Datasheet

LM4810
www.ti.com SNAS125D –FEBRUARY 2001–REVISED APRIL 2013
OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The LM4810 contains circuitry that minimizes turn-on and shutdown transients or “clicks and pop”. For this
discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated.
During turn-on, the LM4810's internal amplifiers are configured as unity gain buffers. An internal current source
charges up the capacitor on the BYPASS pin in a controlled, linear manner. 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. During device turn-on, a transient (pop) is created from a voltage
difference between the input and output of the amplifier as the voltage on the BYPASS pin reaches 1/2 V
DD
. For
this discussion, the input of the amplifier refers to the node between R
I
and C
I
. Ideally, the input and output track
the voltage applied to the BYPASS pin. During turn-on, the buffer-configured amplifier output charges the input
capacitor, C
I
, through the input resistor, R
I
. This input resistor delays the charging time of C
I
thereby causing the
voltage difference between the input and output that results in a transient (pop). Higher value capacitors need
more time to reach a quiescent DC voltage (usually 1/2 V
DD
) when charged with a fixed current. Decreasing the
value of C
I
and R
I
will minimize the turn-on pops at the expense of the desired -3dB frequency.
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. Here are some typical turn-on times for various values
of C
B
:
C
B
T
ON
0.1µF 80ms
0.22µF 170ms
0.33µF 270ms
0.47µF 370ms
0.68µF 490ms
1.0µF 920ms
2.2µF 1.8sec
3.3µF 2.8sec
4.7µF 3.4sec
10µF 7.7sec
In order eliminate “clicks and pops”, all capacitors must be discharged before turn-on. Rapidly switching V
DD
may
not allow the capacitors to fully discharge, which may cause “clicks and pops”. In a single-ended configuration,
the output is coupled to the load by C
O
. This capacitor usually has a high value. C
O
discharges through internal
20kΩ resistors. Depending on the size of C
O
, the discharge time constant can be relatively large. To reduce
transients in single-ended mode, an external 1kΩ–5kΩ resistor can be placed in parallel with the internal 20kΩ
resistor. The tradeoff for using this resistor is increased quiescent current.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32Ω Audio Amplifier
Given:
Power Output 70 mW
Load Impedance 32Ω
Input Level 1 Vrms (max)
Input Impedance 20kΩ
Bandwidth 100 Hz–20 kHz ± 0.50dB
The design begins by specifying the minimum supply voltage necessary to obtain the specified output power.
One way to find the minimum supply voltage is to use Figure 27 in Typical Performance Characteristics. Another
way, using Equation 5, is to calculate the peak output voltage necessary to achieve the desired output power for
a given load impedance. To account for the amplifier's dropout voltage, two additional voltages, based on
Figure 30 in Typical Performance Characteristics, must be added to the result obtained by Equation 5. For a
single-ended application, the result is Equation 6.
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