SA09 Datasheet, PDF (6/7 Pages) Cirrus Logic – Pulse Width Modulation Amplifiers

SA09
Product Innova tionFrom
CURRENT LIMIT
There are two load current sensing pins, I SENSE A and I SENSE B. The two pins can be shorted in the voltage
mode connection but both must be used in the current mode connection (see figures A and B). It is recommended
that R_LIMIT resistors be non-inductive. Load current flows in the I SENSE pins. To avoid errors due to lead lengths
connect the I LIMIT/SHDN pin directly to the R_LIMIT resistors (through the filter network and shutdown divider resistor)
and connect the R_LIMIT resistors directly to the GND pin. Do not connect R_LIMIT sense resistors to the ground plane.
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 be-
low 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
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Suggested
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C_FILTER
=
0.001uF,
R_FILTER
=
5k
.
The required value of R_LIMIT in voltage mode may be calculated by:
R_LIMIT = 0.1 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
condition 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.

SA09U