LM4951 datasheet(12/18 Pages) NSC

12 / 18 page

Application Information (Continued)

lation. Keep the length of leads and traces that connect

capacitors between the LM4951’s power supply pin and

ground as short as possible. Connecting a larger capacitor,

BYPASS, between the BYPASS pin and ground improves

the internal bias voltage’s stability and improves the amplifi-

er’s PSRR. The PSRR improvements increase as the by-

pass pin capacitor value increases. Too large, however,

increases turn-on time and can compromise the amplifier’s

click and pop performance. The selection of bypass capaci-

tor values, especially C

BYPASS, depends on desired PSRR

requirements, click and pop performance (as explained in

the section, SELECTING EXTERNAL COMPONENTS),

system cost, and size constraints.

MICRO-POWER SHUTDOWN

The LM4951 features an active-low micro-power shutdown

mode. When active, the LM4951’s micro-power shutdown

feature turns off the amplifier’s bias circuitry, reducing the

supply current. The low 0.01µA typical shutdown current is

achieved by applying a voltage to the SHUTDOWN pin that

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

GND may increase the shutdown current.

There are a few methods to control the micro-power shut-

down. These include using a single-pole, single-throw switch

(SPST), a microprocessor, or a microcontroller. When using

a switch, connect the SPST switch between the shutdown

pin and V

DD. Select normal amplifier operation by closing the

switch. Opening the switch applies GND to the SHUTDOWN

pin activating micro-power shutdwon.The switch and internal

pull-down resistor guarantee that the SHUTDOWN pin will

not float. This prevents unwanted state changes. In a system

with a microprocessor or a microcontroller, use a digital

output to apply the active-state voltage to the SHUTDOWN

pin.

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 1, the input resistor (R

i) and the input

capacitor (C

i) produce a high pass filter cutoff frequency that

is found using Equation (6).

c = 1/2

πR

iCi

(6)

As an example when using a speaker with a low frequency

limit of 50Hz, C

i, using Equation (6) is 0.159µF. The 0.39µF

INA shown in Figure 1 allows the LM4951 to drive high

efficiency, full range speaker whose response extends below

30Hz.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4951 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 LM4951’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

voltage 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 undesirable output transients (“click and pops”). Af-

ter this delay, the device becomes fully functional.

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1.8W into an 8

Ω BTL

The following are the desired operational parameters:

Power Output

1.8W

RMS

Load Impedance

Ω

Input Level

0.3V

RMS (max)

Input Impedance

20k

Ω

Bandwidth

50Hz–20kHz ± 0.25dB

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 Power Supply Voltage curve in the Typical Performance

Characteristics section. Another way, using Equation (7), is

to calculate the peak output voltage necessary to achieve

the desired output power for a given load impedance. To

account for the amplifier’s dropout voltage, two additional

voltages, based on the Clipping Dropout Voltage vs Power

Supply Voltage in the Typical Performance Characteris-

tics curves, must be added to the result obtained by Equa-

tion (7). The result is Equation (8).

(7)

DD =VOUTPEAK +VODTOP +VODBOT

(8)

The commonly used 7.5V supply voltage easily meets this.

The additional voltage creates the benefit of headroom,

allowing the LM4951 to produce peak output power in ex-

cess of 1.8W without clipping or other audible distortion. The

choice of supply voltage must also not create a situation that

violates of maximum power dissipation as explained above

in the Power Dissipation section. After satisfying the

LM4951’s power dissipation requirements, the minimum dif-

ferential gain needed to achieve 1.8W dissipation in an 8

Ω

BTL load is found using Equation (9).

(9)

Thus, a minimum gain of 12.6 allows the LM4951’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

V-BTL = 13. The amplifier’s

overall BTL gain is set using the input (R

i) and feedback (Rf)

resistors of the first amplifier in the series BTL configuration.

Additionaly, A

V-BTL is twice the gain set by the first amplifier’s

i and Rf. With the desired input impedance set at 20k

Ω, the

feedback resistor is found using Equation (10).

f / Ri =AV-BTL / 2

(10)

The value of R

f is 130k

Ω (choose 191kΩ, the closest value).

The nominal output power is 1.8W.

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