Load drive circuit

Abstract

To implement soft start using a relatively simple circuit.SOLUTION: A load drive circuit includes an n-channel output transistor M13 whose drain is connected to a power supply and which applies output from the source to a load 16 when turned on and a soft start reference block 12 that applies a soft start voltage between the gate and source of the output transistor during a predetermined period starting from the output turn-on. The soft-start reference block 12 applies a soft-start voltage corresponding to the gate-source voltage of a reference transistor, which conducts current from a current source, to the gate-source of the output transistor. The size of a reference transistor is smaller than that of the output transistor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a load driving circuit that drives a load using an n-channel transistor. [Background technology]

[0002] Conventionally, semiconductor integrated circuits have been known that incorporate switch devices that control the driving of various loads such as motors. Switch devices include high-side load switches that are placed upstream of the load and low-side switches that are placed downstream of the load, and either one can be selected depending on the application.

[0003] Also, when using a power transistor, such as a MOSFET device, as a switch device, there are two options: an n-channel transistor (NMOSFET) or a p-channel transistor (PMOSFET). NMOS is suitable for applications where the supply current to the load is relatively large.

[0004] In the case of a high-side load switch, the gate voltage of the NMOS must be higher than the input power supply voltage to control the switching of the NMOS. This requires a power supply with a higher voltage than the input power supply voltage. Such a high-voltage power supply is usually generated by a charge pump circuit. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] ROHM Datasheet 2.0A Current Load Switch ICs for Portable Equipment'' Summary of the Invention [Problem to be solved by the invention]

[0006] Here, the power generated by the charge pump circuit is limited, and low power consumption is desirable for controlling the switch device. High-side load switches may also require a soft-start function to prevent surge currents when turned on. Furthermore, high-side load switches replace mechanical relays and must be able to handle a variety of load conditions, including capacitive loads, inductive loads, and high-current loads.

[0007] As described above, there are various requirements for high-side load switches. [Means for solving the problem]

[0008] The load driving circuit according to the present disclosure comprises: an n-channel output transistor whose drain is connected to a power supply and whose source applies output to a load when the output is on; a soft start reference block that applies a soft start voltage between the gate and source of the output transistor for a predetermined period from the start of output on; Including, The soft start reference block comprises: applying a soft start voltage corresponding to a gate-source voltage of a reference transistor that passes a current from a current source between the gate and source of the output transistor; The reference transistor has a size smaller than that of the output transistor. [Effects of the Invention]

[0009] The load driving circuit according to the present disclosure can perform soft start with low power consumption. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a circuit diagram showing a configuration of a load driving circuit according to an embodiment; [Figure 2] 2 is a timing chart showing the operation of the circuit of FIG. 1. [Figure 3]FIG. 10 is a diagram showing the relationship between the output current Iout during soft start and the current I_vcp supplied from Vcp. [Figure 4] FIG. 10 is a diagram showing waveforms when an output transistor is turned on without soft start. [Figure 5] This is a circuit diagram of the full-on control block, showing the voltages between multiple points. [Figure 6] 10 is a timing chart showing the operation of the full-on control block. [Figure 7] FIG. 10 is a circuit diagram showing a configuration of a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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.

[0012] "Circuit configuration" 1 is a circuit diagram showing the configuration of a load driving circuit according to an embodiment. As shown, the load driving circuit includes an on-off control block 10, a soft-start reference block 12, a full-on control block 14, and an n-channel transistor M13, which is a power transistor that drives a load 16. The load driving circuit is preferably housed in a single semiconductor integrated circuit, such as an LSI. The transistor M13 is called an output transistor.

[0013] The output of this circuit controls the driving of the output transistor M13. The output current of the output transistor M13 drives the load 16. An output voltage Vout is supplied from the upper side of the load 16 (the connection point between the transistor M13 and the load 16).

[0014] <Soft start reference block> The current source I1 outputs a constant current I1. The drain of an n-channel transistor M1 is connected downstream of the current source I1, and the source of the transistor M1 is connected to ground GND. The gate and drain of the transistor M1 are shorted, so that the transistor M1 functions as a diode. Therefore, the constant current I1 flows through the transistor M1, and the voltage on its upstream side (drain side) becomes the Vgs of the transistor M1. It is desirable to be able to set the constant current I1 as desired.

