CA3274

The CA3274 is a controlled current switch and may be used in general purpose switching applications that require specified maximum levels of current. The functional block diagram of the CA3274 is shown and a typical application circuit is shown in Figure 1. An internal emitter follower has 200mA of source drive output capability. The Control Input is a Schmitt trigger buffer amplifier for noise immunity in the environments typical of industrial and automotive control systems.

Current sensing in the emitter circuit of a power-darlington output stage is fed back from a sampling resistor to the sense input of the CA3274 which has a 335mV typical offset. For the example shown in Figure 1, a sampling resistor of 0.056W permits 6.0A (0.335/0.056) of current in the emitter of the output driver. When the current limiter is activated, the flag output changes state conditionally. If the control input is the "0" state, the flag output will remain in a "1" state. If CA3274  control input is in the "1" state and the sense input is less than the voltage reference level of 335mV, the flag output will remain in the "1" state. If the control input is the "1" state and the sense input is equal to or greater than the 335mV reference level, the flag output goes to the "0" state. The output flag switch may be usedto accurately establish dwell timing in automotive applications. When the control input goes to "0", the flag is reset to "1". Noiseimmunity hold-off is used to prevent pre-triggering of the flag output and is noted as tD in the timing diagram of Figure 2.

The flag output may be used for diagnostic feedback via the current sense input to detect a fault mode. In this case the sampled drive current is either from the emitter of the CA3274 internal power transistor or an external output amplifier, such as a darlington power transistor or power-FET output stage. The CA3274 has separate power and signal grounds to minimize transient-loop feedback to the input ground and thus prevent false triggering of the output. Optionally, the output from the CA3274 may be taken from the open collector (DRIVE IN) at pin 6. An external resistor at pin 6 may be used to set the level at which Q2 will saturate, providing additional limiting protection for the maximum forward-drive from the CA3274.