LM4892 datasheet(11/22 Pages) NSC

11 / 22 page

Application Information (Continued)

requirements, click and pop performance (as explained in

the section, Proper Selection of External Components),

system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4892 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the

amplifier off when a logic low is placed on the shutdown pin.

By switching the shutdown pin to ground, the LM4892 supply

current draw will be minimized in idle mode. While the device

will be disabled with shutdown pin voltages less than

0.5V

DC, the idle current may be greater than the typical

value of 0.1µA. (Idle current is measured with the shutdown

pin grounded).

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry to provide a

quick, smooth transition into shutdown. Another solution is to

use a single-pole, single-throw switch in conjunction with an

external pull-up resistor. When the switch is closed, the

shutdown pin is connected to ground and disables the am-

plifier. If the switch is open, then the external pull-up resistor

will enable the LM4892. This scheme guarantees that the

shutdown pin will not float thus preventing unwanted state

changes.

Table 1. Logic Level Truth Table for Shutdown and HP

Sense Operation

Shutdown

HP Sense

Operational Mode

Logic High

Logic Low

Bridged Amplifier

Logic High

Single-Ended Amplifier

Logic Low

Micro-Power Shutdown

Logic Low

Logic High

Micro-Power Shutdown

HP SENSE FUNCTION

Applying a voltage between 4V and V

CC to the LM4892’s

HP-Sense headphone control pin turns off Amp2 and mutes

a bridged-connected load. Quiescent current consumption is

reduced when the IC is in the single-ended mode.

Figure 2 shows the implementation of the LM4892’s head-

phone control function. With no headphones connected to

the headphone jack, the R

4-R6 voltage divider sets the volt-

age applied to the HP-Sense pin (pin3) at approximately

50mV. This 50mV enables the LM4892 and places it in

bridged mode operation.

While the LM4892 operates in bridged mode, the DC poten-

tial across the load is essentially 0V. Since the HP-Sense

threshold is set at 4V, even in an ideal situation, the output

swing can not cause a false single-ended trigger. Connect-

ing headphones to the headphone jack disconnects the

headphone jack contact pin from V

01 and allows R4 to pull

the HP Sense pin up to V

CC. This enables the headphone

function, turns off Amp2, and mutes the bridged speaker.

The amplifier then drives the headphone whose impedance

is in parallel with R

6. Resistor R6 has negligible effect on

output drive capability since the typical impedance of head-

phones is 32

Ω. The output coupling capacitor blocks the

amplifier’s half supply DC voltage, protecting the head-

phones.

A microprocessor or a switch can replace the headphone

jack contact pin. When a microprocessor or switch applies a

voltage greater than 4V to the HP Sense pin, a bridged-

connected speaker is muted and Amp1 drives the head-

phones.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4892 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4892 is unity-gain stable which gives the designer

maximum system flexibility. The LM4892 should be used in

low gain configurations to minimize THD+N values, and

maximize the signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power.

Input signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the

section, Audio Power Amplifier Design, for a more com-

plete explanation of proper gain selection.

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

width is dictated by the choice of external components

shown in Figure 1. The input coupling capacitor, C

i, forms a

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed

frequency response for a few distinct reasons.

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenu-

ation. But in many cases the speakers used in portable

systems, whether internal or external, have little ability to

reproduce signals below 100Hz to 150Hz. Thus, using a

large input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

i. A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 1/2 V

DD). This

charge comes from the output via the feedback and is apt to

20012774

FIGURE 2. Headphone Circuit (Pin #’s apply toM&MM

packages)

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