[0015] The drain of transistor M1 is connected to the positive input terminal of the operational amplifier OPA. The output of the operational amplifier OPA is connected to the gate of n-channel transistor M2. The source of transistor M2 is connected to ground GND via resistor R1. The junction of resistor R1 and the source of transistor M3 is connected to the negative input terminal of the operational amplifier OPA.

[0016] The drain of the transistor M2 is connected to the drain of a p-channel transistor M3, and the source of the transistor M3 is connected to the high-voltage power supply VH. The gate and drain of the transistor M3 are short-circuited, and the transistor M3 functions as a diode.

[0017] Therefore, current flows from the high voltage power supply VH through transistor M3, transistor M2, and resistor R1 to ground GND. The operational amplifier OPA then operates so that the voltage at the negative input terminal becomes the gate-source voltage Vgs_M1 of transistor M1, which is the input voltage at the positive input terminal. As a result, a current I=Vgs_M1 / R1 flows through resistor R1 (resistance value R1). Transistor M1 is called the reference transistor.

[0018] The gate of the transistor M3 is connected to the base of a p-channel transistor M4, and the source of the transistor M4 is connected to the high-voltage power supply VH. Therefore, the transistors M3 and M4 form a current mirror.

[0019] The drain of transistor M4 is connected to the drain of n-channel transistor M5. The source of transistor M5 is connected to the output terminal Vout via resistor R2. The gate and drain of transistor M5 are shorted, and it functions as a diode. Therefore, a current corresponding to the current flowing through transistor M3 flows through transistor M4, transistor M5, and resistor R2.

[0020] The gate of transistor M5 is connected to the gate of n-channel transistor M6. The drain of transistor M6 is connected to the high-voltage power supply VH, and its source is connected to the output terminal vout via resistor R3. The junction point between transistor M6 and resistor R3 is also connected to the gate of output transistor M13. Therefore, the gate-source voltage of output transistor M13 is a voltage corresponding to the voltage drop across resistor R3.

[0021] Because transistors M5 and M6 form a current mirror, a current corresponding to the current flowing through transistor M5 flows through transistor M6 and resistor R3. If the size ratio of transistors M3 and M4 corresponds to the ratio of the currents flowing through resistors R1 and R2, and the size ratio of transistors M5 and M6 corresponds to the ratio of the currents flowing through resistors R2 and R3, the voltage drop across resistor R1 and the voltage drop across resistor R3 become equal, and therefore the voltage drop across resistor R3 becomes equal to Vgs_M1.

[0022] The voltage drop across resistor R3 becomes the gate-source voltage (called the soft-start voltage) of output transistor M13, and the current flowing through output transistor M13n1 at this time becomes the soft-start current I_soft. Therefore, the gate-source voltage Vgs_M13 of output transistor M13 is determined by the gate-source voltage Vgs_M1 of transistor M1, and so the soft-start current I_soft can be determined.

[0023] Therefore, the transistors M1 and M13 are set to have the same characteristics and a predetermined ratio in size, for example, the ratio of the size of the transistors M1 and M13 is set to n (n=M13 / M1). Here, the current through transistor M13 during soft start is I_soft. To match the operating conditions, the current through transistor M1, i.e., I1, is set to I_soft / n. If resistors R1, R2, and R3 have the same resistance, the voltage drop across resistor R1 will be equal to the voltage drop across resistor R3. Furthermore, due to the operation of the OPA circuit, the voltage drop across resistor R1 will be equal to Vgs_M1. Therefore, Vgs_M1 will be equal to Vgs_M13, and I_soft = I1 * n. The resistance values ​​of the resistors R1, R2, and R3 can be selected arbitrarily, so by setting them to large values, the current consumption I_vcp of Vcp can be reduced.

[0024] Therefore, the soft start current I_soft can be determined by the magnitude of the constant current I1 flowing from the current source I1.

[0025] 3 shows the relationship between the output current Iout=I_soft during soft start and the current I_vcp supplied from Vcp when n=300, for example. This current is the current of the power supply Vcp and flows through resistors R1, R2, and R3. When Iout=50mA, I_vcp=5.1μA and Vgs=1.7V, and when Iout=1.3A, I_vcp=8.4μA and Vgs=2.8V.

[0026] In this way, the soft-start current is determined by the gate-source voltage Vgs of transistor M1, which has similar characteristics to output transistor M13, so the soft-start current can be easily set by setting the current of current source I1. Also, by setting the sizes of output transistor M13 and transistor M1, the soft-start reference block 12 can be made relatively small.

