LM4961LQBD/NOPB National Semiconductor, LM4961LQBD/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM4961LQBD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4961LQBD/NOPB

• Lead Free Status / Rohs Status: Compliant

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Application Information
BRIDGE CONFIGURATION EXPLANATION
The Audio Amplifier portion of the LM4961 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 con-
figuration. 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 20kΩ resistors. Figure 1 shows that
the output of amplifier one serves as the input to amplifier two.
This results in both amplifiers producing signals identical in
magnitude, but out of phase by 180°. Consequently, the dif-
ferential gain for the Audio Amplifier is
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 configuration 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. Four times the output power is possible as compared
to a single-ended amplifier under the same conditions.
The bridge configuration also creates a second advantage
over single-ended amplifiers. Since the differential outputs,
Vo1 and Vo2, are biased at half-supply, no net DC voltage
exists across the load. This eliminates the need for an output
coupling capacitor which is required in a single supply, single-
ended amplifier configuration. Without an output coupling
capacitor, the half-supply bias across the load would result in
both increased internal IC power dissipation and also possible
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 LM4961 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 as well, so some derating
needs to be applied when calculating IC power dissipation.
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
LM4961 has two operational amplifiers, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given BTL application
can be derived from Equation 1.
where
P
D(SWITCH)
P
DMAX(AMP)
= DC x I
A
VD
= (2V
IND
= 2 *(Rf/Ri)
(AVE)
DD
2
) / (π
2
x R
2
R
DS
L
)
(ON)
(1)
(2)
8
MAXIMUM TOTAL POWER DISSIPATION
The total power dissipation for the LM4961 can be calculated
by adding Equation 1 and Equation 2 together to establish
Equation 3:
where
The result from Equation 3 must not be greater than the power
dissipation that results from Equation 4:
For the LQA28A, θ
LM4961. Depending on the ambient temperature, T
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 increased,
the load impedance increased or T
application of a 4.2V power supply, with a 2uF+30Ω load, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 109°C pro-
vided that device operation is around the maximum power
dissipation point. Thus, for typical applications, power dissi-
pation is not an issue. Power dissipation is a function of output
power and thus, if typical operation is not around the maxi-
mum power dissipation point, the ambient temperature may
be increased accordingly. Refer to the Typical Performance
Characteristics curves for power dissipation information for
lower output levels.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4961’s exposed-DAP (die attach paddle) package
(LD) provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. The low
thermal resistance allows rapid heat transfer from the die to
the surrounding PCB copper traces, ground plane, and sur-
rounding air. The LD package should have its DAP soldered
to a copper pad on the PCB. The DAP’s PCB copper pad may
be connected to a large plane of continuous unbroken copper.
This plane forms a thermal mass, heat sink, and radiation
area. Further detailed and specific information concerning
PCB layout, fabrication, and mounting an LD (LLP) package
is found in National Semiconductor’s Package Engineering
Group under application note AN1187.
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
BAND SWITCH FUNCTION
The LM4961 features a Band Switch function which allows
the user to use one amplifier for both receiver (earpiece)
mode and ringer/loudspeaker mode. When a logic high
DD
EFF = Efficiency of boost converter
R
L
and Shutdown pins.
= R
P
o
1 + R
DMAX(TOTAL)
P
o
DMAX
2
= (T
JA
= (2V
R
= 66°C/W. T
L
JMAX
= R
DD
o
- T
2
1 + R
) / (π
A
) / θJA
A
o
2
2
reduced. For the typical
EFF
JMAX
2
R
= 125°C for the
L
)
A
, of the
(3)
(4)