Application Information
MUTE FUNCTION
The thermal resistance from the die to the outside air, θJA
(junction to ambient), is a combination of three thermal re-
sistances, θJC (junction to case), θCS (case to sink), and θSA
(sink to ambient). The thermal resistance, θJC (junction to
case), of the LM4702T is 0.8˚C/W. Using Thermalloy Ther-
macote thermal compound, the thermal resistance, θCS
(case to sink), is about 0.2˚C/W. Since convection heat flow
(power dissipation) is analogous to current flow, thermal
resistance is analogous to electrical resistance, and tem-
perature drops are analogous to voltage drops, the power
dissipation out of the LM4702 is equal to the following:
The mute function of the LM4702 is controlled by the amount
of current that flows into the mute pin. If there is less than
1mA of current flowing into the mute pin, the part will be in
mute. This can be achieved by shorting the mute pin to
ground or by floating the mute pin. If there is between 1mA
and 2mA of current flowing into the mute pin, the part will be
in “play” mode. This can be done by connecting a power
supply (Vmute) to the mute pin through a resistor (Rm). The
current into the mute pin can be determined by the equation
Imute = (Vmute – 2.9) / Rm. For example, if a 5V power
supply is connected through a 1.4k resistor to the mute pin,
then the mute current will be 1.5mA, at the center of the
specified range. It is also possible to use Vcc as the power
supply for the mute pin, though Rm will have to be recalcu-
lated accordingly. It is not recommended to flow more than
2mA of current into the mute pin because damage to the
LM4702 may occur.
PDMAX = (TJMAX−TAMB) / θJA
(1)
where TJMAX = 150˚C, TAMB is the system ambient tempera-
ture and θJA = θJC + θCS + θSA
.
It is highly recommended to switch between mute and “play”
modes rapidly. This is accomplished most easily through
using a toggle switch that alternatively connects the mute pin
through a resistor to either ground or the mute pin power
supply. Slowly increasing the mute current may result in
undesired voltages on the outputs of the LM4702, which can
damage an attached speaker.
20158355
Once the maximum package power dissipation has been
calculated using equation 2, the maximum thermal resis-
tance, θSA, (heat sink to ambient) in ˚C/W for a heat sink can
be calculated. This calculation is made using equation 4
which is derived by solving for θSA in equation 3.
THERMAL PROTECTION
The LM4702 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die exceeds 150˚C, the LM4702 shuts
down. It starts operating again when the die temperature
drops to about 145˚C, but if the temperature again begins to
rise, shutdown will occur again above 150˚C. Therefore, the
device is allowed to heat up to a relatively high temperature
if the fault condition is temporary, but a sustained fault will
cause the device to cycle in a Schmitt Trigger fashion be-
tween the thermal shutdown temperature limits of 150˚C and
145˚C. This greatly reduces the stress imposed on the IC by
thermal cycling, which in turn improves its reliability under
sustained fault conditions.
θSA = [(TJMAX−TAMB)−PDMAX(θJC +θCS)] / PDMAX (2)
Again it must be noted that the value of θSA is dependent
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25˚C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet
the design targets of an application. The choice of external
component values that will affect gain and low frequency
response are discussed below.
Since the die temperature is directly dependent upon the
heat sink used, the heat sink should be chosen so that
thermal shutdown is not activated during normal operation.
Using the best heat sink possible within the cost and space
constraints of the system will improve the long-term reliability
of any power semiconductor device, as discussed in the
Determining the Correct Heat Sink section.
The gain of each amplifier is set by resistors Rf and Ri for the
non-inverting configuration shown in Figure 1. The gain is
found by Equation (3) below:
AV = 1 + Rf / Ri (V/V)
(3)
POWER DISSIPATION AND HEAT SINKING
For best noise performance, lower values of resistors are
used. A value of 1kΩ is commonly used for Ri and then
setting the value of Rf for the desired gain. For the LM4702
the gain should be set no lower than 26dB. Gain settings
below 26dB may experience instability.
When in “play” mode, the LM4702 draws a constant amount
of current, regardless of the input signal amplitude. Conse-
quently, the power dissipation is constant for a given supply
voltage and can be computed with the equation PDMAX = Icc
* (Vcc – Vee). For a quick calculation of PDMAX, approximate
the current to be 25mA and multiply it by the total supply
voltage (the current varies slightly from this value over the
operating range).
The combination of Ri with Ci (see Figure 1) creates a high
pass filter. The low frequency response is determined by
these two components. The -3dB point can be found from
Equation (4) shown below:
fi = 1 / (2πRiCi) (Hz)
(4)
DETERMINING THE CORRECT HEAT SINK
If an input coupling capacitor is used to block DC from the
inputs as shown in Figure 5, there will be another high pass
filter created with the combination of CIN and RIN. When
using a input coupling capacitor RIN is needed to set the DC
The choice of a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature at a level such
that the thermal protection circuitry is not activated under
normal circumstances.
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