Methods and apparatus for soft start and soft turnoff of linear voltage regulators

A method of soft starting a linear regulator having a pass transistor and a pass transistor controller comprising:

• providing an RC circuit coupled to the control terminal of the pass transistor;
• during startup, controlling the pass transistor through the RC circuit so that the pass transistor allows current to flow to an output of the linear regulator at a rate responsive to the RC time constant of the RC circuit;
• and
• after startup, de-coupling the RC circuit from the control terminal of the pass transistor, coupling the pass transistor controller to the control terminal of the pass transistor, and controlling the pass transistor by the pass transistor controller to regulate a voltage at the output of the linear regulator.

A soft shutdown linear regulator having an input terminal, an output terminal and a common terminal, comprising:

• a pass transistor and a second transistor, each transistor having first and second terminals and a control terminal, the voltage between the first terminal and the control terminal controlling the conduction between the first and second terminals of the respective transistor;
• a pass transistor controller coupled to the output terminal and the control terminal of the pass transistor;
• the pass transistor having its first terminal coupled to the input terminal and its second terminal coupled to the output terminal;
• the second transistor having its first terminal coupled to the first terminal of the pass transistor, its control terminal coupled to an R-C circuit and its second terminal coupled to the control terminal of the first transistor;
• and,
• a shutdown control circuit responsive to a shutdown signal having two states to control the R-C circuit, the shutdown control circuit responding to a change in the shutdown signal from the first state to the second state to turn on the second transistor with a time constant associated with the R-C circuit.

A soft start and soft shutdown linear regulator having an input terminal, an output terminal and a common terminal, comprising:

• a pass transistor and second and third transistors, each transistor having first and second terminals and a control terminal, the voltage between the first terminal and the control terminal controlling the conduction between the first and second terminals of the respective transistor;
• a pass transistor controller coupled to the output terminal and the control terminal of the pass transistor;
• a resistor;
• a capacitor;
• the pass transistor having its first terminal coupled to the input terminal and its second terminal coupled to the output terminal;
• the second transistor having its first terminal coupled to the first terminal of the pass transistor, its control terminal coupled to the output terminal and its second terminal coupled to the capacitor, the capacitor being coupled to the resistor and the control terminal of the pass transistor, the resistor being coupled to the common terminal;
• the voltage across the resistor controlling the voltage on the control terminal of the pass transistor during startup and the pass transistor controlling the voltage on the control terminal of the pass transistor after startup;
• the third transistor having its first terminal coupled to the first terminal of the pass transistor, its control terminal coupled to an R-C circuit and its second terminal coupled to the control terminal of the first transistor;
• and,
• a shutdown control circuit responsive to a shutdown signal having two states to control the R-C circuit, the shutdown control circuit responding to a change in the shutdown signal from the first state to the second state to turn on the third transistor with a time constant associated with the R-C circuit.