Squib driver diagnostic circuit and method

An air bag deployment system for a vehicle, comprising:

• a squib configured to fire when it receives a fire current;
• an undeployed air bag configured to deploy when the squib fires;
• and
• a fire current supply circuit, including
• a field-effect transistor (FET) having a source connected to a first output node, a drain connected to a supply node, and a gate;
• a first switch configured to selectively connect a current supply node to one of the gate of the FET and a test node;
• a second switch connected between the first output node and a second output node;
• a test and drive current source configured to selectively provide one of a drive current and a test current to the current supply node;
• a fire current source configured to provide a fire current to one of the supply node and the second output node;
• an error-detecting circuit connected to the test node, a reference terminal, and an error node, the error-detecting circuit being configured to generated an error signal to the error node that indicates whether a value of an error-detecting parameter at the test node exceeds a value of a reference parameter at the reference terminal;
• and
• a control circuit configured to generate control signals to control operation of the test and drive current source, the first switch, and the second switch,
• wherein
• a one of the supply node and the second output node that is not connected to the fire current source is connected to a ground voltage,
• the test node is connected to the ground voltage, and
• the squib is connected between the first output node and the second output node.

A method of testing and operating a squib detonation circuit for an air bag system in a vehicle, the circuit containing first and second squib connectors configured to connect to a squib, and a field effect transistor (FET) connected at its drain to a supply node and at its source to the first squib connector, the method comprising:

• connecting the squib to the first and second squib connectors during a first duration;
• providing a test current to a gate of the FET during the first duration;
• determining whether a first error-detecting parameter at a test node is greater than a reference parameter at the end of the first duration;
• and
• sending a first error signal indicating failure of the FET if the first error-detecting parameter at the second squib connector is not determined to be greater than the reference parameter at the end of the first duration;
• providing a drive current to the second squib connector during a second duration;
• determining whether a second error-detecting parameter at the second squib connector is greater than the reference parameter at the end of the second duration;
• and
• sending a second error signal indicating failure of a drive current source if the second error-detecting parameter at the second squib connector is not determined to be greater than the reference parameter at the end of the second duration,
• wherein
• one of the supply node and the second squib connector is connected to a fire current supply,
• a one of the supply node and the second squib connector that is not connected to the fire current supply is connected to a ground voltage,
• the test node is connected to the ground voltage, and
• the first duration is shorter than the turn-on time of the FET.