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%
Application Information (Continued)
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
2.5VP-P (or 1/2 VDD) is present at pin 11, the Beep Detect cir-
cuitry will enable the bridged amplifiers. Beep in signals less
than 2.5VP-P (or 1/2 VDD) will not trigger the Beep Detect cir-
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 200kΩ re-
sistor is placed in series with the input capacitor. This 200kΩ
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.
V
DD), the output swing cannot cause false triggering.
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.
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 VDD to provide
maximum device performance. By switching the shutdown
pin to VDD, the LM4835 supply current draw will be mini-
mized. While the device will be disabled with shutdown pin
voltages less than VDD, the idle current may be greater than
the typical value of 0.7 µA.The shutdown pin should not be
floated, since this may result in an unwanted shutdown con-
dition.
The LM4835 can be used to drive both a bridged 8Ω internal
speaker and a pair of 32Ω speakers without using the HP
sense circuit. In this case the HP sense is controlled by a mi-
croprocessor or a switch.
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.
DS100139-4
FIGURE 4. Headphone Sensing Circuit
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.
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.
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,
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.
R
LFE, and a capacitor, CLFE, in parallel, can be placed in se-
ries with the feedback resistor of the bridged amplifier as
seen in Figure 5.
Figure 4 shows the implementation of the LM4835’s head-
phone control function using a single-supply. The voltage di-
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