LM4951ASD datasheet(14/20 Pages) NSC

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is as near to GND as possible. A voltage that is greater than

GND may increase the shutdown current.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Two quantities determine the value of the input coupling ca-

pacitor: the lowest audio frequency that requires amplification

and desired output transient suppression.

As shown in Figure 1, the input resistor (R

i) and the input ca-

pacitor (C

i) create a high-pass filter. The cutoff frequency can

be found using Equation (6).

c = 1/2πRiCi

(Hz)

(6)

As an example when using a speaker with a low frequency

limit of 50Hz, C

i, using Equation (6) is 0.159µF with Ri set to

20k

Ω. The values for C

i and Ri shown in Figure 1 allow the

LM4951A to drive a high efficiency, full range speaker whose

response extends down to 20Hz.

Selecting Value A For R

The LM4951A is designed for very fast turn on time. The

CHG pin allows the input capacitor to charge quickly to im-

prove click/pop performance. R

C protects the CCHG pin from

any over/under voltage conditions caused by excessive input

signal or an active input signal when the device is in shut-

down. The recommended value for R

C is 1kΩ. If the input

signal is less than V

DD+0.3V and greater than -0.3V, and if the

input signal is disabled when in shutdown mode, R

C may be

shorted out.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4951A contains circuitry that eliminates turn-on and

shutdown transients ("clicks and pops"). For this discussion,

turn-on refers to either applying the power supply voltage or

when the micro-power shutdown mode is deactivated.

As the V

DD/2 voltage present at the BYPASS pin ramps to its

final value, the LM4951A's internal amplifiers are configured

as unity gain buffers. An internal current source charges the

capacitor connected between the BYPASS pin and GND in a

controlled manner. Ideally, the input and outputs track the

voltage applied to the BYPASS pin.

The gain of the internal amplifiers remains unity until the volt-

age on the bypass pin reaches V

DD/2. As soon as the voltage

on the bypass pin is stable, there is a delay to prevent unde-

sirable output transients (“click and pops”). After this delay,

the device becomes fully functional.

THERMAL SHUTDOWN AND SHORT CIRCUIT

PROTECTION

The LM4951A has thermal shutdown and short circuit pro-

tection to fully protect the device. The thermal shutdown

circuit is activated when the die temperature exceeds a safe

temperature. The short circuit protection circuitry senses the

output current. When the output current exceeds the thresh-

old under a short condition, a short will be detected and the

output deactivated until the short condition is removed. If the

output current is lower than the threshold then a short will not

be detected and the outputs will not be deactivated. Under

such conditions the die temperature will increase and, if the

condition persist to raise the die temperature to the thermal

shutdown threshold, initiate a thermal shutdown response.

Once the die cools the outputs will become active.

RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT

Figures 2–4 show the recommended two-layer PC board lay-

out that is optimized for the SD10A. This circuit is designed

for use with an external 7.5V supply 8

Ω (min) speakers.

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