MSA240 Datasheet, PDF (4/5 Pages) Cirrus Logic – PULSE WIDTH MODULATION AMPLIFIER

MSA240
Product Innova tionFrom
GENERAL
Please read Application Note 30 “PWM Basics”. Refer also
to Application Note 1 “General Operating Considerations” for
helpful information regarding power supplies, heat sinking,
mounting, SOA interpretation, and specification interpretation.
Visit www.Cirrus.com for design tools that help automate tasks
such as calculations for stability, internal power dissipation,
current limit, heat sink selection, Apex Precision Power’s com-
plete Application Notes library, Technical Seminar Workbook
and Evaluation Kits.
OSCILLATOR
The MSA240 includes a user frequency programmable
oscillator. The oscillator determines the switching frequency
of the amplifier. The switching frequency of the amplifier is 1/2
the oscillator frequency. Two resistor values must be chosen
to properly program the switching frequency of the amplifier.
One resistor, R_OSC, sets the oscillator frequency. The other
resistor, R_RAMP, sets the internal ramp amplitude. In all cases
the ramp voltage will oscillate between 1.5V and 3.5V. See
Figure 1. If an external oscillator is applied use the equations
to calculate R_RAMP .
To program the oscillator, R_OSC is given by:
R_OSC = (1.32X10⁸ / F) - 2680
where F is the desired switching frequency and:
R_RAMP = 2 X R_OSC
Use 1% resistors with 100ppm drift (RN55C type resistors,
for example). Maximum switching frequency is 50kHz.
Example:
If the desired switching frequency is 22kHz then R_OSC = 3.32K
and R_RAMP = 6.64K. Choose the closest standard 1% values:
R_OSC = 3.32K and R_RAMP = 6.65K.
FIGURE 1. EXTERNAL OSCILLATOR CONNECTIONS
R_OSC
R_RAMP
22
24 21 20
1
PWM AMPLIFIER
SHUTDOWN
The MSA240 output stage can be turned off with a shutdown
command voltage applied to Pin 10 as shown in Figure 2. The
shutdown signal is OR’ed with the current limit signal and
simply overrides it. As long as the shutdown signal remains
high the output will be off.
CURRENT SENSING
The low side drive transistors of the MSA240 are brought
out for sensing the current in each half bridge. A resistor from
each sense line to PWR GND (pin 58) develops the current
sense voltage. Choose R and C such that the time constant
is equal to 10 periods of the selected switching frequency. The
internal current limit comparators trip at 200mV. Therefore,
current limit occurs at I = 0.2/R_SENSE for each half bridge. See
Figure 2. Accurate milliohm power resistors are required and
there are several sources for these listed in the Accessories
Vendors section of the Databook.
FIGURE 2. CURRENT LIMIT WITH OPTIONAL SHUTDOWN
PWM AMPLIFIER
PWR
GND
58 10
9R
5V SHDN
SIGNAL
C
C
7R
R
40-43 54-57
Rs A
Rs B
POWER SUPPLY BYPASSING
Bypass capacitors to power supply terminals +V_S must be
connected physically close to the pins to prevent local parasitic
oscillation and overshoot. All +V_S pins must be connected
together. Place an electrolytic capacitor of at least 10µF per
output amp required midpoint between these sets of pins. In
addition place a ceramic capacitor 1µF or greater directly at
each set of pins for high frequency bypassing. V_CC is bypassed
internally.
GROUNDING AND PCB LAYOUT
Switching amplifiers combine millivolt level analog signals
and large amplitude switching voltages and currents with fast
rise times. As such grounding is crucial. Use a single point
ground at SIG GND (pin 26). Connect signal ground pins 2 and
18 directly to the single point ground on pin 26. Connect the
digital return pin 23 directly to pin 26 as well. Connect PWR
GND pin 58 also to pin 26. Connect AC BACKPLATE pin 28
also to the single point ground at pin 26. Connect the ground
terminal of the V_CC supply directly to pin 26 as well. Make sure
no current from the load return to PWR GND flows in the analog
signal ground. Make sure that the power portion of the PCB
layout does not pass over low-level analog signal traces on
the opposite side of the PCB. Capacitive coupling through the
PCB may inject switching voltages into the analog signal path.
Further, make sure that the power side of the PCB layout does
not come close to the analog signal side. Fast rising output
signal can couple through the trace-to-trace capacitance on
the same side of the PCB.
DETERMINING THE OUTPUT STATE
The input signal is applied to +IN (Pin 13) and varies from
1.5 to 3.5 volts, zero to full scale. As +IN varies from 1.5 to 2.5
volts theA output "high" duty cycle (relative to ground) is greater
than the B output "high" duty cycle. The reverse occurs as the
input signal varies from 2.5 to 3.5 volts. When +IN = 2.5 volts
the duty cycles of both A and B outputs are 50%. Consequently,
when the input voltage is 1.5V the A output is close to 100%
duty cycle and the B output is close to 0% duty cycle. The
reverse occurs with an input voltage of 3.5V. The output duty
cycle extremes vary somewhat with switching frequency and
are internally limited to approximately 5% to 95% at 10kHz and
7% to 93% at 50kHz.
4
MSA240U