P r o d u c t I n n o v a t i o n F r o m

MSA240

4

MSA240U

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.

the ramp voltage will oscillate between 1.5V and 3.5V. See

Figure 1. If an external oscillator is applied use the equations

where F is the desired

switching frequency and:

Use 1% resistors with 100ppm drift (RN55C type resistors,

for example). Maximum

switching frequency is 50kHz.

Example:

FIGURE 1. EXTERNAL OSCILLATOR CONNECTIONS

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,

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

POWER SUPPLY BYPASSING

connected physically close to the pins to prevent local parasitic

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

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

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 bothA 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.

Rs A

Rs B

40-43

54-57

PWR

GND

58

PWM AMPLIFIER

R

R

7

10

C

C

9R

5V SHDN

SIGNAL

24

20

1

22

PWM AMPLIFIER

21