LDO regulator with thermal shutdown system and method

Details LDO regulator with thermal shutdown system and method LDO regulator with thermal shutdown system and method

2001-03-22

2001-10-30

Berhane, Adolf Deneke (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S280000

Reexamination Certificate

active

06310467

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to low-dropout regulators. In particular, the present invention relates to a method and apparatus that provides for a low-dropout regulator that includes thermal protection.
BACKGROUND OF THE INVENTION
Voltage regulators are often used to provide a relatively constant voltage source to other electronic circuits. Some regulators are limited in their effectiveness in a particular application. For example, some regulators have a high “drop-out” voltage. A “drop-out” voltage is the minimum voltage difference between the input voltage and the output voltage that is necessary to maintain proper regulation. Large drop-out voltages result in wasted power, and raise the minimum power supply requirements for maintaining regulation.
A low-dropout regulator (hereinafter referred to as an “LDO regulator”) is useful in applications where it is desired to maintain a regulated voltage that is sufficiently close to the input voltage. For example, LDO regulators are useful in battery-powered applications where the power supply voltage is exceedingly low.
A typical LDO regulator (
100
) is shown in FIG.
1
. The LDO regulator (
100
) includes a PMOS transistor (MP
10
), a first resistor (R
10
), a second resistor (R
11
), and a voltage control block (
110
). The PMOS transistor (MP
10
) has a drain connected to an output terminal (VOUT), a gate connected to node N
10
, and a source connected to an input voltage (VIN). The first resistor (R
10
) is series connected between nodes the regulator output node (VREG) and node N
11
. The second resistor (R
2
) is series connected between nodes N
11
and a circuit ground potential (GND). The voltage control block (
110
) has three input terminals (VIN, VREF, SENSE) and an output terminal (PCTL). In the voltage control block (
110
), the first input terminal (VIN) is connected to the input voltage (VIN), the second input terminal (VREF) is connected to a reference voltage potential (VREF), and the third input terminal (SENSE) is connected to node N
10
.
A load (ZL) is connected to the output terminal (VREG) of the LDO regulator (
100
). The LDO regulator (
100
) controls the gate of the PMOS transistor (MP
10
) to ensure that regulation of the output voltage (VREG) is maintained. The voltage control block (
110
) monitors the SENSE input terminal and controls the gate of the PMOS transistor (MP
10
) through the PCTL output terminal. Resistors R
10
and R
11
form a resistor divider that produces a signal that is related to the regulated output voltage (VREG). When the SENSE input terminal and the reference signal (VREF) are substantially the same, the LDO is properly maintaining regulation of the output voltage to the load (ZL).
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for an improved LDO regulator including a temperature protection circuit. The LDO regulator has improved power supply rejection and improved regulation by selectively operating the LDO regulator components from the regulated voltage. The LDO regulator components are selectively operated from the unregulated power supply when the operating temperature exceeds a predetermined temperature, which is detected by a temperature sensor.
Briefly stated, a method and apparatus is directed to a thermal shut down for a low drop out (LDO) regulator including a MOS transistor. An error amplifier controls the gate of the MOS transistor by comparing the regulator output voltage to a reference voltage that is generated by a reference circuit. To enhance power supply rejection and improve regulation, the error amplifier and the reference circuits are powered by a potential at an internal power supply node. A power control circuit selectively couples the internal power supply node to one of the regulated output voltage and the unregulated supply voltage. A start-up circuit may be employed to ensure that regulation begins when power is applied. A temperature sensor circuit detects when the operating temperature exceeds a predetermined temperature and activates a supply transfer circuit to couple the unregulated supply to the internal power supply node. After the internal power supply node reaches the unregulated power supply potential, a shutdown circuit deactivates the MOS transistor. A diode is coupled between the regulator output and the internal power supply node to prevent current flowing from the internal power supply node to the load when the over-temperature condition exists. A transistor mirror may be configured to couple the unregulated supply voltage to the internal power supply node when activated. The thermal characteristics of a transistors threshold voltage may be employed as a temperature sensor. Another transistor may deactivate the MOS transistor by coupling the gate of the MOS transistor to the unregulated supply voltage.
According to a feature of the invention, an LDO regulator includes an error amplifier and a reference circuit that are powered by an internal power supply node. The internal power supply node is selectively coupled to one of a regulated voltage and an unregulated power supply voltage. The error amplifier compares the regulated voltage to a reference voltage from the reference circuit to control the gate of a MOS device. The operation of the error amplifier and the reference circuit from the regulated power supply enhances power supply rejection of the LDO regulator.
According to a further feature of the invention, the threshold voltage of a transistor is arranged to detect an over-temperature condition and activate a supply transfer circuit. The supply transfer circuit couples the unregulated power supply voltage to an internal power supply node when activated. The gate of the MOS device is coupled to the unregulated power supply voltage when the internal power supply node is the unregulated power supply voltage.
In accordance with an embodiment of the invention, an apparatus regulates a regulation voltage across a load from an unregulated power supply. The apparatus includes a series regulator circuit is arranged to selectively couple power from the unregulated power supply to the load in response to a control signal. A feedback circuit is arranged to provide a feedback voltage from the regulation voltage, wherein the feedback voltage is at least a portion of the regulation voltage. A power control circuit is arranged to selectively couple power from the unregulated power supply to an internal power supply when activated by an over-temperature signal. A temperature sensor circuit is arranged to provide an over-temperature signal when an over-temperature condition is detected. A diode circuit is arranged to couple power from the regulation voltage to the internal power supply when the power control circuit is deactivated such that the internal power supply is regulated when the power control circuit is deactivated. An error amplifier circuit is arranged to produce the control signal in response to a reference voltage and the feedback voltage, wherein the error amplifier is arranged to operate from the internal power supply whereby the regulation voltage has an improved power supply rejection ratio.
In accordance with another embodiment of the invention, an apparatus that regulates a regulation voltage across a load from an unregulated power supply includes a means for regulating selectively couples power from the unregulated power supply to the load in response to a control signal. A means for coupling unregulated power selectively couples power from the unregulated power supply to an internal power supply when activated by an over-temperature signal. A means for sensing produces an over-temperature signal when an over-temperature condition is detected. A means for coupling regulated power couples power from the regulation voltage to the internal power supply when the internal power supply is less than the regulation voltage. A means for providing feedback provides a feedback voltage from the regulation voltage, wherein the feedback voltage is at least a portion of the regulat

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