SA09 datasheet(6/7 Pages) CIRRUS

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

SA09

SA09U

CURRENTLIMIT

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 RLIMITsense 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 CFILTER 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

exactly at the GND terminal of the amplifier. Suggested starting values are C

FILTER = 0.001uF, RFILTER = 5k .

The required value of R

LIMIT in voltage mode may be calculated by:

LIMIT = 0.1 V / ILIMIT

where R

LIMIT is the required resistor value, and ILIMIT 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 RSHDN 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.

PROTECTIONCIRCUITS

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.

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