lm4919 National Semiconductor Corporation, lm4919 Datasheet - Page 12

• Manufacturer Part Number: lm4919
• Description: 1.5v, Mono 85mw Btl Output, 14mw Stereo Headphone Audio Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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 LM4919 has two opera-
tional amplifiers in one package, 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 the power dissipation graphs or from
Equation 1.
When operating in Single Ended mode, Equation 2 states
the maximum power dissipation point for a single-ended
amplifier operating at a given supply voltage and driving a
specified output load.
Since the LM4919 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number that results from Equation 2. From
Equation 2, assuming a 1.5V power supply and a 16Ω load,
the maximum power dissipation point is 7mW per amplifier.
Thus the maximum package dissipation point is 14mW.
The maximum power dissipation point obtained from either
Equations 1, 2 must not be greater than the power dissipa-
tion that results from Equation 3:
For package MUB10A, θ
the LM4919. Depending on the ambient temperature, T
the system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 or 2 is greater than that
of Equation 3, then either the supply voltage must be de-
creased, the load impedance increased or T
the typical application of a 1.5V power supply, with a 16Ω
load, the maximum ambient temperature possible without
violating the maximum junction temperature is approximately
146˚C provided that device operation is around the maxi-
mum power dissipation point. Thus, 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 accordingly. Refer to the
Typical Performance Characteristics curves for power dissi-
pation information for lower output powers.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be
as close to the device as possible. Typical applications em-
ploy a battery (or 1.5V regulator) with 10µF tantalum or
electrolytic capacitor and a ceramic bypass capacitor that
aid in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4919. A bypass ca-
pacitor value in the range of 0.1µF to 1µF is recommended.
P
P
P
DMAX
DMAX
DMAX
= 4*(V
= (T
= (V
JA
JMAX
DD
= 175˚C/W. T
DD
)
)
2
- T
2
/ (2π
/ (2π
A
) / θ
2
2
R
R
L
JA
(Continued)
L
)
JMAX
)
A
reduced. For
= 150˚C for
A
, of
(1)
(2)
(3)
12
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4919’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4919’s micro-power shutdown fea-
ture turns off the amplifier’s bias circuitry, reducing the sup-
ply current. The trigger point varies depending on supply
voltage and is shown in the Shutdown Hysteresis Voltage
graphs in the Typical Performance Characteristics section.
The low 0.02µA (typ) shutdown current is achieved by ap-
plying a voltage that is as near as ground as possible to the
SHUTDOWN pin. A voltage that is higher than ground may
increase the shutdown current. There are a few ways to
control the micro-power shutdown. These include using a
single-pole, single-throw switch, a microprocessor, or a mi-
crocontroller. When using a switch, connect an external
100kΩ pull-up resistor between the SHUTDOWN pin and
V
ground. Select normal amplifier operation by opening the
switch. Closing the switch connects the SHUTDOWN pin to
ground, activating micro-power shutdown. The switch and
resistor guarantee that the SHUTDOWN pin will not float.
This prevents unwanted state changes. In a system with a
microprocessor or microcontroller, use a digital output to
apply the control voltage to the SHUTDOWN pin. Driving the
SHUTDOWN pin with active circuitry eliminates the pull-up
resistor.
MUTE
When in single ended mode, the LM4919 also features a
mute function that enables extremely fast turn-on/turn-off
with a minimum of output pop and click with a low current
consumption (≤20µA, typical). The mute function leaves the
outputs at their bias level, thus resulting in higher power
consumption than shutdown mode, but also provides much
faster turn on/off times. Providing a logic low signal on the
MUTE pin enables mute mode. Threshold voltages and ac-
tivation techniques match those given for the shutdown func-
tion as well. Mute may not appear to function when the
LM4919 is used to drive high impedance loads. This is
because the LM4919 relies on a typical headphone load
(16-32Ω) to reduce input signal feed-through through the
input and feedback resistors. Mute attenuation can thus be
calculated by the following formula:
Parallel load resistance may be necessary to achieve satis-
factory mute levels when the application load is known to be
high impedance. The mute function, described above, is not
necessary when the LM4919 is operating in BTL mode since
the shutdown function operates quickly in BTL mode with
less power consumption than mute. In these modes, the
Mute signal is equivalent to the Shutdown signal. Mute may
be enabled during shutdown transitions, but should not be
toggled for a brief period immediately after exiting or entering
shutdown. These brief time periods are labeled X1 (time
after returning from shutdown) and X2 (time after entering
shutdown) and are shown in the timing diagram given in
Figure 5. X1 occurs immediately following a return from
shutdown (TWU) and lasts 40ms
part is placed in shutdown and the decay of the bias voltage
has occurred (2.2*250k*CB) and lasts for 100ms
timing of these transition periods relative to X1 and X2 is also
shown in Figure 5. While in single ended mode, mute should
not be toggled during these time periods, but may be toggled
DD
Mute Attenuation (dB) = 20Log[R
. Connect the switch between the SHUTDOWN pin and
±
25%. X2 occurs after the
L
/ (R
i
+R
F
)]
±
25%. The