one serves as the input to
amplifier two. This results in both amplifiers producing sig-
nals identical in magnitude, but out of phase by 180˚. Con-
sequently, the differential gain for the Audio Amplifier is
A
VD = 2 *(Rf/Ri)
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classic single-ended amplifier configura-
tion where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration. It provides differential drive
to the load, thus doubling the output swing for a specified
supply voltage.
AMPLIFIER POWER DISSIPATION
Power dissipation is a major concern when designing a
successful 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
LM4962 has two operational amplifiers, the maximum inter-
nal power dissipation is 4 times that of a single-ended am-
plifier. The maximum power dissipation for a given BTL
application can be derived from Equation 1.
P
DMAX(AMP) = 4(VDD)
2 /(2
π2Z
L)
(1)
where
Z
L =Ro1+Ro2 +1/2
πfc
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 LM4962 FET switch. The
switch power dissipation from ON-time conduction is calcu-
lated by Equation 2.
P
DMAX(SWITCH) =DCxIIND(AVE)
2 xR
DS(ON)
(2)
where DC is the duty cycle.
There will be some switching losses as well, so some derat-
ing needs to be applied when calculating IC power dissipa-
tion.
TOTAL POWER DISSIPATION
The total power dissipation for the LM4962 can be calculated
by adding Equation 1 and Equation 2 together to establish
Equation 3:
P
DMAX(TOTAL) = [4*(VDD)
2/
2
π2Z
L]+[DCxIIND(AVE)
2xR
DS(ON)]
(3)
The result from Equation 3 must not be greater than the
power dissipation that results from Equation 4:
P
DMAX =(TJMAX -TA)/
θ
JA
(4)
For the LQA28A,
θ
JA = 73˚C/W. TJMAX = 125˚C for the
LM4962. Depending on the ambient temperature, T
A,ofthe
system surroundings, Equation 4 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 3 is greater than that of
Equation 4, then either the supply voltage must be in-
creased, the load impedance increased or T
A reduced. For
typical applications, power dissipation is not an issue. Power
dissipation is a function of output power and thus, if typical
operation is not around the maximum power dissipation
point, the ambient temperature may be increased accord-
ingly.
START-UP SEQUENCE
For the LM4962 correct start-up sequencing is important for
optimal device performance. Using the correct start up se-
quence will improve click/pop performance as well as avoid
transients that could reduce battery life. For ringer/
loudspeaker mode, the supply voltage should be applied first
and both the boost converter and the amplifier should be in
shutdown. The boost converter can then be activated fol-
lowed by the amplifier (see timing diagram Figure 3). If the
boost converter shutdown is toggled while the amplifier is
active a very audible pop will be heard.
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 connected between
V
DD and Shutdown pins.
BAND SWITCH FUNCTION
The LM4962 features a Band Switch function which allows
the user to use one amplifier for both receiver (earpiece)
mode and ringer/loudspeaker mode. When the boost con-
verter and the amplifier are both active the device is is in
ringer mode. This enables the boost converter and sets the
externally configurable closed loop gain selection to BW1. If
the boost converter is in the shutdown and the amplifier is
active the device is in receiver mode. In this mode the gain
selection is switched to BW2. This allows the amplifier to be
powered directly from the battery minus the voltage drop
across the Schottky diode.
SD Boost
SD Amp
Receiver Mode (BW2)
Low
High
Boosted Ringer Mode
(BW1)
High
High
Shutdown
Low
Low
BOOTSTRAP PIN
The bootstrap pin, featured in the LM4962, provides a volt-
age supply for the internal switch driver. Connecting the
bootstrap pin to V1 (See Figure 1) allows for a higher voltage
to drive the gate of the switch thereby reducing the Ron. This
configuration is necessary in applications with heavier loads.
The bootstrap pin can be connected to VDD when driving
lighter loads to improve device performance (Iddq, THD+N,
Noise, etc.).
www.national.com
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