LM4651 Datasheet, PDF (12/34 Pages) National Semiconductor (TI) – LM4651 & LM4652 Overture™ Audio Power Amplifier 170W Class D Audio Power Amplifier Solution

LM4651, LM4652
OBSOLETE
SNOS507H – MAY 2000 – REVISED APRIL 2013
APPLICATION INFORMATION
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GENERAL FEATURES
System Functional Information
The LM4651 is a conventional pulse width modulator/driver. As Figure 20 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 proportional 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.
Figure 20. Conventional Pulse Width Modulation
Standby Function
The standby function of the LM4651 is CMOS compatible, allowing the user to perform a muting of the music by
shutting down the pulse width waveform. Standby has the added advantage of minimizing the quiescent current.
Because standby shuts down the pulse width waveform, the attenuation of the music is complete (>120dB), EMI
is minimized, and any output noise is eliminated since there is no modulation waveform. When in Standby mode,
the outputs of the LM4652 will both be at V_CC. 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, eliminating 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 21) 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 Equation 1 or Equation 2:
t_START = (8.4x10⁴)C_START (seconds)
(1)
C_START = T_START/(8.5x10⁴) (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 C_START 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.
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