Datasheet

LM4950
SNAS174E –JULY 2003–REVISED MAY 2013 www.ti.com
the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier's PSRR.
The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases
turn-on time and can compromise the amplifier's click and pop performance. The selection of bypass capacitor
values, especially C
BYPASS
, depends on desired PSRR requirements, click and pop performance (as explained in
SELECTING EXTERNAL COMPONENTS), system cost, and size constraints.
MICRO-POWER SHUTDOWN
The LM4950 features an active-low micro-power shutdown mode. When active, the LM4950's micro-power
shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The low 40µA typical
shutdown current is achieved by applying a voltage to the SHUTDOWN pin that is as near to GND as possible. A
voltage that is greater than GND may increase the shutdown current.
There are a few methods to control the micro-power shutdown. These include using a single-pole, single-throw
switch (SPST), a microprocessor, or a microcontroller. When using a switch, connect a 100kΩ pull-up resistor
between the SHUTDOWN pin and V
DD
and a second 100kΩ resistor in parallel with the SPST switch connected
between the SHUTDOWN pin and GND. The two resistors form a voltage divider that ensures that the voltage
applied to the SHUTDOWN pin does not exceed V
DD
/2. Select normal amplifier operation by opening the switch.
Closing the switch applies GND to the SHUTDOWN pin, activating micro-power shutdown. The switch and
resistor ensure that the SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a
microprocessor or a microcontroller, use a digital output to apply the active-state voltage to the SHUTDOWN pin.
Again, ensure that the microcontroller or microprocessor logic-high signal does not exceed the LM4950's V
DD
/2
SHUTDOWN signal limit.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Two quantities determine the value of the input coupling capacitor: the lowest audio frequency that requires
amplification and desired output transient suppression.
As shown in Figure 65, the input resistor (R
IN
) and the input capacitor (C
IN
) produce a high pass filter cutoff
frequency that is found using Equation 7.
f
c
= 1/2πR
i
C
i
(7)
As an example when using a speaker with a low frequency limit of 50Hz, C
i
, using Equation 7 is 0.159µF. The
0.39µF C
INA
shown in Figure 65 allows the LM4950 to drive high efficiency, full range speaker whose response
extends below 30Hz.
Bypass Capacitor Value
Besides minimizing the input capacitor size, careful consideration should be paid to value of C
BYPASS
, the
capacitor connected to the BYPASS pin. Since C
BYPASS
determines how fast the LM4950 settles to quiescent
operation, its value is critical when minimizing turn-on pops. The slower the LM4950's outputs ramp to their
quiescent DC voltage (nominally V
DD
/2), the smaller the turn-on pop. Choosing C
BYPASS
equal to 10µF along with
a small value of C
IN
(in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As
discussed above, choosing C
IN
no larger than necessary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4950 contains circuitry that eliminates turn-on and shutdown transients ("clicks and pops"). For this
discussion, turn-on refers to either applying the power supply voltage or when the micro-power shutdown mode
is deactivated.
As the V
DD
/2 voltage present at the BYPASS pin ramps to its final value, the LM4950's internal amplifiers are
configured as unity gain buffers and are disconnected from the AMP
A
and AMP
B
pins. An internal current source
charges the capacitor connected between the BYPASS pin and GND in a controlled manner. Ideally, the input
and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until
the voltage applied to the BYPASS pin.
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