BEEP DETECT FUNCTION

The Beep Detect pin (pin 11) is a mono input that detects the

presence of a beep signal. When a signal greater than

2.5V

P-P

(or 1/2 V

cuitry will enable the bridged amplifiers. Beep in signals less

than 2.5V

cuitry. When triggered, the Beep Detect circuitry will enable

the bridged amplifiers regardless of the state of the mute,

mode, or HP sense pins. As shown in the Fig. 2, a 200k

Ω re-

sistor is placed in series with the input capacitor. This 200k

Ω

resistor can be changed to vary the amplitude of the beep in

signal. Higher values of the resistor will reduce the amplifier

gain and attenuate the beep in signal. These resistors are re-

quired in order for the beep signal to pass to the output. The

Beep Detect pin will not pass the beep signal to the output.

In cases where system beeps are required when the system

is in a suspended mode, the LM4835 must be brought out of

shutdown before the beep in signal is input.

SHUTDOWN FUNCTION

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

LM4835 contains a shutdown pin to externally turn off the

bias circuitry. The LM4835 will shutdown when a logic high is

placed on the shutdown pin. The trigger point between a

logic low and logic high level is typically half supply. It is best

to switch between ground and the supply V

maximum device performance. By switching the shutdown

pin to V

mized. While the device will be disabled with shutdown pin

voltages less than V

the typical value of 0.7 µA.The shutdown pin should not be

floated, since this may result in an unwanted shutdown con-

dition.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry which pro-

vides a quick, smooth transition into shutdown. Another solu-

tion is to use a single-pole, single-throw switch in conjuction

with an external pull-up resistor. When the switch is

closed,the shutdown pin is connected to ground and enables

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

resistor will shutdown the LM4835. This scheme prevents

the shutdown pin from floating.

MODE FUNCTION

The LM4835 was designed to operate in two modes. In

mode 0 (lineout mode),where the mode pin (pin 4) is given a

logic level low, the attenuation of the outputs are fixed at

unity gain. In mode 1 (adjustable mode), where the mode pin

is given a logic level high, the attenuation of the amplifier out-

puts is controlled through the DC voltage at pin 7.

MUTE FUNCTION

By placing a logic level high on the mute pin (pin5), the out-

puts of the amplifiers and pins 9 and 13 will be muted. The

beep in signal will be output even if the LM4835 is muted.

The mute pin must not be floated.

HP SENSE FUNCTION

The LM4835 possesses a headphone sense pin (pin 21) that

mutes the bridged amplifier, when given a logic high, so that

headphone or line out operation can occur while the bridged

connected load will be muted.

Figure 4 shows the implementation of the LM4835’s head-

phone control function using a single-supply. The voltage di-

vider of R1, R2, and R4 sets the voltage at the HP sense pin

(pin 21) to be approximately 50 mV when there are no head-

phones plugged into the system. This logic-low voltage at

the HP sense pin enables the bridged power amplifiers. Re-

sistor R4 limits the amount of current flowing out of the HP

sense pin when the voltage at that pin goes below ground re-

sulting from the music coming from the headphone amplifier.

Since the threshold of the HP sense pin is set at 4V ( or 80%

V

When a set of headphones are plugged into the system, the

contact pin of the headphone jack is disconnected from the

signal pin, interrupting the voltage divider set up by resistors

R1, R2, and R4. Resistor R1 then pulls up the HP sense pin,

enabling the headphone function and disabling the bridged

amplifier. The headphone amplifier then drives the head-

phones, whose impedance is in parallel with resistor R2 and

R3. Also shown in

Figure 4 are the electrical connections for

the headphone jack and plug. A 3-wire plug consists of a Tip,

Ring and Sleeve, where the Tip and Ring are signal carrying

conductors and the Sleeve is the common ground return.

One control pin contact for each headphone jack is sufficient

to indicate that the user has inserted a plug into a jack and

that another mode of operation is desired.

The LM4835 can be used to drive both a bridged 8

Ω internal

speaker and a pair of 32

Ω speakers without using the HP

sense circuit. In this case the HP sense is controlled by a mi-

croprocessor or a switch.

GAIN SELECT FUNCTION (Bass Boost)

External gain/internal gain can be toggled by changing the

logic at pin 3. A logic high will switch the power amplifiers to

external gain mode. In external gain mode the gain of the

amplifier is set by the external resistors. Whereas the inter-

nal gain mode sets the amplifiers to unity gain.

In some cases a designer may want to improve the low fre-

quency 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

ries with the feedback resistor of the bridged amplifier as

seen in

Figure 5.

DS100139-4

FIGURE 4. Headphone Sensing Circuit

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