LM4810 datasheet(15/25 Pages) TI

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LM4810

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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 VDD. 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 VDD. For

this discussion, the input of the amplifier refers to the node between RI and CI. 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, CI, through the input resistor, RI. This input resistor delays the charging time of CI 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 VDD) when charged with a fixed current. Decreasing the

value of CI and RI 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 CB alters the device's turn-on time

and the magnitude of “clicks and pops”. Increasing the value of CB reduces the magnitude of turn-on pops.

However, this presents a tradeoff: as the size of CB increases, the turn-on time increases. There is a linear

relationship between the size of CB and the turn-on time. Here are some typical turn-on times for various values

of CB:

TON

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 VDD 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 CO. This capacitor usually has a high value. CO discharges through internal

20k

Ω resistors. Depending on the size of CO, the discharge time constant can be relatively large. To reduce

transients in single-ended mode, an external 1k

Ω–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/32

Ω Audio Amplifier

Given:

Power Output

70 mW

Load Impedance

Ω

Input Level

1 Vrms (max)

Input Impedance

20k

Ω

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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