LM4842 datasheet(20/31 Pages) NSC

20 / 31 page

Application Information (Continued)

(7)

As an example when using a speaker with a low frequency

limit of 150Hz, the input coupling capacitor using Equation

(7), is 0.053µF. The 0.33µF input coupling capacitor shown

in Figure 1 allows the LM4842 to drive high efficiency, full

range speaker whose response extends below 30Hz.

TURN ON Characteristics

The LM4842 contains advanced circuitry that minimizes

turn-on and shutdown transients or “clicks and pops”. For

this discussion, turn-on refers to either applying the power

supply voltage or when the shutdown mode is deactivated.

While the power supply is ramping to its final value, the

LM4842’s internal amplifiers are configured as unity gain

buffers. An internal current source changes the voltage of

the BYPASS pin in a controlled, linear manner. Ideally, the

input and outputs track the voltage applied to the BYPASS

pin. The gain of the internal amplifiers remains unity until the

voltage on the BYPASS pin reaches 1/2 V

DD. As soon as the

voltage on the BYPASS pin is stable, the LM4842 is ready to

be fully turned on. To turn the device on, the input signal

must exceed 22mV

rms.

This is accomplished through a

threshold detect circuit that enables all appropriate output

amplifiers after the 22mVrms limit is reached. Until this

threshold is reached, some of the amplifiers remain in a

tri-state mode. This insures that there is no current flowing

through to the speakers or headphones during power up.

Without current flow, the speakers or headphones remain

silent. During headphone mode, A+, B-, and B+ are in tri-

state mode during power up. During speaker mode, A+ and

B+ are in tri-state mode during power up.

Although the BYPASS pin current cannot be modified,

changing the size of C

BYPASS alters the device’s turn-on

time. As the size of C

BYPASS increases, the turn-on time

increases. There is a linear relationship between the size of

BYPASS and the turn-on time. Here are some typical turn-on

times for various values of C

BYPASS:

BYPASS

0.01µF

2ms

0.1µF

20ms

0.22µF

44ms

0.47µF

94ms

1.0µF

200ms

DOCKING STATION INTERFACE

Applications such as notebook computers can take advan-

tage of a docking station to connect to external devices such

as monitors or audio/visual equipment that sends or receives

line level signals. The LM4842 has two outputs, Dock A and

Dock B, which connect to outputs of the internal input am-

plifiers that drive the volume control inputs. These input

amplifiers can drive loads of >1k

Ω (such as powered speak-

ers) with a rail-to-rail signal. Since the output signal present

on the Dock A and Dock B pins are biased to V

DD/2, coupling

capacitors should be connected in series with the load when

using these outputs. Typical values for the output coupling

capacitors are 0.33µF to 1.0µF. If polarized coupling capaci-

tors are used, connect their ’+’ terminals to the respective

output pin.

Since the Dock outputs precede the internal volume control,

the signal amplitude will be equal to the input signal’s mag-

nitude and cannot be adjusted. However, the input amplifi-

er’s closed-loop gain can be adjusted using external resis-

tors. These 20k resistors (R

FA and RFB) are shown in Figure

2 and they set each input amplifier’s gain to -1. Use Equation

7 to determine the input and feedback resistor values for a

desired gain.

-A

v =RF /RIN

(8)

Adjusting the input amplifier’s gain sets the minimum gain for

that channel. Although the single ended output of the Bridge

Output Amplifiers can be used to drive line level outputs, it is

recommended that the A & B Dock Outputs simpler signal

path be used for better performance.

MUX CONTROL AND MODE PINS

The LM4842 has two pairs of stereo inputs. The MUX CON-

TROL pin determines which of these stereo pairs of inputs

are active. Applying 0V to the MUX CONTROL pin (with the

Mode pin tied low) selects inputs A0 and B0. Applying V

DD to

the MUX CONTROL (with the Mode pin tied low) pin selects

inputs A1 and B1 (See Truth Table for Logic Inputs).

When the Mode pin is pulled high the MUX Control is dis-

abled and the Headphone Sense Pin controls the A and B

input pairs as follows:

Headphone Sense enabled (Headphone plugged in) se-

lects inputs A1 and B1.

Headphone Sense disabled (Headphone not plugged in)

selects inputs A0 and B0.

When the MODE pin is high the HP SENSE controls the

MUX inputs. This configuration can be very useful, for ex-

ample, when the headphone acoustic output needs to be

much lower than the BTL Speaker Output. Two different

gains can be hardwired into the two pairs of MUX input

circuits. This allows the user to have the HP SENSE auto-

matically change the output level for either of the MUX input

pairs by switching between the two pairs of MUX inputs, if

the same input signal is connected to both MUX inputs.

When the MODE pin is low the MUX pin switches the MUX

input pairs directly.

BEEP DETECT FUNCTION

Computers and notebooks produce a system ’beep’ signal

that drives a small speaker. The speaker’s auditory output

signifies that the system requires user attention or input. To

accommodate this system alert signal, the LM4842’s beep

input pin is a mono input that accepts the beep signal.

Internal level detection circuitry at this input monitors the

beep signal’s magnitude. When a signal level greater than

DD/2 is detected on the BEEP IN pin, the bridge output

amplifiers are enabled. The beep signal is amplified and

applied to the load connected to the output amplifiers. A valid

beep signal will be applied to the load even when MUTE is

active. Use the input resistors connected between the BEEP

IN pin and the stereo input pins to accommodate different

beep signal amplitudes. These resistors are shown as

200k

Ω devices in Figure 1. Use higher value resistors to

reduce the gain applied to the beep signal. The resistors

must be used to pass the beep signal to the stereo inputs.

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