Datasheet

LM4951A
www.ti.com SNAS453C –AUGUST 2008–REVISED APRIL 2013
If the result of Equation 2 is greater than that of Equation 3, then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. Further, ensure that speakers rated at a nominal 8Ω do not fall
below 6Ω. If these measures are insufficient, a heat sink can be added to reduce θ
JA
. The heat sink can be
created using additional copper area around the package, with connections to the ground pins, supply pin and
amplifier output pins. Refer to the Typical Performance Characteristics curves for power dissipation information at
lower output power levels.
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 voltage regulator typically use a 10µF in parallel with a 0.1µF filter
capacitors to stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient
response. However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance
connected between the LM4951A's supply pins and ground. Do not substitute a ceramic capacitor for the
tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors between
the LM4951A's power supply pin and ground as short as possible. Connecting a larger capacitor, C
BYPASS
,
between 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,
system cost, and size constraints.
MICRO-POWER SHUTDOWN
The LM4951A features an active-low micro-power shutdown mode. When active, the LM4951A's micro-power
shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The low 0.01µA 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.
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 1, the input resistor (R
i
) and the input capacitor (C
i
) create a high-pass filter. The cutoff
frequency can be found using Equation 6.
f
c
= 1/2πR
i
C
i
(Hz) (6)
As an example when using a speaker with a low frequency limit of 50Hz, C
i
, using Equation 6 is 0.159µF with R
i
set to 20kΩ. The values for C
i
and R
i
shown in Figure 1 allow the LM4951A to drive a high efficiency, full range
speaker whose response extends down to 20Hz.
Selecting Value A For R
C
The LM4951A is designed for very fast turn on time. The C
CHG
pin allows the input capacitor to charge quickly to
improve click/pop performance. R
C
protects the C
CHG
pin from any over/under voltage conditions caused by
excessive input signal or an active input signal when the device is in shutdown. The recommended value for R
C
is 1kΩ. If the input signal is less than V
DD
+0.3V and greater than -0.3V, and if the input signal is disabled when in
shutdown mode, R
C
may be shorted out.
OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4951A 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 LM4951A's internal amplifiers are
configured as unity gain buffers. 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.
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