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 utilizing a p-channel output transistor. [Background technology]

[0002] Output circuits that drive a load by supplying an output current from an output transistor to the load are widely used. In this case, a P-channel MOSFET is used as the output transistor, and the output of a gate drive circuit is supplied to the gate of the output transistor.

[0003] If the source of a p-channel MOSFET is connected to a high-voltage power supply, the p-channel MOSFET can typically be turned off by pulling up its gate with a resistor or another p-channel MOSFET.

[0004] FIG. 1 shows the configuration of a conventional output circuit. The output of a gate drive circuit A1 is supplied to the gate of a p-channel output transistor MP1. The source of the output transistor MP1 is connected to a power supply Vhv, and the drain of the output transistor MP1 is connected to ground via a load resistor RL and serves as the output terminal for the output voltage Vout. The gate of the output transistor MP1 is connected to the power supply Vhv via a pull-up resistor R1. Therefore, when the output of the gate drive circuit A1 becomes high impedance when the output transistor MP1 is turned off, the gate of the output transistor MP1 is pulled up by the pull-up resistor R1, and the output transistor MP1 is turned off. Summary of the Invention [Problem to be solved by the invention]

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

[0006] Furthermore, if a separate 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 that a current be passed through a resistor to generate a clamp voltage, which can have disadvantages in terms of operating characteristics, turn-off speed, or current consumption during turn-off. [Means for solving the problem]

[0007] The output circuit according to the present disclosure comprises: a p-channel output transistor having a drain connected to the output terminal; a gate drive circuit that drives a gate voltage of the output transistor; an n-channel pull-up transistor connected to the gate of the output transistor, and turned on when the output transistor is off to pull up the gate voltage; Including, the output transistor is an enhanced type having a relatively high threshold voltage for turning on, The pull-up transistor is a native type with a relatively low threshold voltage for turning on. [Effects of the Invention]

[0008] According to the output circuit of the present disclosure, it is possible to reduce power consumption and quickly turn off the p-channel output transistor without adversely affecting gate drive. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a circuit diagram showing a configuration of a conventional output circuit. [Figure 2] FIG. 2 is a circuit diagram showing a configuration of an output circuit according to an embodiment. [Figure 3] 3 is a timing chart illustrating the operation of the circuit of FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the following embodiments do not limit the present disclosure, and configurations formed by selectively combining multiple examples are also included in the present disclosure.

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

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

[0013] The output terminal of the gate drive circuit A1 is connected to the gate of the output transistor MP1. For example, the gate drive circuit A1 is composed of an operational amplifier and performs feedback control of the gate drive voltage so that the output voltage Vout becomes a target voltage. The operation of the gate drive circuit A1 is turned on and off by a signal EN, which will be described later. When the gate drive circuit A1 is off, its output terminal becomes 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 the high-voltage power supply Vhv. 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] One end of resistor R3 is connected to the high-voltage power supply Vhv, and the other end is connected to ground via n-channel transistor MN2. A signal EN is supplied to the gate of transistor MN2. When signal EN is high level (EN=H), transistor MN2 turns on, and when signal EN is low level (EN=L), transistor MN2 turns off. Note that while the diagram shows only transistor MN2 as the current source that flows current Ibias, transistor MN2 can also be used as a transistor that flows the final stage current of the current source.

[0016] The source of the pull-up transistor NAT is connected to the gate of an n-channel protection transistor MN1. The drain of the protection transistor MN1 is connected to the high-voltage power supply Vhv, and the source is connected to the junction of resistor R3 and 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] The junction point between resistor R3 and transistor MN2 is connected to the gate of pull-up transistor NAT. Therefore, when transistor MN2 is on, the current flowing through transistor MN2 flows to resistor R3, and the voltage dropped from the high-voltage power supply Vhv by resistor R3 is applied to the gate of pull-up transistor NAT. On the other hand, when transistor MN2 is off, the gate of pull-up transistor NAT becomes the voltage of the high-voltage power supply Vhv via resistor R3.

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

[0019] "Circuit operation" FIG. 3 is a timing chart illustrating the operation of the circuit of FIG.

[0020] The transistor MN2 and the gate drive circuit A1 are turned on or off in response to the high level (EN=H) or low level (EN=L) of the signal EN.

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

[0022] The gate (MP1_gate) of the output transistor MP1 is controlled by a gate drive circuit A1 and is set to a voltage that is a predetermined voltage lower than the high voltage power supply Vhv so that the output voltage Vout of the output transistor MP1 becomes a predetermined value (for example, 5V).

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

[0024] The voltage drop across resistor R3 can be adjusted by changing the magnitude of the current Ibias through transistor MN2. In one example configuration, a voltage of up to 8 V can develop across resistor R3. Because protection transistor MN1 is provided, the gate voltage of pull-up transistor NAT is maintained at a voltage lower than the gate voltage of output transistor MP1 by the gate-source voltage Vgs of protection transistor MN1. In other words, the voltage relationship is NAT_gate = MP1_gate - Vgs, and protection transistor MN1 clamps NAT_gate to prevent it from dropping too low. The gate-source voltage Vgs of protection transistor MN1 in the on state is equal to its threshold voltage Vth.

[0025] Next, when EN goes low, the output of the gate drive circuit A1 goes high impedance. Also, the current Ibias of transistor MN2 turns off. This eliminates the voltage drop across resistor R3, and the gate voltage NAT_gate of the pull-up transistor NAT quickly rises to the voltage of the high-voltage power supply Vhv. This causes the gate-source voltage Vgs of the pull-up transistor NAT to fall between 0V and 0.1V.

[0026] The threshold voltage of the pull-up transistor NAT is −0.3V to +0.1V, and 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 (for example, 15V), the gate-source voltage Vgs of the output transistor MP1 becomes 0V to 0.1V, and the output transistor MP1 is quickly turned off. Therefore, the output voltage Vout also becomes 0V.

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

[0028] When the circuit operates normally with EN=H, the gate of the output transistor MP1 is driven to a minimum of (Vhv-|maxVgs_p|), where |maxVgs_p| is the maximum allowable gate-source voltage of the output transistor MP1.

[0029] The values ​​of the current Ibias and R3 are determined so that Ibias × R3 is slightly greater than |max.Vgs_p|, which ensures that the gate voltage of the pull-up transistor NAT is always smaller than the gate voltage of the output transistor MP1. In the circuit of Figure 2, the protection transistor MN1 prevents the gate voltage of the pull-up transistor NAT from dropping too low and causing damage.

[0030] The circuit of this embodiment can achieve the following three things when turning off the output transistor MP1, which is a p-channel MOSFET. (1) The current consumption of the output transistor MP1 in the off state is set to zero. (2) The output transistor MP1 is quickly turned off. (3) When the output transistor MP1 is normally driven, the circuit for pulling up the output transistor MP1 does not adversely affect the driving 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

1. a p-channel output transistor having a drain connected to an output terminal; a gate drive circuit that drives a gate voltage of the output transistor; an n-channel pull-up transistor connected to the gate of the output transistor, and turned on when the output transistor is off to pull up the gate voltage; Including, the output transistor is an enhanced type having a relatively high threshold voltage for turning on, The pull-up transistor is a native type having a relatively low threshold voltage for turning on. Output circuit.

2. 2. The output circuit according to claim 1, the drain of the pull-up transistor 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 a junction of a resistor and a current source connected to the high voltage power supply; Output circuit.

3. 3. The output circuit according to claim 2, further comprising an n-channel enhanced-type protection transistor having a gate connected to the gate of the output transistor, a drain connected to the high-voltage power supply, and a source connected to a junction point of the resistor and the current source; Output circuit.

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