Technique for combining in-rush current limiting and short circuit current limiting
AssigneeFairchild Semiconductor Corporation
InventorJames Garrett
Life of this patent
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Abstract
A circuit that protects from high power-on in-rush currents and short circuits. The circuit has a pass transistor and a parallel smaller transistor. A comparator senses when an output voltage crosses a reference and turns off the pass transistor and turns on the parallel smaller transistor. The parallel smaller transistor has a higher “on” resistance so that the short circuit or the in-rush current does not harm the electronics. When the short circuit or in-rush current condition is removed, the comparator senses this condition and returns to the normal operation where the pass transistor is on and the parallel small transistor is off.
Claims9
A circuit that limits current and protects against short circuits, the circuit comprising:
- a pass transistor defining a first input, a first output and a first control, wherein the input is connected to an input voltage, the first output delivers an output current to a load;
- a second transistor defining a second input, a second output and a second control, wherein the first and second inputs and outputs are connected together, and wherein the second transistor is smaller than the pass transistor;
- a reference;
- a comparator with one input connected to the first and second outputs, a second input connected to the reference, and the comparator having a compare output that drives the second control;
- a logic gate with an input connected to the compare output, wherein the logic gate output drives the first control, wherein when the first and second outputs are at a lower voltage than the reference, the compare output turns on the second transistor and turns off the pass transistor.
A process for protecting a system from excessive output currents higher, the process comprising the steps of:
- passing the output current through a pass transistor;
- setting an output voltage level that correlates to the excessive output current level;
- sensing the output voltage level passing the set output voltage level;
- and in response,
- turning off the pass transistor and turning on a second transistor.
Dependencies are inferred from the claim text.