While the LM4875 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 cannot cause a false single-ended trigger. Connecting

headphones to the headphone jack disconnects the head-

phone jack contact pin from V

HP Sense pin up to V

tion, turns off Amp2, and mutes the bridged speaker. The

amplifier then drives the headphones, whose impedance is

in parallel with resistor R2. Resistor R2 has negligible effect

on output drive capability since the typical impedance of

headphones 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 bridge-

connected speaker is muted and Amp1 drives the head-

phones.

SELECTING EXTERNAL COMPONENTS

Optimizing the LM4875’s performance requires properly se-

lecting external components. Though the LM4875 operates

well when using external components having wide toler-

ances, the best performance is achieved by optimizing com-

ponent values.

Input Capacitor Value Selection

Amplification of the lowest audio frequencies requires high

value input coupling capacitors. These high value capacitors

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. In application

5 using speakers with this limited frequency response, a

large input capacitor will offer little improvement in system

performance.

Figure 1 shows that the nominal input impedance (R

10k

Ω at maximum volume and 110kΩ at minimum volume.

Together, the input capacitor, C

high pass filter cutoff frequency that is found using Equation

(4).

(4)

As the volume changes from minimum to maximum, R

IN

decrease from 110k

Ω to 10kΩ. Equation (4) reveals that the

-3dB frequency will increase as the volume increases. The

nominal value of C

should be calculated with R

Ω . As an example when

using a speaker with a low frequency limit of 150Hz, C

using Equation (4) is 0.1µF. The 0.22µF C

is optimized for a speaker whose response extends down to

75Hz.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of the bypass capacitor C

Since C

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

the LM4875’s outputs ramp to their quiescent DC voltage

(nominally V

equal to 1.0µF, along with a small value of C

0.1µF to 0.39µF), produces a clickless and popless shut-

down function. Choosing C

mize clicks and pops.

CLICK AND POP CIRCUITRY

The LM4875 contains circuitry that minimizes turn-on and

shutdown transients or "clicks and pops". For this discus-

sion, turn-on refers to either applying the power supply volt-

age or when the shutdown mode is deactivated. While the

power supply is ramping to its final value, the LM4875’s

internal amplifiers are configured as unity gain buffers. An

internal current source changes the voltage of the bypass

pin in a controlled, linear 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 1/2 V

the bypass pin is stable, the device becomes fully opera-

tional and the gain is set by the external voltage applied to

the DC Vol/SD pin.

Although the bypass pin current cannot be modified, chang-

ing the size of C

magnitude of "clicks and pops". Increasing the value of C

B

reduces the magnitude of turn-on pops. However, this pre-

sents a tradeoff: as the size of C

increases. There is a linear relationship between the size of

CB and the turn-on time. Shown below are some typical

turn-on times for various values of C

C

B

T

ON

0.01µF

3ms

0.1µF

30ms

0.22µF

65ms

0.47µF

135ms

1.0µF

280ms

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

discharged before turn-on. Rapidly switching V

allow the capacitors to fully discharge, which may cause

"clicks and pops". In a single-ended configuration, the output

coupling capacitor, C

pacitor discharges through an internal 20k

Ω resistor. De-

pending on the size of C

relatively large. To reduce transients in single-ended mode,

10104234

FIGURE 3. Headphone Circuit

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