[0027] Figure 4 shows the waveforms when the output transistor M13 is turned on without soft start. As shown, a large surge current I_surge flows at the start of operation. The maximum value of this surge current is I_surge_max = Vin / Rdson, which is the input voltage Vin divided by the on-resistance Rdson of the transistor M13.

[0028] <Full-on control block> The drain of a p-channel transistor M9 is connected to the gate of M13, and the source of the transistor M9 is connected to the high voltage power supply VH.

[0029] The gate of the transistor M9 is connected to a p-channel transistor M10. The source of the transistor M10 is connected to the high-voltage power supply VH, and the gate and source of the transistor M10 are shorted. Therefore, the transistors M10 and M9 form a current mirror.

[0030] The drain of transistor M10 is connected to the drain of n-channel transistor M11 via resistor R4. The source of transistor M11 is connected to the source of n-channel transistor M12, and the drain of transistor M12 is connected to the input terminal Vin for an external input voltage Vin. A boost power supply Vcp is disposed between the input terminal Vin and the high-voltage power supply VH. Therefore, the voltage of the high-voltage power supply VH is higher than the input voltage Vin by the boost voltage Vcp.

[0031] As will be described later, when the input voltage Vin and the output voltage Vout have a predetermined relationship, the voltage of the high-voltage power supply VH is higher than the input voltage Vin by the boost voltage Vcp, causing a current to flow through transistor M10. This current then flows toward the input terminal Vin via resistor R4 and transistors M11 and M12. A current corresponding to the current through transistor M10 flows through transistor M9, which then flows toward the output terminal Vout via resistor R3. The voltage drop across resistor R3 becomes Vgs_M13 of the output transistor M13. This voltage Vgs_M13 determines the boost voltage Vcp so that the output transistor M13 is fully on. Therefore, when the full-on control block 14 is operating, the output transistor M13 is fully on. This boost voltage Vcp is called the full-on voltage.

[0032] <Output section> In addition to the drain of the transistor M9, the gates of the transistors M11 and M12 and the gate of the output transistor M13 are connected to the connection point between the transistor M6 and the resistor R3 in the soft-start reference block 12. In other words, the gates of the transistors M11, M12 and M13 are commonly connected to the connection point between the transistor M6 and the resistor R3.

[0033] As described above, the boost power supply Vcp is disposed between the input terminal Vin and the high-voltage power supply VH, and the voltage of the high-voltage power supply is set to VH=Vin+Vcp. The boost power supply Vcp can be configured using a charge pump circuit or the like.

[0034] The source of the output transistor M13 serves as the output terminal Vout, and the output voltage Vout is applied to the load 16.

[0035] <On / Off Control Block> To drive the load 16, i.e., to apply the output voltage Vout, an on signal ON is supplied via an inverter INV to the gates of n-channel transistors M7 and M8. The drain of transistor M7 is connected to the gates of transistors M5 and M6, and its source is connected to ground GND. The drain of transistor M8 is connected to the gates of transistors M11 and M12 and output transistor M13, and its source is connected to ground GND.

[0036] Therefore, when the ON signal is ON (high level), the transistors M7 and M8 are OFF, and the soft start reference block 12 and the full ON control block 14 operate. On the other hand, when the ON signal is OFF (low level), the transistors M7 and M8 are ON, The transistors M5, M6, M11, M12, and M13 are all turned off, and the operation of the soft-start reference block 12 and the full-on control block 14 is stopped.

[0037] <About operation> Fig. 2 is a timing chart showing the operation of the circuit of Fig. 1. When the on signal ON goes high (output on), the soft start reference block 12 starts operating. Due to the control of the operational amplifier OPA, the current mirror operation of transistors M3 and M4, and the current mirror operation of transistors M5 and M6, the voltage drop across resistor R3 approaches Vgs_M1. The current through transistor M6 becomes larger than the specified value due to the charging of the gate capacitance of transistor M13. When the gate capacitance charging period ends, the current of transistor M6 flows into resistor R3, and the current becomes Vgs_M1 / R3. The voltage drop across resistor R3 due to this current generates Vgs_soft, which controls the output current to I_softstart. During the soft start period, the voltage of Vgs_M13 remains constant.

[0038] In this state, the charging of the capacitive component of the load 16 continues, and the output voltage Vout gradually increases.

