ducing Cb will decrease turn-on time and increase “click and

pop”. If Cb is too small, the LM4831 can develop a

low-frequency oscillation (“motorboat”) when used at high

gains.

In order to eliminate “click and pop”, all coupling capacitors

must be discharged before turn-on. Rapid on/off switching of

the device or shutdown function may cause the “click and

pop” circuitry to not operate fully, resulting in increased “click

and pop” noise. For single-ended (headphone) circuitry, the

output coupling cap, C

down, this capacitor is discharged through an internal 20k

Ω

resistor. Depending on the size of C

constant can be quite large. To reduce the time constant, an

external 1k

Ω-5kΩ resistor can be placed in parallel with the

internal 20k

Ω resistor. The tradeoff for using this resistor is

an increase in quiescent current and an increase in turn-off

“click and pop”.

Changing the bypass capacitor size also affects the amount

of time that the beep circuitry turns on the LM4831. Increas-

ing the bypass capacitor size increases the turn-on time,

which reduces the amount of time that the LM4831 is fully on

for during the RC-timed beep period.

The bypass capacitor also helps determine the power supply

rejection ratio. The smaller the bypass capacitor, the more

the power supply ripples couple onto the half supply and

then to all circuitry which uses the half supply for biasing.

COUPLING CAPACITORS

Since the LM4831 is a single supply circuit, all audio signals

(excepting the bridged outputs) must be capacitor coupled to

the chip to remove the 2.5V

20k

Ω input impedance, so the AC-coupling capacitor will cre-

ate a high-pass filter with f

−3dB point at 20Hz, C

Single-ended and line-out loads need to be AC-coupled back

to the LM4831 amplifiers. This high-pass filter is comprised

of the output load and the coupling capacitor, where the filter

cutoff is at f

−3dB point at 20Hz, C

EQUALIZER INPUT/OUTPUT

In some systems, the internal speakers require filtering to

improve their frequency response. The LM4831 provides the

system designer with external access to the signal using the

equalizer output and equalizer inputpins. When the DS_IN

and HP_IN pins are low (ie. the system is not in the docking

station and no headphone are plugged in), an internal mux

routes the audio signal to the equalizer output pin. After the

signal is filtered, it is returned to the LM4831 audio path

through the equalizer inputpin.

The input impedance to the equalizer input pin is 20k

Ω.Ifthe

external filter’s bias voltage is not derived from the half sup-

ply pin on the LM4831, AC-coupling capacitors must be used

on the equalizer input and output pins. If no equalization is

required, the equalizer out pin can be connected directly to

the equalizer in pin without any coupling capacitors.

LINE OUT

The line out pins are designed for use with a docking station

system. When the computer is plugged into the docking sta-

tion, the DS_IN pin should be forced high, thereby turning off

the power amplifier outputs and turning on the line out ampli-

fiers. All audio amplification and filtering is then done by the

docking station. The line out pins must be AC-coupled to the

docking station audio inputs.

POWER AMPLIFIERS

The power amplifiers in the LM4831 are designed to drive

8

Ω or 32Ω loads at 1W (continuous) or 250mW(continuous),

respectively, with 1% THD+N. If the power amplifiers are

used to drive single-ended loads, such as headphones, the

amplifier inverting outputs should be AC-coupled to the out-

put load. When the LM4831 is in headphone (single-ended)

mode, the amplifier non-inverting inputs are in a high-

impedance state.

In low gain applications (A

small feedback capacitance to prevent oscillation. Typically,

5-10pF will prevent oscillation.

MICROPHONE AMPLIFIER

The microphone amplifier is an uncommitted op-amp which

is intended to amplify low-level signals. The microphone in-

puts are very high impedance (R

rectly connected to microphone networks. The microphone

amplifier has enough output capability to drive a 1k

Ω load.

All microphone inputs and outputs must be AC-coupled.

As shown in

Figure 1, the microphone amplifier is typically

configured as an inverting amplifier. The positive terminal is

connected to the half-supply bypass pin to properly bias the

amplifier output to interface with the other inputs on the

LM4831. The microphone input pin is connected to the in-

verting node of a CMOS op amp, so the input impedance is

Ω)

BEEP CIRCUITRY

The beep circuitry is designed to allow a “sleeping” system to

temporarily power-up the LM4831 and output an audio alert

(“beep”). This feature might be used in a computer which is

“sleeping”, but needs to notify the user that the computer

batteries are low or that the user has new e-mail.

The beep circuitry is activated by any edge which occurs on

the BEEP A-D pins. With a resistor, R

C

will be activated for R

R

The BEEP OUT pin is designed to signal other audio circuitry

that the LM4831 is powering up. Generally a CODEC will re-

ceive this signal and begin sending audio information to the

LM4831. Logically, the BEEP OUT signal is the result of an

XOR of the BEEP A-D pins.

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