Output circuit

Abstract

To quickly turn off a p-channel output transistor without adversely affecting the gate drive and with reduced power consumption.SOLUTION: An output circuit includes a p-channel output transistor MP1 whose drain is connected to the output terminal, a gate drive circuit A1 that drives the gate voltage of the output transistor, and an n-channel pull-up transistor NAT that is connected to the gate of the output transistor MP1 and turns on when the output transistor is off to pull up the gate voltage. The output transistor MP1 is an enhanced type with a relatively high threshold voltage for turning on, and the pull-up transistor NAT is a native type with a relatively low threshold voltage for turning on.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to an output circuit using a p-channel output transistor.

Background Art

[0002] An output circuit that supplies an output current from an output transistor to a load and drives the load is widely used. In this case, a p channel MOSFET may be used, and the output of a gate drive circuit may be supplied to its gate.

[0003] When the source of a p-channel MOSFET is connected to a high-voltage power supply, to turn off the p-channel MOSFET, its gate is generally pulled up by a resistor or another p channel MOSFET.

[0004] FIG. 1 is a diagram showing the configuration of a conventional output circuit. The output of gate drive circuit A1 is supplied to the gate of p-channel output transistor MP1. The source of output transistor MP1 is connected to power supply Vhv, and the drain of output transistor MP1 is connected to ground via load resistor RL and is the output terminal of output voltage Vout. And the gate of output transistor MP1 is connected to power supply Vhv via pull-up resistor R1. Therefore, when the output of gate drive circuit A1 becomes high impedance when turning off output transistor MP1, the gate of output transistor MP1 is pulled up by pull-up resistor R1, and output transistor MP1 turns off.

Summary of the Invention

Problems to be Solved by the Invention

[0005] When pulling up using pull-up resistor R1, if the resistance value of pull-up resistor R1 is small, it will affect the circuit characteristics for driving the gate of output transistor MP1. On the other hand, if the resistance value of pull-up resistor R1 is large, it will take time for output transistor MP1 to turn off.

[0006] Furthermore, if a different p-channel transistor is used for pull-up instead of the pull-up resistor R1, a voltage clamp circuit is required to protect the gate of that transistor. Voltage clamping requires current to flow through a resistor to generate a clamp voltage, which can result in disadvantages in operating characteristics, turn-off speed, or current consumption during turn-off. [Means for solving the problem]

[0007] The output circuit relating to this disclosure is A p-channel output transistor with its drain connected to the output terminal, A gate drive circuit that drives the gate voltage of the output transistor, The drain is connected to a high-voltage power supply, the source is connected to the gate of the output transistor, and the gate is connected to the connection point of a resistor connected to the high-voltage power supply and a current source. An n-channel pull-up transistor that turns on when the output transistor is off to pull up the gate voltage, An n-channel enhanced protection transistor, wherein the gate is connected to the gate of the output transistor, the drain is connected to the high-voltage power supply, and the source is connected to the connection point between the resistor and the current source, Includes, The output transistor is an enhanced type with a relatively high threshold voltage for turning it on. The aforementioned pull-up transistor is a native type with a relatively low threshold voltage for turning it on. [Effects of the Invention]

[0008] According to the output circuit described herein, power consumption can be reduced and the p-channel output transistor can be quickly turned off without adversely affecting gate driving. [Brief explanation of the drawing]

[0009] [Figure 1] This is a circuit diagram showing the configuration of a conventional output circuit. [Figure 2] This is a circuit diagram showing the configuration of the output circuit according to the embodiment. [Figure 3] This is a timing chart illustrating the operation of the circuit shown in Figure 2. [Modes for carrying out the invention]

[0010] The embodiments of this disclosure will be described below with reference to the drawings. The embodiments described below are not limiting to this disclosure, and configurations formed by selectively combining multiple examples are also included in this disclosure.

[0011] Circuit configuration Figure 2 is a circuit diagram showing the configuration of the output circuit according to the embodiment.

