LX1721 datasheet(7/14 Pages) MICROSEMI

7 / 14 page

Microsemi

Linfinity Microelectronics Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 7

Rev. 1.2a,2004-06-10

AudioMAX™

LX1721 / 1722

Class-D Stereo Power Amplifier Controller

PRODUCTION

I N T E GRA T ED

PR ODUC T S

APPLICATION INFORMATION

Frequency Synchronization

Two or more LX1721 / LX1722 oscillators can be configured for

synchronous operation. One unit, the master, is programmed for

the desired frequency with the RPWM and CPWM as usual.

Additional units will be slave units, and their oscillators will be

disabled by leaving the RPWM pin disconnected. The CLOCK pin

and the CPWM pin of the slave units should be tied to the CLOCK

pin and the CPWM pin of the master unit respectively. In this

configuration, the CLOCK pins of the slave units begin receiving

instead of transmitting clock pulses. Also, the CPWM pins quit

driving the PWM capacitor in the slave units. Note that for

optimum performance, all slave units should be located within a

few inches of the master unit.

Oscillator Configuration (RPWM and CPWM selection)

The oscillator is programmed by the external timing components

RPWM and CPWM. For a nominal frequency of 333kHz, RPWM and

CPWM should be set to 49.9kOhms and 100pF respectively. Note that

in order to keep the slope of the PWM ramp voltage proportional to

the supply voltage, both the ramp peak and valley voltages, and the

charge and discharge currents are proportional to the supply voltage.

This keeps the frequency relatively constant while keeping the slope

of the PWM ramp proportional to the voltage on the VDD pin. For

operating frequencies other than 333kHz, the frequency can be

approximated by the following equation:

PWM

320

)

)(

577

(

Frequency

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