Datasheet

LM4936
www.ti.com
SNAS269A APRIL 2005REVISED APRIL 2013
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4936's performance requires properly selecting external components. Though the LM4936
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
The LM4936 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 affecting system cost and size, the input coupling capacitor has an effect on the LM4936'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 (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 LM4936 to drive a high
efficiency, full range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4936 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 LM4936'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
unity until the voltage on the BYPASS pin reaches 1/2 V
DD
. As soon as the voltage on the BYPASS pin is
stable, the device becomes fully operational. Although the BYPASS pin current cannot be modified, changing the
size of C
B
alters the device's turn-on time and the magnitude of “clicks and pops”. Increasing the value of C
B
reduces the magnitude of turn-on pops. However, this presents a tradeoff: as the size of C
B
increases, the turn-
on time increases. There is a linear relationship between the size of C
B
and the turn-on time. Below are some
typical turn-on times for various values of C
B
:
C
B
T
ON
0.01µF 2ms
0.1µF 20ms
0.22µF 44ms
0.47µF 94ms
1.0µF 200ms
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