CAP-LESS HEADPHONE (SINGLE-ENDED) AMPLIFIER

OPERATION

An internal pull−up circuit is connected to the HP Sense (Pin

21 HP-IN) headphone amplifier control pin. When this pin is

left unconnected, V

Amp B +OUT (not seen in Fig 5, see Fig 2 Pin 15) and

switches Amp A +OUT’s input signal from an audio signal to

the V

result is muted bridge-connected loads. Quiescent current

consumption is reduced when the IC is in this single−ended

mode.

Figure 5 above shows the implementation of the LM4841’s

headphone control function. An internal comparator with a

nominal 400mV offset monitors the signal present at the

−OUT B output. It compares this signal against the signal

applied to the HP−IN pin (Notice in Figure 5, Pin 21 is

shorted to Pin 17 without a headphone plugged in). When

these signals are equal, as in the case when a BTL is

connected to the amplifier, an internal comparator forces the

LM4841 to maintain bridged−amplifier operation. When the

HP−IN pin is externally floated, such as when headphones

are connected to the jack shown in Figure 5, an internal

pull−up forces V

puts. This changes the comparator’s output state and en-

ables the headphone function: it turns off Amp B +OUT (not

seen in Fig 5, see Fig 2 Pin 15), switches the Amp A +OUT

input signal from an audio signal to the V

present on pin 28, and mutes the bridge-connected loads.

Amp A -OUT and Amp B -OUT drive the headphones.

Figures 2 and 6 also show suggested headphone jack elec-

trical connections. The jack is designed to mate with a

three−wire plug. The plug’s tip and ring should each carry

one of the two stereo output signals, whereas the sleeve

provides the return to Amp A +OUT. A headphone jack with

one control pin contact is sufficient to drive the HP−IN pin

when connecting headphones

A switch can replace the headphone jack contact pin. When

a switch shorts the HP−IN pin to V

contact will accomplish this because there is an internal

pull-up resistor), the bridge−connected speakers are muted

and Amp A -OUT and Amp B -OUT drive the stereo head-

phones. When a switch shorts the HP−IN pin to GND (pulling

down the internal pull-up resistor), the LM4841 operates in

bridge mode. If headphone drive is not needed, short the

HP−IN pin to the −OUTB pin.

ESD Protection

As stated in the Absolute Maximum Ratings, pin 28 on the

MT/MH packages and pin 25 on the LQ package, have a

maximum ESD susceptibility rating of 6500V. For higher

ESD voltages, the addition of a PCDN042 dual transil (from

California Micro Devices), as shown in Figure 6, will provide

additional protection.

GAIN SELECT FUNCTION (Bass Boost)

The LM4841 features selectable gain, using either internal or

external feedback resistors. Either set of feedback resistors

set the gain of the output amplifiers. The voltage applied to

the GAIN SELECT pin controls which gain is selected. Ap-

plying V

mode. Applying 0V to the GAIN SELECT pin selects the

internally set unity gain.

In some cases a designer may want to improve the low

frequency response of the bridged amplifier or incorporate a

bass boost feature. This bass boost can be useful in systems

where speakers are housed in small enclosures. A resistor,

R

with the feedback resistor of the bridged amplifier as seen in

Figure 7.

20028094

FIGURE 6. The PCDN042 provides additional ESD protection beyond the 6500V shown in the

Absolute Maximum Ratings for the AMP2A output

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