Datasheet

LM4818
www.ti.com SNAS163B –APRIL 2002–REVISED MAY 2013
Besides affecting system cost and size, C
i
has an effect on the LM4818'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 value. Higher value
capacitors need more time to reach a quiescent DC voltage (usually 1/2 V
DD
) when charged with a fixed current.
The amplifier's output charges the input capacitor through the feedback resistor, R
F
. Thus, selecting an input
capacitor value that is no higher than necessary to meet the desired -3dB frequency can minimize pops.
As shown in Figure 1, the input resistor (R
i
) and the input capacitor, C
i
produce a -3dB high pass filter cutoff
frequency that is found using Equation 5.
f
-3dB
= 1/(2 πR
i
C
i
) (Hz) (5)
As an example when using a speaker with a low frequency limit of 150Hz, C
i
, using Equation 5 is 0.063µF. The
0.39µF C
i
shown in Figure 1 allows the LM4818 to drive a high efficiency, full range speaker whose response
extends down to 20Hz.
Besides optimizing the input capacitor value, the bypass capacitor value, C
B
requires careful consideration. The
bypass capacitor's value is the most critical to minimizing turn-on pops because it determines how fast the
LM4818 turns on. The slower the LM4818's outputs ramp to their quiescent DC voltage (nominally 1/2V
DD
), the
smaller the turn-on pop. While the device will function properly (no oscillations or motorboating), with C
B
less
than 1.0µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of C
B
equal to or
greater than 1.0µF is recommended in all but the most cost sensitive designs.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to the value of C
B
, the capacitor
connected to the BYPASS pin. Since C
B
determines how fast the LM4818 settles to quiescent operation, its
value is critical when minimizing turn-on pops. The slower the LM4818's outputs ramp to their quiescent DC
voltage (nominally 1/2V
DD
), the smaller the turn-on pop. Choosing C
B
equal to 1.0µF along with a small value of
C
i
(in the range of 0.1µF to 0.39µF) produces a click-less and pop-less shutdown function. As discussed above,
choosing C
i
no larger than necessary for the desired bandwidth helps minimize clicks and pops. If using the
optional capacitor, C
B2
, the total capacitance see at the BYPASS pin is C
B
+ C
B2
. When using the values shown
in Figure 1 for C
B
and C
B2
the change in the capacitance seen by the BYPASS pin is not significant relative to
capacitor value tolerances.
Optimizing Click and Pop Reduction Performance
The LM4818 contains circuitry that minimizes turn-on and shutdown transients or "clicks and pops". For this
discussion, turn on refers to either applying the power or supply voltage or when the shutdown mode is
deactivated. While the power supply is ramping to it's final value, the LM4818's internal amplifiers are configured
as unity gain buffers. An internal current source charges the voltage of the bypass capacitor, C
B
, connected to
the BYPASS pin in a controlled, linear manner. Ideally, the input and outputs track the voltage charging on the
bypass capacitor. The gain of the internal amplifiers remains unity until the bypass capacitor is fully charged to
1/2V
DD
. As soon as the voltage on the bypass capacitor is stable, the device becomes fully operational. Although
the BYPASS pin current cannot be modified, changing the size of the bypass capacitor, C
B
, alters the device's
turn-on time and 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 (Ton) increases. There is a
linear relationship between the size of C
B
and the turn on time. If using the optional capacitor, C
B2
, the total
capacitance see at the BYPASS pin is C
B
and C
B2
. The total capacitance see at the BYPASS pin must be
considered for the table below and when optimizing click and pop performance. Below are some typical turn-on
times for various values of C
B
:
C
B
T
ON
0.01µF 20ms
0.1µF 200ms
0.22µF 440ms
0.47µF 940ms
1.0µF 2S
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