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.
OPERATING
CONSIDERATIONS
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
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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
amplifier. One resistor, R
OSC
, sets the oscillator frequency.
The other resistor, R
RAMP
, sets the 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
= (1.32X10
8
/ 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 or simply use two
of selected R
OSC
in series for R
RAMP
.
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Vendors section of the Databook.
FIGURE 2. CURRENT LIMIT WITH OPTIONAL SHUT-
DOWN
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
must be connected together.
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
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.
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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.
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:
Ramp > +IN A
OUT
> B
OUT
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.
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
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