LM4651 datasheet(10/20 Pages) NSC

10 / 20 page

Application Information

GENERAL FEATURES

System Functional Information: The LM4651 is a conven-

tional pulse width modulator/driver. As Figure 2 shows the

incoming audio signal is compared with a triangle waveform

with a much higher frequency than the audio signal (not

drawn to scale). The comparator creates a variable duty

cycle squarewave. The squarewave has a duty cycle propor-

tional to the audio signal level. The squarewave is then

properly conditioned to drive the gates of power MOSFETs in

an H-bridge configuration, such as the LM4652. The pulse

train of the power MOSFETs are then fed into a low pass

filter (usually a LC) which removes the high frequency and

delivers an amplified replica of the audio input signal to the

load.

Standby Function: The standby function of the LM4651 is

CMOS compatible, allowing the user to perform a muting of

the music as well as turning off all power MOSFETs by

shutting down the pulse width waveform. Standby has the

added advantage of minimizing the quiescent current. Be-

cause standby shuts down the pulse width waveform, the

attenuation of the music is complete (>120dB), EMI is mini-

mized, and any output noise is eliminated since there is no

modulation waveform. By placing a logic ’1’ or 5V at pin 13,

the standby function will be enabled. A logic ’0’ or 0V at pin

13 will disable the standby function allowing modulation by

the input signal.

Under Voltage Protection: The under voltage protection

disables the output driver section of the LM4651 while the

supply voltage is below ± 10.5V. This condition can occur as

power is first applied or when low line, changes in load

resistance or power supply sag occurs. The under voltage

protection ensures that all power MOSFETs are off, eliminat-

ing any shoot-through current and minimizing pops or clicks

during turn-on and turn-off. The under voltage protection

gives the digital logic time to stabilize into known states

providing a popless turn on.

Start Up Sequence and Self-Diagnostic Timing: The

LM4651 has an internal soft start feature (see Figure 3) that

ensures reliable and consistent start-up while minimizing

turn-on thumps or pops. During the start-up cycle the system

is in standby mode. This start-up time is controlled externally

by adjusting the capacitance (C

START) value connected to

the START pin. The start-up time can be controlled by the

capacitor value connected to the START pin given by Equa-

tion (1) or (2):

START = (8.4x10

4)C

START

(seconds)

(1)

START =TSTART/(8.5x10

(Farads)

(2)

The value of C

START sets the time it takes for the IC to go

though the start-up sequence and the frequency that the

diagnostic circuitry checks to see if an error condition has

been corrected. An Error condition occurs if current limit,

thermal shut down, under voltage detection, or standby are

sensed. The self-diagnostic circuit checks to see if any one

of these error flags has been removed at a frequency set by

the C

START capacitor. For example, if the value of CSTART is

10µF then the diagnostic circuitry will check approximately

every second to see if an error condition has been corrected.

If the error condition is no longer present, the LM4651/52 will

return to normal operation.

Current Limiting and Short Circuit Protection: The resis-

tor value connected between the SCKT pin and GND deter-

mines the maximum output current. Once the output current

is higher than the set limit, the short circuit protection turns

all power MOSFETs off. The current limit is set to a minimum

of 10A internally but can be increased by adjusting the value

of the R

SCKT resistor. Equation (3) shows how to find RSCKT.

SCKT = 1X10

5/(10k

Ω\ R

SCKT)

(Amps)

(3)

This feature is designed to protect the MOSFETs by setting

the maximum output current limit under short circuit condi-

tions. It is designed to be a fail-safe protection when the

output terminals are shorted or a speaker fails and causes a

short circuit condition.

Thermal Protection The LM4651 has internal circuitry (pin

12) that is activated by the thermal shutdown output signal

from the LM4652 (pin 4). The LM4652 has thermal shut

down circuitry that monitors the temperature of the die. The

voltage on the TSD pin (pin 4 of the LM4652) goes high (6V)

once the temperature of the LM4652 die reaches 150˚C.

This pin should be connected directly to the TSD pin of the

LM4651 (pin 12). The LM4651 disables the pulse width

waveform when the LM4652 transmits the thermal shutdown

flag. The pulse width waveform remains disabled until the

TSD flag from the LM4652 goes low, signaling the junction

temperature has cooled to a safe level.

Dead Time Setting The DELAY pin on the LM4651 allows

the user to set the amount of dead time or break before

make of the system. This is the amount of time one pair of

FETs are off before another pair is switched on. Increased

dead time will reduce the shoot through current but has the

disadvantage of increasing THD. The dead time should be

DS101277-1

FIGURE 2. Conventional Pulse Width Modulation

DS101277-70

FIGURE 3. Startup Timing Diagram

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