Soft start circuit and method for DC-DC voltage regulator

A voltage regulator control circuit configured to couple to and control a PNP pass transistor of a voltage regulator, the pass transistor including a first node to receive an input voltage, a second node to provide a regulated voltage, and a control node, wherein the first node include an emitter, the second node includes a collector, and the control node includes a base, the control circuit comprising:

• an amplifier circuit coupled to produce a control voltage on a control line that is coupled to control a voltage at the control node of the pass transistor, based at least in part upon a reference voltage and the regulated voltage;
• a switch that includes a transistor configured to transition between a first switch state in which the switch operatively couples the control line to a turn-off voltage having a value to turn off the pass transistor and a second switch state in which the switch decouples the control line from the turn-off voltage;
• and
• a switch control circuit that includes a signal delay circuit configured to maintain the switch in the first switch state during a first time interval while the input voltage ramps up and to transition the switch to the second switch state after the first time interval;
• wherein the switch control circuit includes a field effect transistor and a delay circuit;
• wherein a source of the field effect transistor is coupled to receive the input voltage and a drain of the field effect transistor is coupled to provide a switch control signal to a control terminal of the switch transistor;
• and
• wherein an input of the delay circuit is coupled to receive the input voltage and an output of the delay circuit is operatively coupled to the gate of field effect transistor.

In a voltage regulator control circuit configured to couple to and control a pass transistor of a voltage regulator, the voltage regulator to receive a supply voltage and to provide a regulated voltage, the voltage regulator including an amplifier circuit to receive a reference voltage and an indication of the regulated voltage and to provide a control signal on a control line that is coupled to control turn on of the pass transistor, a method comprising:

• receiving a supply voltage ramp up at a supply node of the pass transistor;
• in response to receiving the supply voltage ramp up, coupling the control line to a turn-off voltage;
• and
• blocking current flow in the pass transistor during the supply voltage ramp up, to turn off the pass transistor for a time interval that is long enough for the control line to charge to at least a normal steady state value range;
• decoupling the turn-off voltage from the control line after the time interval;
• and
• regulating current flow within the pass transistor based upon a voltage of the control line after the supply voltage ramp up.