LM4950TS datasheet(17/25 Pages) NSC

17 / 25 page

Application Information (Continued)

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Two quantities determine the value of the input coupling

capacitor: the lowest audio frequency that requires amplifi-

cation and desired output transient suppression.

As shown in Figure 3, the input resistor (R

IN) and the input

capacitor (C

IN) produce a high pass filter cutoff frequency

that is found using Equation (7).

c = 1/2

πR

iCi

(7)

As an example when using a speaker with a low frequency

limit of 50Hz, C

i, using Equation (7) is 0.159µF. The 0.39µF

INA shown in Figure 3 allows the LM4950 to drive high

efficiency, full range speaker whose response extends below

30Hz.

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of C

BYPASS, the capacitor

connected to the BYPASS pin. Since C

BYPASS determines

how fast the LM4950 settles to quiescent operation, its value

is critical when minimizing turn-on pops. The slower the

LM4950’s outputs ramp to their quiescent DC voltage (nomi-

nally V

DD/2), the smaller the turn-on pop. Choosing CBYPASS

equal to 10µF along with a small value of C

IN (in the range of

0.1µF to 0.39µF), produces a click-less and pop-less shut-

down function. As discussed above, choosing C

IN no larger

than necessary for the desired bandwidth helps minimize

clicks and pops.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4950 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 LM4950’s internal amplifiers are configured

as unity gain buffers and are disconnected from the AMP

and AMP

B pins. An internal current source charges the ca-

pacitor 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 voltage applied to the BY-

PASS pin.

The gain of the internal amplifiers remains unity until the

voltage on the bypass pin reaches V

DD/2. As soon as the

voltage on the bypass pin is stable, the device becomes fully

operational and the amplifier outputs are reconnected to

their respective output pins. Although the BYPASS pin cur-

rent cannot be modified, changing the size of C

BYPASS alters

the device’s turn-on time. Here are some typical turn-on

times for various values of C

BYPASS:

B (µF)

ON (ms)

1.0

120

2.2

120

4.7

200

440

In order eliminate "clicks and pops", all capacitors must be

discharged before turn-on. Rapidly switching V

DD may not

allow the capacitors to fully discharge, which may cause

"clicks and pops".

There is a relationship between the value of C

and

BYPASS that ensures minimum output transient when power

is applied or the shutdown mode is deactivated. Best perfor-

mance is achieved by setting the time constant created by

IN and Ri +Rf to a value less than the turn-on time for a

given value of C

BYPASS as shown in the table above.

DRIVING PIEZO-ELECTRIC SPEAKER TRANSDUCERS

The LM4950 is able to drive capacitive piezo-electric trans-

ducer loads that are less than equal to 200nF. Stable opera-

tion is assured by placing 33pF capacitors in parallel with the

20k

Ω feedback resistors. The additional capacitors are

shown in Figure 4.

When driving piezo-electric tranducers, sound quality and

accoustic power is entirely dependent upon a transducer’s

frequency response and efficiency. In this application, power

dissipated by the LM4950 is very low, typically less than

250mW when driving a 200nF piezo-electric transduce

DD = 12V).

LM4950TS Datasheet(PDF) 17 Page - National Semiconductor (TI)

LM4950TS Datasheet(HTML) 17 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Datasheet Download

Link URL