Application Information

GROUNDING

Certain grounding techniques should be followed when lay-

ing out the LM4831 circuit.

Figure 4 shows how to setup the

grounds for the LM4831. The half-supply bypass ground

should be tied with the input source grounds and brought

back to the power supply ground separately from the output

load grounds. in with the input grounds. Bringing the output

load grounds back to the supply separately will keep large

signal currents from interfering with the stable 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 relative to

each other. The output load ground returns should be physi-

cally located as far as reasonably possible from low signal

level lines and their ground return lines. The layout of the mi-

crophone amplifier signal lines is critical, since these lines

generally work at very low signal levels.

SUPPLY BYPASSING

As with all op amps and power op amps, the LM4831 re-

quires the power supplies to be bypassed to reduce distor-

tion and avoid oscillation. To avoid high frequency instabili-

ties, a 0.1µF metallized-film or ceramic capacitor should be

used to bypass each supply pin as near to the chip as pos-

sible. For low frequency considerations, a 10µF or greater

tantalum or electrolytic capacitor should be paralleled with

the high frequency bypass capacitor.

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, which 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.

Under certain conditions, the LM4831 may refuse to come

out of shutdown. A 1M

Ω resistor connected from the power

supply to the bypass pin, as shown in the Typical Applica-

tion section circuit,

Figure 3, will guarantee startup.

CLICK & POP CIRCUITRY AND THE BYPASS

CAPACITOR

The LM4831 contains circuitry to minimize turn-on tran-

sients. In this case, turn-on refers to either power supply

turn-on or the device coming out of shutdown mode. During

turn-on, an internal current source charges the bypass ca-

pacitor on the bypass pin. Both the inputs and outputs track

the voltage at the bypass pin. As soon as the bypass node is

stable at 1/2 V

Although the bypass pin current source cannot be modified,

the size of the bypass capacitor, Cb, can be changed to alter

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

The relationship between the size of Cb and the turn-on time

is linear. By increasing Cb, the amount of turn-on pop can be

reduced. However, the trade-off for using a larger bypass ca-

pacitor is an increase in the turn-on time for the device. Re-

Equalizer Output

Noise Floor

DS100057-8

DS100057-99

FIGURE 4. Grounding Strategy for LM4831

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