SA07 Datasheet, PDF (5/5 Pages) Cirrus Logic – PULSE WIDTH MODULATION AMPLIFIERS

OPERATING
CONSIDERATIONS
SA07
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Switching noise spikes will invariably be found at the I
SENSE pins. The noise spikes could trip the current limit
threshold which is only 100 mV. R_FILTER and C_FILTER should be
adjusted so as to reduce the switching noise well below 100
mV to prevent false current limiting. The sum of the DC level
plus the noise peak will determine the current limiting value.
As in most switching circuits it may be difficult to determine
the true noise amplitude without careful attention to grounding
of the oscilloscope probe. Use the shortest possible ground
lead for the probe and connect exactly at the GND terminal of
the amplifier. Suggested starting values are C_FILTER = .001uF,
R_FILTER = 5k .
The required value of R_LIMIT in voltage mode may be cal-
culated by:
R_LIMIT = .1 V / I_LIMIT
where R_LIMIT is the required resistor value, and I_LIMIT is the
maximum desired current. In current mode the required value
of each R_LIMIT is 2 times this value since the sense voltage is
divided down by 2 (see Figure B). If R_SHDN is used it will further
divide down the sense voltage. The shutdown divider network
will also have an effect on the filtering circuit.
SHUTDOWN
The shutdown circuitry makes use of the internal current
limiting circuitry. The two functions may be externally combined
in voltage and current modes as shown below in Figures A
and B. The R_LIMIT resistors will normally be very low values and
can be considered zero for this application. In Figure A, R_SHDN
and 1K form a voltage divider for the shutdown signal. After a
suitable noise filter is designed for the current limit, adjust the
value of R_SHDN to give a minimum 110 mV of shutdown signal
at the I LIMIT/SHDN pin when the shutdown signal is high.
Note that C_FILTER will filter both the current limit noise spikes
and the shutdown signal. Shutdown and current limit operate
on each cycle of the internal switching rate. As long as the
shutdown signal is high the output will be disabled.
PROTECTION CIRCUITS
Circuits monitor the temperature and load on each of the
bridge output transistors. On each cycle should any fault condi-
tion be detected all output transistors in the bridge are shut off.
Faults protected against are: shorts across the outputs, shorts
to ground, and over temperature conditions. Should any of
these faults be detected, the output transistors will be latched
off.* In addition there is a built in dead time during which all the
output transistors are off. The dead time removes the possibility
of a momentary conduction path through the upper and lower
transistors of each half bridge during the switching interval.
Noise or flyback may be observed at the outputs during this
time due to the high impedance of the outputs in the off state.
This will vary with the nature of the load.
* ^{To restart the SA07 remove the fault and recycle V}_CC ^or,
alternatively, toggle the I_LIMIT/SHDN (PIN16) with a shut
down pulse.
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SA07U REV. B DECEMBER 1998 © 1998 Apex Microtechnology Corp.
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