[0012] The output transistor MP1 is the output transistor that supplies drive current to the load resistor RL. The output transistor MP1 is composed of a p-channel MOSFET, with its source connected to a high-voltage power supply Vhv and its drain connected to ground via the load resistor RL. The drain of the output transistor MP1 is also the output terminal of the output voltage Vout.

[0013] The output terminal of gate drive circuit A1 is connected to the gate of output transistor MP1. For example, gate drive circuit A1 is composed of an operational amplifier or the like, and it feedback-controls the gate drive voltage so that the output voltage Vout becomes the target voltage. Note that the operation of gate drive circuit A1 is turned on and off by the signal EN, which will be described later. When gate drive circuit A1 is off, its output terminal is in a high-impedance state.

[0014] The gate of the output transistor MP1 is connected to the source of an n-channel pull-up transistor NAT, whose drain is connected to a high-voltage power supply Vhv. Here, the pull-up transistor NAT is a native-type n-channel MOSFET with a threshold voltage of -0.3V to +0.1V. Note that the threshold voltage of a typical enhanced-type MOSFET is 0.5V to 1.0V.

[0015] Also, one end of resistor R3 is connected to the high-voltage power supply Vhv, and the other end is connected to the ground via an n-channel transistor MN2. A signal EN is supplied to the gate of transistor MN2. When the signal EN is at a high level (EN = H), the transistor MN2 is turned on, and when the signal EN is low at a low level (EN = L), the transistor MN2 is turned off. In the figure, only the transistor MN2 is shown as the current source that conducts the current Ibias, but the transistor MN2 can be the transistor that conducts the current at the final stage of the current source.

[0016] The gate of an n-channel protection transistor MN1 is connected to the source of the pull-up transistor NAT. The drain of the protection transistor MN1 is connected to the high-voltage power supply Vhv, and the source is connected to the connection point of the resistor R3 and the transistor MN2. Therefore, when the protection transistor MN1 is on, the voltage difference between the gate of the output transistor MP1 and the gate of the pull-up transistor NAT is maintained at the threshold voltage Vth of the protection transistor MN1.

[0017] And the connection point of the resistor R3 and the transistor MN2 is connected to the gate of the pull-up transistor NAT. Therefore, when the transistor MN2 is on, the current flowing through the transistor MN2 flows through the resistor R3, and the voltage dropped by the resistor R3 from the high-voltage power supply Vhv is applied to the gate of the pull-up transistor NAT. On the other hand, when the transistor MN2 is off, the gate of the pull-up transistor NAT becomes the voltage of the high-voltage power supply Vhv via the resistor R'.

[0018] Note that the output transistors MP1, MN1, and MN2 other than the pull-up transistor NAT are enhancement-type MOSFETs.

[0019] "Circuit Operation" Figure 3 is a timing chart for explaining the operation of the circuit in Figure 2.

[0020] In conjunction with the high level (EN = H) and low level (EN = L) of the signal EN, the transistor MN2 and the gate drive circuit A1 turn on or off.

[0021] When EN = H, the transistor MN2 turns on and the current Ibias flows. Therefore, the gate of the pull-up transistor NAT becomes a voltage lower than the high-voltage power supply Vhv by the voltage drop across the resistor R3. The source of the pull-up transistor NAT is Nat_gate the gate-source voltage Vgs of the protection transistor MN1 Only expensive voltage. Therefore, the gate-source voltage Vgs_NAT of the pull-up transistor NAT becomes a predetermined negative voltage (for example, -0.8V), and the pull-up transistor NAT turns off.

[0022] The gate (MP1_g a te) of the output transistor MP1 is controlled by the gate drive circuit A1 and is set to a voltage lower than the high-voltage power supply Vhv by a predetermined voltage so that the output voltage Vout of the output transistor MP1 becomes a predetermined value (for example, 5V).

[0023] And the output voltage Vout is the target voltage (for example, 5V) determined by the gate drive circuit A1 as described above.

