LM4681SQ datasheet(13/19 Pages) NSC

13 / 19 page

General Features

SYSTEM FUNCTIONAL INFORMATION

Modulation Technique

Unlike typical Class D amplifiers that use single-ended com-

parators to generate a pulse-width modulated switching

waveform and RC timing circuits to set the switching fre-

quency, the LM4681 uses a balanced differential floating

modulator. Oscillation is a result of injecting complimentary

currents onto the respective plates of a floating, on-die ca-

pacitor. The value of the floating capacitor and value of the

components in the modulator’s feedback network set the

nominal switching frequency at 450kHz. Modulation results

from imbalances in the injected currents. The amount of

current imbalance is directly proportional to the applied input

signal’s magnitude and frequency.

Using a balanced, floating modulator produces a Class D

amplifier that is immune to common mode noise sources

such as substrate noise. This noise occurs because of the

high frequency, high current switching in the amplifier’s out-

put stage. The LM4681 is immune to this type of noise

because the modulator, the components that set its switch-

ing frequency, and even the load all float with respect to

ground.

The balanced modulator’s pulse width modulated output

drives the gates of the LM4681’s H-bridge configured output

power MOSFETs. The pulse-train present at the power

MOSFETs’ output is applied to an LC low pass filter that

removes the 450kHz energy component. The filter’s output

signal, which is applied to the driven load, is an amplified

replica of the audio input signal.

Shutdown Function

The LM4681’s digitally controlled shutdown function allows

the user to place the amplifier in a shutdown mode while the

system power supply remains active. Activating shutdown

deactivates the output switching waveform and minimizes

the quiescent current. Through the SPI/I

2C digital interface,

the Mode control register’s Bit 0 is used to control the

LM4681’s shutdown function. A logic “0” activates shutdown,

whereas a logic “1” returns the amplifier to its operational

quiescent state. When the power supply voltage is first ap-

plied, the LM4681 is operating in the shutdown mode. For

more information on the digital interface, see the section

titled “SPI/I

2C Serial Digital Interface.”

Mute Function

The LM4681’s digitally-controlled mute function allows the

user to place the amplifier outputs in a muted mode while the

amplifier’s analog input signals remain active. Activating

mute internally removes the analog input signal from the

Class D and headphone amplifier inputs. While muted, the

amplifier inputs and outputs retain their V

DD/2 operational

bias. Through the SPI/I

2C digital interface, the Mode control

function. A logic “0” activates mute, whereas a logic “1”

deactivates mute. When the power supply voltage is first

applied, the LM4681’s headphone and Class D amplifier

outputs are muted. More information on the digital interface

is found in the section titled “SPI/I

2C Serial Digital Interface.”

Stereo Headphone Amplifier

The LM4681’s stereo headphone amplifier operates continu-

ously, even while the Class D amplifiers are active. When

using headphones to listen to program material, it is usually

desirable to stop driving external speakers. This is easily

achieved by using the active low HPSEL input. As shown in

typical application schematic in Figure 1, with no head-

phones connected to the headphone jack the input voltage

applied to the HPSEL pin is a logic low. In this state, the

Class D amplifiers are active and able to drive external

speakers. When headphones are plugged into the head-

phone jack, the switch internal to the jack is opened. This

changes the voltage applied to the HPSEL pin to a logic

high, shutting off the LM4681’s Class D amplifiers.

Under Voltage Proctection

The under voltage protection disables the output driver sec-

tion of the LM4681 while the supply voltage is below 8V. This

condition may occur as power is first applied or during low

line conditions, changes in load resistance, or when power

supply sag occurs. The under voltage protection ensures

that all of the LM4681’s power MOSFETs are off. This action

eliminates shoot-through current and minimizes output tran-

sients during turn-on and turn-off. The under voltage protec-

tion gives the digital logic time to stabilize into known states,

further minimizing turn output transients.

Power Supply Sequencing

To ensure best performance, please observe the following

power-up sequence. The I

2CV

DD supply voltage should be

applied first. Do not send any data to the LM4681’s internal

registers until the V

DD is applied. This takes advantage of the

LM4681’s power-on reset, which activates the amplifier’s

shutdown and mute. Once I

2CV

DD is applied, apply the VDD

supply voltage. Prior to removing the two supply voltages,

activate shutdown and mute.

Turn-On Time

The LM4681 has an internal timer that determines the am-

plifier’s turn-on time. After power is first applied or the part

returns from shutdown, the nominal turn-on time is 600ms.

This delay allows all externally applied capacitors to charge

to a final value of V

DD/2. Further, during turn-on, the outputs

are muted. This minimizes output transients that may occur

while the part settles into is quiescent operating mode.

Output Stage Current Limit and Fault Detection

Protection

The output stage MOSFETs are protected against output

conditions that could otherwise compromise their operational

status. The first stage of protection is output current limiting.

When conditions that require high currents to drive a load,

the LM4681’s current limit circuitry clamps the output current

at a nominal value of 2.5A. The output waveform is present,

but may be clipped or its amplitude reduced. The same 2.5A

nominal current limit also occurs if the amplifier outputs are

shorted together or either output is shorted to V

DD or GND.

The second stage of protection is an onboard fault detection

circuit that continuously monitors the signal on each output

MOSFET’s gate and compares it against the respective

drain voltage. When a condition is detected that violates a

MOSFET’s Safe Operating Area (SOA), the drive signal is

disconnected from the output MOSFETs’ gates. The fault

detect circuit maintains this protective condition for approxi-

mately 600ms, at which time the drive signal is reconnected.

If the fault condition is no longer present, normal operation

resumes. If the fault condition remains, however, the drive

signal is again disconnected.

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