APPLICATION NOTES CONT'D
CURRENT LIMIT
The current limit function sets the peak limit on current flow in pin 8. This circuit can trip at anytime during the conduction
period and will hold the output transistors off for the remainder of that conduction period. It is recommended that RLIMIT
resistor be non-inductive. Load current flows in the ISENSE pin. To avoid errors due to lead length connect the SHDN/FILTER
pin directly to the RLIMIT resistor (through the filter network and shutdown divider resistor) and connect the RLIMIT
resistor directly to the +VS return. Switching noise spikes will invariably be found at the ISENSE pin. The noise spikes could
trip the current limit threshold which is only 200mV. RFILTER and CFILTER should be adjusted so as to reduce the switching
noise well below 200mV 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 pin of the amplifier. Suggested starting values are CFILTER=0.01µF, RFILTER=5K. The required value of RLIMIT
in voltage mode may be calculated by:
RLIMIT=0.2V/ILIMIT where RLIMIT is the required resistor value and ILIMIT is the maximum desired current. 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.
FIGURE 1
ERROR AMPLIFIER
The internal amplifier can be used to integrate the difference between command signals and feedback signals. For highest
loop accurancy it is best to configure the error amplifier as an integrator. Poles and zeros must be selected to provide circuit
stability while maximizing system respones. Specific component values will vary from system to system. The 7.5V refer-
ence output can be used to bias the non-inverting input of the error amplifier to the middle of its 2 to 8V common mode
voltage range. (See typical application circuit).
PROTECTION CIRCUITS
In addition to the externally programmable current limit there is also a fixed internal current limit which senses only the
highside current. The DC trip level is approximately 35A. Should either of the outputs be shorted to ground the highside
current limit will latch off the output transistors. It will be necessary to remove the fault condition and recycle power to
+VS to restart the circuit.
4
Rev. B 2/10
MSK [ M.S. KENNEDY CORPORATION ]