[0039] Then, when Vout rises and the difference between Vin, i.e., the drain-source voltage of transistor M13, reaches a predetermined small value, the detection voltage Vdet, which is the difference between Vout and the source voltage of transistor M11 (the voltage obtained by subtracting the gate-source voltage of transistor M11 from the gate-source voltage of transistor M13), reaches a predetermined value. This causes current to begin flowing through transistors M11 and M12. Therefore, current also begins to flow through transistor M10, and current also begins to flow from transistor M9 to resistor R3.

[0040] As Vout approaches Vin, the current flowing through transistor M9 causes the voltage drop across resistor R3 to become Vcp. This causes the gate-source voltage of transistor M13 to become Vcp, turning transistor M13 fully on. At this time, the fully on load current I_load flows through load 16.

[0041] When the on signal ON goes low (output off), transistors M7 and M8 turn on, stopping the voltage supply to the gate of output transistor M13. Then, output transistor M13 turns off, and output current Iout disappears. As a result, output voltage Vout gradually becomes zero as the current from Cout decreases. The time constant for the decrease in output voltage Vout is t = Cout * Rout. Here, Cout is the capacitance of load 16, and Rout is the resistance of load 16.

[0042] In this embodiment, as described above, the operation of the full-on control block 14 is initiated by the detection voltage Vdet, but the predetermined period of the soft start may be determined by measuring a predetermined time from output on with a timer or by comparing the voltage value of the output Vout with a predetermined value as long as it is possible to prevent a surge current to the load 16. Then, after the predetermined period has elapsed, the full-on control block 14 turns the output transistor M13 fully on.

[0043] <Full-on control> The operation of the full-on control block 14 will now be described with reference to Figures 3 and 4. Figure 5 is a circuit diagram of the full-on control block 14, showing voltages between multiple points. Figure 6 is a timing chart showing the operation of the full-on control block 14.

[0044] As shown in FIG. 5, the difference between the gate-source voltage Vgs_M13 of the transistor M13 and the gate-source voltage of the transistor M11 is defined as a detection voltage Vdet (Vdet=Vgs_M13−Vgs_M11).

[0045] When the output voltage Vout is low, if the source voltage of transistor M12 is initially equal to GND, it rises toward the input voltage Vin, rising to Vgs_M12 = Vt (threshold voltage) and turning off. Note that if the gate-source voltage Vgs_M12 of transistor M12 is initially equal to its threshold voltage Vt, it is off from the start. Since the gate-source voltage Vgs_M11 of transistor M11 is equal to the gate-source voltage Vgs_M12 of transistor M12, transistor M11 is also turned off.

[0046] When the output voltage Vout rises, the gate voltages of transistors M11, M12, and M13 also rise, and these gate voltages become higher than the input voltage Vin. As the drain-source voltage of transistor M12 approaches Vds_M12=0V, the source voltage of transistors M11 and M12 becomes the same as the input voltage Vin.

[0047] Furthermore, when the output voltage Vout approaches the input voltage Vin, Vds of the transistor M12 decreases and Vgs of the transistor M11 becomes larger than Vt, so that current flows through the transistor M10, resistor R4, and transistors M11 and M12.

[0048] Thus, when the gate-source voltage of transistor M12 is equal to its threshold voltage (Vgs_M12 = Vt), no current flows through resistor R4. However, when the output voltage Vout rises, the gate voltage of transistor M13 rises while the gate-source voltage Vgs_M13 of M13 remains constant. As a result, the gate voltage of transistor M12, which is commonly connected to the gate of transistor M13, also rises and becomes higher than the input voltage Vin. As this rises further, the drain-source voltage Vds_M12 of transistor M12 becomes 0. When the drain-source voltage Vds_M12 of transistor M12 becomes negative, current flows from resistor R4.

[0049] When the transistors M11 and M12 are fully turned on, their drain-source voltages Vds_M11 and Vds_M12 become nearly 0. Therefore, the source and drain of the transistors M11 and M12 have the same voltage as the input voltage Vin. In reality, the drain-source voltages Vds_M11 and Vds_M12 of the transistors M11 and M12 depend on the ratio of the on-resistance Rdson of the transistors M11 and M12 to the resistor R4, so the condition for Vds_M11=Vds_M12=0 is when Rdson< <R4である。

[0050] In this way, current flows through transistor M10, which in turn flows through transistor M9. This causes the voltage across resistor R3 to become greater than the voltage across resistor R2, and the gate-source voltage of transistor M5 becomes greater than the gate-source voltage of transistor M6, Vgs_M5>Vgs_M6, causing the current through transistor M6 to decrease. When the difference further increases, the gate-source voltage of transistor M6 becomes less than the threshold voltage, Vgs_M6 <Vtとなり、トランジスタM6はオフする。

