Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
Copyright © 2001
Rev. 1.2a,2004-06-10
WWW.Microsemi .COM
AudioMAX™ LX1721 / 1722
Class-D Stereo Power Amplifier Controller
INTEGRATED PRODUCTS
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 R
PWM
and C
PWM
as usual.
Additional units will be slave units, and their oscillators will be
disabled by leaving the R
PWM
pin disconnected. The CLOCK pin
and the C
PWM
pin of the slave units should be tied to the CLOCK
pin and the C
PWM
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 C
PWM
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 (R
PWM
and C
PWM
selection)
The oscillator is programmed by the external timing components
R
PWM
and C
PWM
. For a nominal frequency of 333kHz, R
PWM
and
C
PWM
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:
nsCR PWMPWM 320))()(577.0(
1
Frequency
+
=
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
