Voltage regulator with improved line regulation transient response
View Patent ↗A significant reduction of the amplitude of the transient response is obtained by keeping a low dropout regulator circuit in a closed loop condition. This is achieved by manipulation of the reference voltage level when an open loop condition arises due to a falling input voltage. In this case, the reference voltage level is tracked with the input voltage level, keeping the output voltage regulated. As a consequence, the power pass element of the regulator is not forced into the linear region (in the case of a MOSFET) or deep saturation (in the case of a bipolar transistor).
1. A voltage regulator circuit, comprising:
an input node configured to receive an input voltage;
a power transistor having a conduction path coupled between the input node and an output node;
an amplifier having an output driving a control terminal of the power transistor and a first input coupled to the output node to form a regulator feedback loop, said amplifier further having a second input; and
a voltage generator supplied by the input voltage and configured to generate a variable reference voltage applied to the second input of the amplifier, said variable reference voltage varying correspondingly with changes in the input voltage.
2. The voltage regulator circuit of claim 1 , further comprising:
a current sensing circuit configured to sense current flowing in the conduction path of the power transistor and generate a sense current at an intermediate node;
a first resistor coupled between the input node and the intermediate node; and
a first transistor having a conduction path coupled between the intermediate node and the second input of the amplifier.
3. The voltage regulator circuit of claim 2 , further comprising a low pass filter coupled between the first transistor and the second input of the amplifier.
4. The voltage regulator circuit of claim 2 , further comprising a zener diode coupled between the first transistor and a reference voltage node.
5. The voltage regulator circuit of claim 4 , wherein the reference voltage node is a ground node.
6. The voltage regulator circuit of claim 4 , further comprising:
a second transistor coupled to the first transistor to form a current mirroring circuit; and
a current source configured to supply a bias current to the second transistor.
7. The voltage regulator circuit of claim 2 , further comprising a bandgap reference voltage generator including said first transistor.
8. The voltage regulator circuit of claim 7 , wherein said bandgap reference voltage generator comprises:
a pair of MOSFET transistor coupled to the input node and configured in a current mirror relationship; and
a pair of bipolar transistors respectively coupled in series with the pair of MOSFET transistors;
wherein a series connect node between one of the pair of MOSFET transistors and one of the pair of bipolar transistors is coupled to a control terminal of the first transistor.
9. The voltage regulator circuit of claim 8 , wherein said bandgap reference voltage generator further comprises a resistive voltage divider circuit coupled between the first transistor and a reference voltage node, an output of the resistive voltage divider circuit coupled to control terminals of the pair of bipolar transistors.
10. The voltage regulator circuit of claim 9 , wherein the reference voltage node is a ground node.
11. The voltage regulator circuit of claim 2 , further comprising:
a bandgap reference voltage generator configured to generate a bandgap voltage;
a resistive voltage divider circuit coupled between the first transistor and a reference voltage node; and
an additional amplifier having an output driving a control terminal of the first transistor, a first input coupled to an output of the resistive voltage divider to form a feedback loop and a second input coupled to receive said bandgap voltage.
12. The voltage regulator circuit of claim 2 , wherein said current sensing circuit comprises a second transistor which is a scaled copy of the power transistor, said second transistor having a conduction path coupled between the intermediate node and the output node and having a control terminal coupled to the output of the amplifier.
13. A voltage regulator circuit, comprising:
an input node configured to receive an input voltage;
a power transistor having a conduction path coupled between the input node and an output node;
a current sensing circuit configured to sense current flowing in the conduction path of the power transistor and generate a sense current;
an amplifier having an output driving a control terminal of the power transistor and a first input coupled to the output node to form a regulator feedback loop, said amplifier further having a second input; and
a voltage generator supplied by the input voltage and configured to generate a variable reference voltage applied to the second input of the amplifier in response to said input voltage and the sense current.
14. The voltage regulator circuit of claim 13 , wherein the sense current is generated at an intermediate node, the voltage generator comprising:
a first resistor coupled between the input node and the intermediate node; and
a first transistor having a conduction path coupled between the intermediate node and the second input of the amplifier.
15. The voltage regulator circuit of claim 14 , further comprising:
a zener diode coupled between the first transistor and a reference voltage node;
a second transistor coupled to the first transistor to form a current mirroring circuit; and
a current source configured to supply a bias current to the second transistor.
16. The voltage regulator circuit of claim 14 , wherein the voltage generator circuit comprises a bandgap reference voltage generator including said first transistor.
17. The voltage regulator circuit of claim 14 , further comprising:
a bandgap reference voltage generator configured to generate a bandgap voltage;
a resistive voltage divider circuit coupled between the first transistor and a reference voltage node; and
an additional amplifier having an output driving a control terminal of the first transistor, a first input coupled to an output of the resistive voltage divider to form a feedback loop and a second input coupled to receive said bandgap voltage.
18. The voltage regulator circuit of claim 14 , wherein said current sensing circuit comprises a second transistor which is a scaled copy of the power transistor, said second transistor having a conduction path coupled between the intermediate node and the output node and having a control terminal coupled to the output of the amplifier.
19. A method for operating a voltage regulator circuit, comprising:
determining an error between a feedback voltage and a reference voltage;
driving a control terminal of a power transistor with a control voltage derived from the determined error to regulate an output voltage, wherein said feedback voltage is derived from the output voltage;
supplying an input voltage to the power transistor; and
decreasing said reference voltage correspondingly to a decrease in the input voltage so as to maintain the output voltage in regulation.
20. A method for operating a voltage regulator circuit, comprising:
determining an error between a feedback voltage and a reference voltage;
driving a control terminal of a power transistor with a control voltage derived from the determined error to regulate an output voltage, wherein said feedback voltage is derived from the output voltage;
generating a sense current corresponding to a current flowing through the power transistor; and
decreasing the reference voltage in response to change in the sense current so as to maintain the output voltage in regulation.
21. The voltage regulator circuit of claim 1 , wherein said variable reference voltage decreases in response to a decrease in the input voltage to a voltage level that will provide for a regulated voltage at said output node.
22. The voltage regulator circuit of claim 1 , wherein said variable reference voltage decreases in response to a decrease in the input voltage to a voltage level preventing the voltage regulator circuit from dropping out of regulation.
23. The voltage regulator circuit of claim 1 , wherein said variable reference voltage changes in response to changes in the input voltage so as to maintain the regulator feedback loop in a closed loop operating condition.
24. The voltage regulator circuit of claim 13 , wherein said variable reference voltage decreases in response to change in the sense current and a decrease in the input voltage to a voltage level that will provide for a regulated voltage at said output node.
25. The voltage regulator circuit of claim 13 , wherein said variable reference voltage decreases in response to change in the sense current and a decrease in the input voltage to a voltage level preventing the voltage regulator circuit from dropping out of regulation.
26. The voltage regulator circuit of claim 13 , wherein said variable reference voltage changes in response to changes in the sense current and input voltage so as to maintain the regulator feedback loop in a closed loop operating condition.