LM4870 Datasheet, PDF (5/8 Pages) National Semiconductor (TI) – 1.1W Audio Power Amplifier with Shutdown Mode

Application Information (Continued)
ing excessive clipping which will damage high frequency
transducers used in loudspeaker systems, please refer to
the Audio Power Amplifier Deslgn section.
A bridge configuration, such as the one used in Boomer Au-
dio Power Amplifiers, also creates a second advantage over
single-ended amplifiers. Since the differential outputs, V_O1
and V_O2, are biased at half-supply, no net DC voltage exists
across the load. This eliminates the need for an output cou-
pling capacitor which is required in a single supply,
single-ended amplifier configuration. Without a large output
coupling capacitor in a single supply single-ended amplifier,
the half-supply bias across the load would result in both in-
creased internal IC power dissipation.
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. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.
P_DMAX = 4 * (V_DD) ²/(2π²R_L)
(1)
Since the LM4870 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended amplifier. Even with this substantial in-
crease in power dissipation, the LM4870 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8Ω load, the maximum power dissipation point is 625
mW. The maximum power dissipation point obtained from
Equation 1 must not be greater than the power dissipation
that results from Equation 2:
P_DMAX = (T_JMAX − T _A)/θ_JA
(2)
For the LM4870 surface mount package, θ_JA = 100˚C/W and
T_JMAX = 150˚C. Depending on the ambient temperature, T_A,
of the system surroundings, Equation 2 can be used to find
the maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 is greater than that of
Equation 2, then either the supply voltage must be de-
creased or the load impedance increased. For the typical ap-
plication of a 5V power supply, with an 8Ω load, the maxi-
mum ambient temperature possible without violating the
maximum junction temperature is approximately 88˚C, pro-
vided that device operation is around the maximum power
dissipation point. 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 can
be increased. Refer to the Typical Performance Character-
istics curves for power dissipation information for lower out-
put powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. As
displayed in the Typical Performance CharacterIstIcs sec-
tion, the effect of a larger half-supply bypass capacitor is im-
proved low frequency THD+N due to increased half-supply
stability. Typical applications employ a 5V regulator with
a10 µF tantalum and a 0.1 µF film bypass capacitors which
aid in supply stability, but do not eliminate the need for by-
passing the supply nodes of the LM4870. The selection of
bypass capacitors, especially C_B, is thus dependant upon
desired low frequency THD+N, system cost, and size con-
straints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4870 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. The shutdown feature turns the am-
plifier off when a logic high is placed on the shutdown pin.
Upon going into shutdown, the output is immediately discon-
nected from the speaker. There is a built-in threshold which
produces a drop in quiescent current to 500 µA typically. For
a 5V power supply, this threshold occurs when 2V–3V is ap-
plied to the shutdown pin. A typical quiescent current of
0.6 µA results when the supply voltage is applied to the shut-
down pin. In many applications, a microcontroller or micro-
processor output is used to control the shutdown circuitry
which provides a quick, smooth transition into shutdown. An-
other solution is to use a single-pole, single-throw switch that
when closed, is connected to ground and enables the ampli-
fier. If the switch is open, then a soft pull-up resistor of 47 kΩ
will disable the LM4870. There are no soft pull-down resis-
tors inside the LM4870, so a definite shutdown pin voltage
must be applied externally, or the internal logic gate will be
left floating which could disable the amplifier unexpectedly.
HEADPHONE CONTROL INPUTS
The LM4870 possesses two headphone control inputs that
disable the amplifier and reduce I_DD to less than 1 mA when
either one or both of these inputs have a logic-high voltage
placed on their pins.
Unlike the shutdown function, the headphone control func-
tion does not provide the level of current conservation that is
required for battery powered systems. Since the quiescent
current resulting from the headphone control function is
1000 times more than the shutdown function, the residual
currents in the device may create a pop at the output when
coming out of the headphone control mode. The pop effect
may be eliminated by connecting the headphone sensing
output to the shutdown pin input as shown in Figure 2. This
solution will not only eliminate the output pop, but will also
utilize the full current conservation of the shutdown function
by reducing I_DD to 0.6 µA. The amplifier will then be fully
shutdown. This configuration also allows the designer to use
the control inputs as either two headphone control pins or a
headphone control pin and a shutdown pin where the lowest
level of current consumption is obtained from either function.
Figure 3 shows the implementation of the LM4870’s head-
phone control function using a single-supply headphone am-
plifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN1 pin to be approximately 50 mV when there are
no headphones plugged into the system. This logic-low volt-
age at the HP-IN1 pin enables the LM4870 to amplify AC sig-
nals. Resistor R3 limits the amount of current flowing out of
the HP-IN1 pin when the voltage at that pin goes below
ground resulting from the music coming from the headphone
amplifier. The output coupling cap protects the headphones
by blocking the amplifier’s half-supply DC voltage. The ca-
pacitor also protects the headphone amplifier from the low
voltage set up by resistors R1 and R2 when there aren’t any
headphones plugged into the system. The tricky point to this
setup is that the AC output voltage of the headphone ampli-
fier cannot exceed the 2.0V HP-IN1 voltage threshold when
there aren’t any headphones plugged into the system, as-
suming that R1 and R2 are 100k and 1k, respectively. The
LM4870 may not be fully shutdown when this level is ex-
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