LM4917SD datasheet(12/18 Pages) NSC

12 / 18 page

Application Information (Continued)

operation by opening the switch. Closing the switch con-

nects the SD_LC/SD_RC pins to ground, activating micro-

power shutdown. The switch and resistor guarantee that the

SD_LC/SD_RC pins will not float. This prevents unwanted

state changes. In a system with a microprocessor or micro-

controller, use a digital output to apply the control voltage to

the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins

with active circuitry eliminates the pull-up resistor.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4917’s performance requires properly se-

lecting external components. Though the LM4917 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

The LM4917 is unity-gain stable, giving a designer maximum

design flexibility. The gain should be set to no more than a

given application requires. This allows the amplifier to

achieve minimum THD+N and maximum signal-to-noise ra-

tio. These parameters are compromised as the closed-loop

gain increases. However, low gain demands input signals

with greater voltage swings to achieve maximum output

power. Fortunately, many signal sources such as audio

CODECs have outputs of 1V

RMS (2.83VP-P). Please refer to

the Audio Power Amplifier Design section for more infor-

mation on selecting the proper gain.

Charge Pump Capacitor Selection

Choose low ESR (<100m

Ω) ceramic capacitors for optimum

performance. Low ESR capacitors keep the charge pump

output impedance to a minimum, extending the headroom

on the negative supply. Choose capacitors with an X7R

dielectric for best performance over temperature.

Charge pump load regulation and output resistance is af-

fected by the value of the flying capacitor (C1). A larger

valued C1 improves load regulation and minimizes charge

pump output resistance. The switch on-resistance and ca-

pacitor ESR dominates the output resistance for capacitor

values above 2.2µF.

The output ripple is affected by the value and ESR of the

output capacitor (C2). Larger valued capacitors reduce out-

put ripple on the negative power supply. Lower ESR capaci-

tors minimizes the output ripple and reduces the output

resistance of the charge pump.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitor (C

i in Figure 2). A high value capaci-

tor can be expensive and may compromise space efficiency

in portable designs. In many cases, however, the speakers

used in portable systems, whether internal or external, have

little ability to reproduce signals below 150Hz. Applications

using speakers with this limited frequency response reap

little improvement by using high value input and output ca-

pacitors.

Besides affecting system cost and size, C

i has an effect on

the LM4917’s click and pop performance. The magnitude of

the pop is directly proportional to the input capacitor’s size.

Thus, pops can be minimized by selecting an input capacitor

value that is no higher than necessary to meet the desired

−3dB frequency.

As shown in Figure 2, the input resistor, R

i and the input

capacitor, C

i, produce a -3dB high pass filter cutoff frequency

that is found using Equation (3).

i-3dB =1/2

πR

iCi

(3)

Also, careful consideration must be taken in selecting a

certain type of capacitor to be used in the system. Different

types of capacitors (tantalum, electrolytic, ceramic) have

unique performance characteristics and may affect overall

system performance.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 90mW/16

Ω Audio Amplifier

Given:

Power Output

90mW

Load Impedance

Ω

Input Level

1Vrms (max)

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.50dB

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Supply Voltage curve in the Typical Performance Char-

acteristics section. Another way, using Equation (5), is to

calculate the peak output voltage necessary to achieve the

desired output power for a given load impedance. For a

single-ended application, the result is Equation (5).

(4)

≥ [2V

OPEAK +(VDOTOP +VDOBOT)]

(5)

The Output Power vs Supply Voltage graph for a 16

Ω load

indicates a minimum supply voltage of 3.1V. This is easily

met by the commonly used 3.3V supply voltage. The addi-

tional voltage creates the benefit of headroom, allowing the

LM4917 to produce peak output power in excess of 90mW

without clipping or other audible distortion. The choice of

supply voltage must also not create a situation that violates

maximum power dissipation as explained above in the

Power Dissipation section. Remember that the maximum

power dissipation point from Equation (1) must be multiplied

by two since there are two independent amplifiers inside the

package. Once the power dissipation equations have been

addressed, the required gain can be determined from Equa-

tion (6).

(6)

Thus, a minimum gain of 1.2 allows the LM4917 to reach full

output swing and maintain low noise and THD+N perfro-

mance. For this example, let A

V = 1.5.

The amplifiers overall gain is set using the input (R

i ) and

feedback (R

f ) resistors. With the desired input impedance

set at 20k

Ω, the feedback resistor is found using Equation

(7).

V =Rf /Ri

(7)

LM4917SD Datasheet(PDF) 12 Page - National Semiconductor (TI)

LM4917SD Datasheet(HTML) 12 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL