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4

OPERATING

CONSIDERATIONS

MSA260

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 interpreta-

tion. Visit www.apexmicrotech.com for design tools that help

automate tasks such as calculations for stability, internal power

dissipation, current limit, heat sink selection, Apex’s complete

Application Notes library, Technical Seminar Workbook and

Evaluation Kits.

OSCILLATOR

The MSA260 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

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:

If the desired

FIGURE 1. EXTERNAL OSCILLATOR CONNECTIONS

SHUTDOWN

The MSA260 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 MSA260 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 SHUT-

DOWN

POWER SUPPLY BYPASSING

connected physically close to the pins to prevent local parasitic

Place and electrolytic capacitor of at least 10µF per output amp

required midpoint between these sets of pins. In addition place

a ceramic capacitor 1.0µF or greater directly at

each set of pins

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

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. The ramp also varies over

the same range. When:

The output duty cycle extremes vary somewhat with switch-

ing frequency and are internally limited to approximately 5%

to 95% at 10kHz and 7% to 93% at 50kHz.

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