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 LM4680 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 and sets

the nominal switching frequency at 450kHz. Modulation re-

sults 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 LM4680 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 LM4680’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 LM4680’s active-low shutdown function allows the user

to place the amplifier in a shutdown mode while the system

power supply remains active. Activating shutdown deacti-

vates the output switching waveform and minimizes the

quiescent current. Applying logic 0 (GND) to pin 8 enables

the shutdown function. Applying logic 1 (4V

≤ V

LOGIC

≤ V

to pin 8 disables the shutdown function and restores full

amplifier operation.

Under Voltage Proctection

The under voltage protection disables the output driver sec-

tion of the LM4680 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 LM4680’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.

Turn-On Time

The LM4680 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

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 LM4680’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

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) and 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.

Thermal Protection

The LM4680 has thermal shutdown circuitry that monitors

the die temperature. Once the LM4680 die temperature

reaches 170˚C, the LM4680 disables the output switching

waveform and remains disabled until the die temperature

falls below 140˚C (typ).

Over-Modulation Protection

The LM4680’s over-modulation protection is a result of the

preamplifier’s (AMP1 and AMP2, Figure 1) inability to pro-

duce signal magnitudes that equal the power supply volt-

ages. Since the preamplifier’s output magnitude will always

be less than the supply voltage, the duty cycle of the ampli-

fier’s switching output will never reach zero. Peak modula-

tion is limited to a nominal 95%.

Application Hints

SUPPLY BYPASSING

Correct power supply bypassing has two important goals.

The first is to reduce noise on the power supply lines and

minimize deleterious effects that the noise may cause to the

amplifier’s operation. The second is to help stabilize an

unregulated power supply and to improve the supply’s tran-

sient response under heavy current demands. These two

goals require different capacitor value ranges. Therefore,

various types and values are recommended for supply by-

passing. For noise de-coupling, generally small ceramic ca-

pacitors (0.01µF to 0.1µF) are recommended. Larger value

(1µF to 10µF) tantalum capacitors are needed for the tran-

sient current demands. These two capacitors in parallel will

do an adequate job of removing most noise from the supply

rails and providing the necessary transient current. These

capacitors should be placed as close as possible to each

IC’s supply pin(s) using leads as short as possible.

The LM4680 has two V

signal V

ceramic (0.1µF) and high value tantalum (10µF) should be

used to bypass the PV

or tantalum can be used to bypass the SV

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