Datasheet

LM4875

www.ti.com

SNAS042C –JANUARY 2002–REVISED MAY 2013

HP-Sense FUNCTION

Applying a voltage between 4V and V

to the LM4875'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 this single-ended

mode.

Figure 27 shows the implementation of the LM4875's headphone control function. With no headphones

connected to the headphone jack, the R1-R2 voltage divider sets the voltage applied to the HP-Sense pin (pin 3)

at approximately 50mV. This 50mV enables the LM4875 and places it in bridged mode operation.

Figure 27. Headphone Circuit

While the LM4875 operates in bridged mode, the DC potential 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 headphone jack contact pin from V

and allows R1 to pull the HP Sense pin up to V

. This enables the headphone function, 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 32Ω. The output coupling capacitor blocks the amplifier's half supply DC voltage, protecting the

headphones.

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 headphones.

PROPERLY SELECTING EXTERNAL COMPONENTS

Optimizing the LM4875's performance requires properly selecting external components. Though the LM4875

operates well when using external components having wide tolerances, the best performance is achieved by

optimizing component 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

) is 10kΩ at maximum volume and 110kΩ at minimum

volume. Together, the input capacitor, C

, and R

, produce a -3dB high pass filter cutoff frequency that is found

using Equation 4.

(4)

Product Folder Links: LM4875