VOLTAGE REGULATOR CIRCUIT AND METHOD FOR LIMITING INRUSH CURRENT
A voltage regulator circuit with two series connected resistors connected together at a common resistor node. An initial conducting path has a first pass transistor with a first terminal connected to a first supply rail and a second terminal connected to a regulated output voltage. A main conducting path has a second pass transistor with a first terminal connected to the first supply rail and a second terminal connected to the regulated output voltage. A comparator has an input connected to the common resistor node output is connected to a gate of the first pass transistor. A voltage threshold switch is connected to the common resistor node, and based on the voltage at the common resistor node the voltage threshold switch selectively connects a gate of the second pass transistor to the comparator output.
1 . A voltage regulator circuit comprising:
two series connected resistors connected together at a common resistor node;
a first pass transistor with a first terminal connected to a first supply rail and a second terminal connected to a second supply rail through the two series connected resistors, the second terminal of the first pass transistor being connected to a regulated voltage output node;
a second pass transistor with a first terminal connected to the first supply rail and a second terminal connected to the second supply rail through the two series connected resistors, the second terminal of the second pass transistor being connected to the regulated voltage output node;
a comparator with a first comparator input connected to a reference voltage line, a second comparator input connected to the common resistor node and a comparator output connected to a control node of the first pass transistor; and
a voltage threshold switch with a voltage sensing input node connected to the common resistor node, an input switch node connected to the comparator output and an output switch node connected to a control node of the second pass transistor,
wherein the voltage regulator circuit is configured so that when the first pass transistor and second pass transistor are both conducting, current flowing through the first pass transistor is less than current flowing through the second pass transistor, and wherein when the voltage sensing input node reaches a threshold voltage value, the voltage threshold switch connects the control node of the second pass transistor to an error signal voltage at the comparator output so that the comparator output controls conductivity of the second pass transistor, thereby regulating the output voltage at the regulated voltage output node.
2 . The voltage regulator circuit of claim 1 , wherein during initial activation of the voltage regulator circuit an inrush current is regulated by the error signal voltage at the comparator output that controls conductivity of the first pass transistor by monitoring a feedback voltage value at the common resistor node.
3 . The voltage regulator circuit of claim 2 , wherein the conductivity of the first pass transistor limits the inrush current.
4 . The voltage regulator circuit of claim 1 , wherein the second pass transistor has an aspect ratio greater than that of the first pass transistor.
5 . The voltage regulator circuit of claim 1 , further comprising a first control switch connected to the first pass transistor, the first control switch being arranged to selectively switch off the first pass transistor.
6 . The voltage regulator circuit of claim 5 , wherein the first control switch is connected across the control node and first terminal of the first pass transistor.
7 . The voltage regulator circuit of claim 6 , further comprising a second control switch connected to the second pass transistor, wherein the second control switch selectively switches off the second pass transistor.
8 . The voltage regulator circuit of claim 7 , wherein the second control switch is connected across the control node and first terminal of the second pass transistor.
9 . The voltage regulator circuit of claim 1 , further comprising a capacitor connected between the regulated voltage output node and the second supply rail.
10 . The voltage regulator circuit of claim 1 , wherein the voltage threshold switch comprises:
a switch transistor connected across the input switch node and output switch node, and
an inverter having an inverter input providing the voltage sensing input node and an inverter output connected to a control terminal of the switch transistor, wherein in operation when the voltage sensing input node reaches the threshold voltage value a logic value at the inverter output toggles thereby causing the switch transistor to conduct.
11 . The voltage regulator circuit of claim 1 , wherein the voltage threshold switch comprises:
a switch transistor connected across the input switch node and output switch node, and
a threshold voltage detecting comparator having a reference input connected to a node providing the threshold voltage value, a monitoring input providing the voltage sensing input node and comparator output connected to a control terminal of the switch transistor, wherein in operation when the voltage sensing input node reaches the threshold voltage value, a voltage at the comparator output causes the switch transistor to conduct.
12 . A method of limiting inrush current in a voltage regulator circuit, the method comprising:
comparing a voltage at a common resistor node with a reference voltage to provide an error signal voltage;
controlling an inrush current in an initial conducting path with the error signal voltage, which controls a first pass transistor that has a first terminal connected to a first supply rail and a second terminal connected to a regulated voltage output node; and
controlling an inrush current in a main conducting path with the error signal voltage when the common resistor node reaches a threshold voltage value, wherein the main conducting path includes a second pass transistor with a first terminal connected to the first supply rail and a second terminal connected to the regulated voltage output node,
wherein when the first pass transistor and second pass transistor are both conducting, current flowing through the first pass transistor is less than current flowing through the second pass transistor, and wherein when the voltage sensing input node reaches a threshold voltage value, the voltage threshold switch connects the control node of the second pass transistor to the error signal voltage at the comparator output so that the comparator output controls conductivity of the second pass transistor, thereby regulating the output voltage at the regulated voltage output node.
13 . The method of limiting inrush current of claim 12 , wherein during initial activation of the voltage regulator circuit the inrush current through the first pass transistor is regulated by varying the error signal voltage at the comparator output that controls conductivity of the first pass transistor by monitoring a feedback voltage value at the common resistor node.
14 . The method of limiting inrush current of claim 13 , wherein the conductivity of the first pass transistor limits the inrush current.
15 . The method of limiting inrush current of claim 12 , wherein the second pass transistor has an aspect ratio greater than that of the first pass transistor.