[0051] Assume that the on-resistance of transistor M9 is sufficiently smaller than the resistance value of resistor R3 (Rdson << R3). In this case, the current flowing through transistor M9 is the current flowing through resistor R3, which is Vcp / R3. Also, due to the current flowing through resistor R4, the absolute value of the gate-source voltage Vgs_M10 of transistor M10 increases, causing transistor M9 to turn on and its resistance value to become sufficiently smaller than that of resistor R3. Therefore, the relationship between the resistance values of resistor R4 and resistor R3 should be R4 > R3, and the relationship between the sizes of transistor M9 and transistor M10 should be M9 > M10.

[0052] "Modified Example" Figure 7 is a circuit diagram showing the configuration of the modified example. In this modified example, transistor M14 and resistor R5 are added to the circuit of Figure 1.

[0053] One end of resistor R5 is connected to the high-voltage power supply VH, and the drain of n-channel transistor M12 is connected to the other end of the resistor. The gate and drain of transistor M14 are short-circuited. The gate of transistor M14 is connected to the gate of transistor M11, and its source is connected to the output terminal Vout.

[0054] In such a circuit, when the output voltage Vout approaches the input voltage Vin and the difference between Vgs_M11 of transistor M11 and Vgs_M14 of transistor M14 becomes the detection voltage Vdet, transistor M11 turns on. As a result, current flows through transistors M10 and M9 via transistor M11. When the output voltage becomes almost equal to the input voltage (Vout = Vin), the gate-source voltage Vgs_M13 of transistor M13 becomes Vcp, and transistor M13 turns fully on.

[0055] In this way, in this circuit, the detection voltage Vdet is defined by the difference between Vgs_M11 of transistor M11 and Vgs_M14 of transistor M14, and is independent of Vgs_M13 of transistor M13, which changes with the soft-start current. Therefore, more accurate full-on control can be achieved.

[0056] "Effects of the embodiment" According to this embodiment, the reference for the soft-start current is first converted to a current proportional to the Vgs of the low-side transistor M1, thereby reducing the current consumption of the circuit for controlling the soft-start current.

[0057] Furthermore, the output current of the transistor M13, which is a large power MOS transistor, can be detected by detecting the difference between the output voltage Vout and the input voltage Vin, allowing a wide range of selection for the current conversion ratio at the output.

[0058] Furthermore, the soft start control starts automatically when the power is turned on, and therefore can be reliably performed without being affected by the configuration of the load 16 or the like.

[0059] Furthermore, Vgs_M13 of the output transistor M13 can be set to a preset boost voltage Vcp, and Vds of the output transistor M13 can be maintained at approximately 0 V, thereby achieving operation in full-on mode. [Explanation of symbols]

[0060] 10 On-Off Control Block, 12 Soft Start Reference Block, 14 Full-On Control Block, 16 Load.

Claims

1. an n-channel output transistor having a drain connected to a power supply and applying an output from a source to a load when the output is on; a soft start reference block that applies a soft start voltage between the gate and source of the output transistor for a predetermined period from the start of output on; Including, The soft start reference block comprises: applying a soft start voltage corresponding to a gate-source voltage of a reference transistor that passes a current from a current source between the gate and source of the output transistor; The size of the reference transistor is smaller than that of the output transistor. Load drive circuit.

2. 2. The load driving circuit according to claim 1, the reference transistor has similar characteristics to the output transistor; Load drive circuit.

3. 2. The load driving circuit according to claim 1, The soft start reference block comprises: a current corresponding to the gate-source voltage of a reference transistor is passed through a resistor disposed between the gate and source of the output transistor, thereby applying a soft start voltage to the gate and source of the output transistor; Load drive circuit.

4. 2. The load driving circuit according to claim 1, the predetermined period from the start of output-on is a period from the output-on until the source voltage of the output transistor reaches a predetermined value; Load drive circuit.

5. 2. The load driving circuit according to claim 1, moreover, a full-on control block for applying a full-on voltage between the gate and source of the output transistor after a predetermined period has elapsed since the start of output on; include, Load drive circuit.

6. 6. The load driving circuit according to claim 5, The full-on control block is a predetermined current is passed through a resistor disposed between the gate and source of the output transistor, thereby applying a full-on voltage between the gate and source of the output transistor; Load drive circuit.

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