LM4832 datasheet(13/22 Pages) NSC

13 / 22 page

Application Information

GROUNDING

In order to achieve the best possible performance, certain

grounding techniques should be followed. All input reference

grounds should be tied with their respective source grounds

and brought back to the power supply ground separately

from the output load ground returns. These input grounds

should also be tied in with the half-supply bypass ground.

Bringing the ground returns for the output loads back to the

supply separately will keep large signal currents from inter-

fering with the stable AC input ground references.

LAYOUT

As stated in the Grounding section, placement of ground re-

turn lines is critical for maintaining the highest level of sys-

tem performance. It is not only important to route the correct

ground return lines together, but also important to be aware

of where those ground return lines are routed in conjunction

with each other. The output load ground returns should be

physically located as far as reasonably possible from low

signal level lines and their ground return lines. Critical signal

lines are those relating to the microphone amplifier section,

since these lines generally work at very low signal levels.

SUPPLY BYPASSING

As with all op amps and power op amps, the LM4832 re-

quires the supplies to be bypassed to avoid oscillation. To

avoid high frequency instabilities, a 0.1 µF metallized-film or

ceramic capacitor should be used to bypass the supplies as

close to the chip as possible. For low frequency consider-

ations, a 10 µF or greater tantalum or electrolytic capacitor

should be paralleled with the high frequency bypass capaci-

tor.

If power supply bypass capacitors are not sufficiently large,

the current in the power supply leads, which is a rectified ver-

sion of the output current, may be fed back into internal cir-

cuitry. This internal feedback signal can cause high fre-

quency distortion and oscillation.

If power supply lines to the chip are long, larger bypass ca-

pacitors could be required. Long power supply leads have in-

ductance and resistance associated with them, that could

prevent peak low frequency current demands from being

met. The extra bypass capacitance will reduce the peak cur-

rent requirements from the power supply lines.

POWER-UP STATUS

On power-up or after a hard reset, the LM4832 registers will

be initialized with the default values listed in the truth tables.

By default, the LM4832 power and microphone outputs are

muted, the tone controls are all flat, National 3D Enhance is

off, the chip is in stereo mode, and the microphone input 1 is

selected with +30 dB of gain.

CLICK AND POP CIRCUITRY

The LM4832 contains circuitry to minimize turn-on transients

or “click and pops”. In this case, turn-on refers to either

power supply turn-on or the device coming out of shutdown

mode. When the deviceis turning on, the amplifiers are inter-

nally configured as unity gain buffers. An internal current

source charges the bypass capacitor on the bypass pin.

Both the inputs and outputs ideally track the voltage at the

bypass pin. The device will remain in buffer mode until the

bypass pin has reached its half supply voltage, 1/2 V

DD.As

soon as the bypass node is stable, the device will become

fully operational.

Although the bypass pin current source cannot be modified,

the size of the bypass capacitor, C

B, can be changed to alter

the device turn-on time and the amount of “click and pop”. By

increasing C

B, the amount of turn-on pop can be reduced.

However, the trade-off for using a larger bypass capacitor is

an increase in the turn-on time for the device. Reducing C

will decrease turn-on time and increase “click and pop”. If C

is too small, the LM4832 can develop a low-frequency oscil-

lation (“motorboat”) when used at high gains.

There is a linear relationship between the size of C

B and the

turn-on time. Some typical turn-on times for different values

of C

B are:

0.01 µF

20 ms

0.1 µF

200 ms

0.22 µF

420 ms

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

discharged before turn-on. Rapid on/off switching of the de-

vice or shutdown function may cause the “click and pop” cir-

cuitry to not operate fully, resulting in increased “click and

pop” noise. The output coupling cap, C

O, is of particular con-

cern. This capacitor discharges through an internal 20 k

Ω re-

sistor. Depending on the size of C

O, the time constant can be

quite large. To reduce transients, an external 1 k

Ω–5 kΩ re-

sistor can be placed in parallel with the internal 20 k

Ω resis-

tor. The tradeoff for using this resistor is an increase in quies-

cent current.

COUPLING CAPACITORS

Because the LM4832 is a single supply circuit, all audio sig-

nals must be capacitor coupled to the chip to remove the

2.5 V

DC bias. All audio inputs have 20 kΩ input impedances,

so the AC-coupling capacitor will create a high-pass filter

with

−3dB = 1/(2π*20 kΩ*CIN).

The amplifier outputs also need to be AC-coupled to the

loads.The high-pass filter is comprised of the output load

and the coupling capacitor,where the filter cutoff is at

−3dB = 1/(2π*RLOAD*COUT).

POWER AMPLIFIER

The power amplifiers in the LM4832 are designed to drive

Ω or 32Ω loads at 200 mW (continuous) and 75 mW (con-

tinuous), respectively, with 1% THD+N. As shown in the Typi-

cal Performance Characteristics, the power amplifiers typi-

cally drive 4

Ω loads at 350 mW, but with a slight increase in

high-frequency THD. As discussed above, these outputs

should be AC-coupled to the output load.

MICROPHONE AMPLIFIER

The microphone preamplifier is intended to amplify low-level

signals for signal conditioning. The microphone inputs can

be directly connected to microphone networks. The micro-

phone amplifier has enough output capability to drivea1k

Ω

load. All microphone inputs and outputs must be AC-

coupled.

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