Datasheet

LM4938
SNAS245B –FEBRUARY 2005–REVISED MAY 2013 www.ti.com
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 5V regulator typically use a 10 µF in parallel with a 0.1 µF filter capacitor 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 LM4938'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 LM4938's power
supply pin and ground as short as possible. Connecting a 1µF capacitor, C
B
, between the BYPASS pin and
ground improves the internal bias voltage's stability and the amplifier's PSRR. The PSRR improvements increase
as the BYPASS pin capacitor value increases. Too large a capacitor, however, increases turn-on time and can
compromise the amplifier's click and pop performance. The selection of bypass capacitor values, especially C
B
,
depends on desired PSRR requirements, click and pop performance (as explained in the following section,
SELECTING PROPER EXTERNAL COMPONENTS), system cost, and size constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4938's performance requires properly selecting external components. Though the LM4938
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
The LM4938 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more
than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-
noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain circuits
demand input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal
sources such as audio CODECs have outputs of 1V
RMS
(2.83V
P-P
). Please refer to the AUDIO POWER
AMPLIFIER DESIGN section for more information on selecting the proper gain.
INPUT CAPACITOR VALUE SELECTION
Amplifying the lowest audio frequencies requires a high value input coupling capacitor (0.33µF in Figure 3), but
high value capacitors can be expensive and may compromise space efficiency in portable designs. In many
cases, however, the speakers used in portable systems, whether internal or external, have little ability to
reproduce signals below 150 Hz. Applications using speakers with this limited frequency response reap little
improvement by using a large input capacitor.
Besides effecting system cost and size, the input coupling capacitor has an affect on the LM4938's click and pop
performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input
capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional to the input
capacitor's size. Higher value capacitors need more time to reach a quiescent DC voltage (usually V
DD
/2) when
charged with a fixed current. The amplifier's output charges the input capacitor through the feedback resistor, R
f
.
Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the
desired −6dB frequency.
As shown in Figure 3, the input resistor (R
IR
, R
IL
= 20k) ( and the input capacitor (C
IR
, C
IL
= 0.33µF) produce a
−6dB high pass filter cutoff frequency that is found using Equation 7.
(7)
As an example when using a speaker with a low frequency limit of 150Hz, the input coupling capacitor, using
Equation 7, is 0.053µF. The 0.33µF input coupling capacitor shown in Figure 3 allows the LM4938 to drive a high
efficiency, full range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4938 contains circuitry that minimizes turn-on and shutdown transients or “clicks and pops”. For this
discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated.
While the power supply is ramping to its final value, the LM4938's internal amplifiers are configured as unity gain
buffers. An internal current source changes the voltage of the BYPASS pin in a controlled, linear manner. Ideally,
the input and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains
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