[0024] Here, by changing the magnitude of the current Ibias of the transistor MN2, the voltage drop across the resistor R3 can be adjusted. In one example configuration, a voltage up to 8V can occur across the resistor R3. Here, since the protection transistor MN1 is provided, the gate voltage of the pull-up transistor NAT is maintained at a voltage lower than the gate voltage of the output transistor MP1 by the gate-source voltage Vgs of the protection transistor MN1. That is, the voltage relationship is NAT_gate = MP1_gate - Vgs, and the protection transistor MN1 clamps the NAT_gate so that it does not drop too much. Note that the on-state gate-source voltage Vgs of the protection transistor MN1 is equal to its threshold voltage Vth.

[0025] Next, when EN=L, the output of gate drive circuit A1 becomes high impedance. Also, the current Ibias of transistor MN2 is turned off. Therefore, the voltage drop across resistor R3 is eliminated, and the gate NAT_g of pull-up transistor NAT is turned off. a te quickly rises to the voltage of the high-voltage power supply Vhv. Therefore, the gate-source voltage Vgs of the pull-up transistor NAT becomes 0V to 0.1V.

[0026] The threshold voltage of the pull-up transistor NAT is -0.3V to +0.1V. When the pull-up transistor NAT turns on, the gate of the output transistor MP1 is pulled up to the voltage of the high-voltage power supply Vhv (e.g., 15V), the gate-source voltage Vgs of the output transistor MP1 becomes 0V to 0.1V, and the output transistor MP1 quickly turns off. Consequently, the output voltage Vout also becomes 0V.

[0027] Thus, in this embodiment, the output transistor MP1 can be turned off without consuming current from the high-voltage power supply Vhv. Furthermore, since the pull-up transistor NAT operates as a source follower during turn-off, the output transistor MP1 can be turned off sufficiently quickly.

[0028] When EN=H and the circuit is operating under normal conditions, the gate of output transistor MP1 is at its minimum (Vhv-|max . Driven by Vgs_p|). |max . Vgs_p| is the maximum allowable gate-source voltage of the output transistor MP1.

[0029] The values ​​of current Ibias and R3 are determined such that Ibias × R3 is slightly greater than |max.Vgs_p|, thereby ensuring that the gate voltage of the pull-up transistor NAT is always lower than the gate voltage of the output transistor MP1. In the circuit shown in Figure 2, the protection transistor MN1 prevents the gate voltage of the pull-up transistor NAT from dropping too low and damaging it.

[0030] The circuit of this embodiment makes it possible to achieve the following three things when turning off the output transistor MP1, which is a p-channel MOSFET. (1) Set the current consumption of output transistor MP1 to zero when it is in the off state. (2) Quickly turn off the output transistor MP1. (3) When the output transistor MP1 is driven under normal conditions, the circuit for pulling up the output transistor MP1 does not adversely affect the drive of the gate of the output transistor MP1. [Explanation of Symbols]

[0031] A1 gate drive circuit, MN1 protection transistor, MP1 output transistor, NAT pull-up transistor, R1 pull-up resistor, Vhv high voltage power supply.

Claims

[Claim 1] A p-channel output transistor with its drain connected to the output terminal, A gate drive circuit that drives the gate voltage of the output transistor, An n-channel pull-up transistor is provided, the drain of which is connected to a high-voltage power supply, the source of which is connected to the gate of the output transistor, the gate of which is connected to the connection point of a resistor connected to the high-voltage power supply and a current source, and which turns on when the output transistor is off to pull up the gate voltage. An n-channel enhanced protection transistor, the gate of which is connected to the gate of the output transistor, the drain of which is connected to the high-voltage power supply, and the source of which is connected to the connection point between the resistor and the current source, Includes, The output transistor is an enhanced type with a relatively high threshold voltage for turning it on. The aforementioned pull-up transistor is a native type with a relatively low threshold voltage for turning it on. Output circuit.

Images