LM4840 datasheet(12/19 Pages) NSC

12 / 19 page

Application Information (Continued)

severe attenuation. In many cases the speakers used in

portable systems, whether internal or external, have little

ability to reproduce signals below 100 Hz–150 Hz. In this

case, usinga large input or output capacitor may not in-

crease system performance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

i. A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 1/2 V

DD.) This

charge comes from the output through the feedback and is

apt to create pops once the device is enabled. By minimizing

the capacitor size based on necessary low frequency re-

sponse, turn-on pops can be minimized.

CLICK AND POP CIRCUITRY

The LM4840 contains circuitry to minimize turn-on transients

or “click and pops”. In this case, turn-on refers to either

power supply turn-on or the device coming out of shutdown

mode. When the device is turning on, the amplifiers are

internally muted. An internal current source ramps up the

voltage of the bypass pin. Both the inputs and outputs ideally

track the voltage at the bypass pin. The device will remain in

mute mode until the bypass pin has reached its half supply

voltage, 1/2 V

DD. As soon as the bypass node is stable, the

device will become fully operational.

Although the bypass pin current source cannot be modified,

the size of the bypass capacitor, C

B, can be changed to alter

the device turn-on time and the amount of “click and pop”. By

increasing C

B, the amount of turn-on pop can be reduced.

However, the trade-off for using a larger bypass capacitor is

an increase in the turn-on time for the device. Reducing C

will decrease turn-on time and increase “click and pop”.

There is a linear relationship between the size of C

B and the

turn-on time. Here are some typical turn-on times for differ-

ent values of C

0.01 µF

2 ms

0.1 µF

20 ms

0.22 µF

42 ms

0.47 µF

84 ms

1.0 µF

200 ms

4.7 µF

1sec

In order to eliminate “click and pop”, all capacitors must be

discharged before turn-on. Rapid on/off switching of the

device or shutdown function may cause the “click and pop”

circuitry to not operate fully, resulting in increased “click and

pop” noise.

DOCKING STATION

In an application such as a notebook computer, docking

station or line level outputs may be required. Pin 9 and Pin

13 can drive loads greater than 1k

Ω rail to rail. These pins

are tied to the output of the input op-amp to drive powered

speakers and other high impedance loads. Output coupling

capacitors need to be placed in series with the load. The

recommended values of the capacitors are between 0.33µF

to 1.0µF with the positive side of the capacitors toward the

IC. The outputs of the docking station pins cannot be attenu-

ated with the DC volume control. However the gain of the

outputs can be configured by adjusting the feedback and

input resistors for the input op-amp. The input op-amp is in

an inverting configuration where the gain is:

F /Ri =- Av

Note that by adjusting the gain of the input op-amp the

overall gain of the output amplifiers are also affected. Al-

though the single ended outputs of the output amplifiers can

be used to drive line level outputs, it is recommended to use

Pins 9 and 13 to achieve better performance.

BEEP DETECT FUNCTION

The Beep Detect pin (Beep In) 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

DD) is present at Beep In, the Beep Detect

circuitry will enable the bridged amplifiers. Beep In signals

less than 2.5V

P-P (or 1/2 VDD) will not trigger the Beep

Detect circuitry. 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. 1, a

200k

Ω resistor 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 required 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 LM4840 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

LM4840 contains a shutdown pin to externally turn off the

bias circuitry. The LM4840 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

DD to provide

maximum device performance. By switching the shutdown

pin to V

DD, the LM4840 supply current draw will be mini-

mized. While the device will be disabled with shutdown pin

voltages less than V

DD, 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

condition.

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 so-

lution is to use a single-pole, single-throw switch in conjuc-

tion 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 LM4840. This scheme prevents

the shutdown pin from floating.

HP-IN FUNCTION

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

headphone amplifier control pin. When this pin is left uncon-

nected, V

DD is applied to the HP−Sense. This turns off

Amp2B and switches Amp2A’s input signal from an audio

signal to the V

DD/2 voltage present on Bypass. The result is

muted bridge-connected loads. Quiescent current consump-

tion is reduced when the IC is in this single−ended mode.

Figure 2 shows the implementation of the LM4840’s head-

phone control function. An internal comparator with a nomi-

nal 400mV offset monitors the signal present at the −OUTB

output. It compares this signal against the signal applied to

the HP−Sense pin. When these signals are equal, as is the

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