Datasheet

LM4953, LM4953SDBD
SNAS299E –SEPTEMBER 2005–REVISED FEBRUARY 2006 www.ti.com
P
DMAX
= (T
JMAX
- T
A
) / (θ
JA
) (3)
Depending on the ambient temperature, T
A
, of 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 2 is greater than
that of Equation 3, then either the supply voltage must be decreased, the load impedance increased or T
A
reduced. 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.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. Applications that employ a 3V power supply typically use a 4.7µF capacitor in parallel with a 0.1µF
ceramic filter capacitor to stabilize the power supply's output, reduce noise on the supply line, and improve the
supply's transient response. Keep the length of leads and traces that connect capacitors between the LM4953's
power supply pin and ground as short as possible.
AUTOMATIC STANDBY MODE
The LM4953 features Automatic Standby Mode circuitry (patent pending). In the absence of an input signal, after
approximately 3 seconds, the LM4953 goes into low current standby mode. The LM4953 recovers into full power
operating mode immediately after a signal, which is greater than the input threshold voltage, is applied to either
the left or right input pins. The input threshold voltage is not a static value, as the supply voltage increases, the
input threshold voltage decreases. This feature reduces power supply current consumption in battery operated
applications.
To ensure correct operation of Automatic Standby Mode, proper layout techniques should be implemented.
Separating PGND and SGND can help reduce noise entering the LM4953 in noisy environments. It is also
important to use correct power off sequencing. The device should be in shutdown and then powered off in order
to ensure proper functionality of the Auto-Standby feature. While Automatic Standby Mode reduces power
consumption very effectively during silent periods, maximum power saving is achieved by putting the device into
shutdown when it is not in use.
MICRO POWER SHUTDOWN
The voltage applied to the SD controls the LM4953’s shutdown function. When active, the LM4953’s micropower
shutdown feature turns off the amplifiers’ bias circuitry, reducing the supply current. The trigger point is
0.3*CPV
DD
for a logic-low level, and 0.7*CPV
DD
for logic-high level. The low 0.01µA (typ) shutdown current is
achieved by applying a voltage that is as near as ground a possible to the SD pins. 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 microcontroller. When using a switch, connect an external 100kΩ pull-up resistor
between the SD pins and V
DD
. Connect the switch between the SD pins and ground. Select normal amplifier
operation by opening the switch. Closing the switch connects the SD pins to ground, activating micro-power
shutdown. The switch and resistor ensure that the SD pins 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 SD
pins. Driving the SD pins with active circuitry eliminates the pull-up resistor.
EXPOSED-DAP CONSIDERATIONS
It is essential that the exposed Die Attach Paddle (DAP), for the LM4953, is NOT connected to GND. For optimal
operation it should be connected to AVss and VCP-OUT (Pins 6 and 8).
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4953's performance requires properly selecting external components. Though the LM4953
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
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