A programmable soft-start circuit and method suitable for NMOS load switch

Abstract

The present invention discloses a programmable soft-start circuit and method suitable for an NMOS load switch, comprising: a third current source for providing a load current to a transconductance amplifier, a feedback sampling module and an NMOS load switch; a ramp voltage reference module for generating a programmable ramp voltage reference, which is transmitted to the positive input terminal of the transconductance amplifier; a feedback sampling module for collecting the gate voltage of the NMOS load switch, which is transmitted to the negative input terminal of the transconductance amplifier, and negative feedback control is performed on the transconductance amplifier; the transconductance amplifier generates a corresponding feedback signal according to the input voltages of the positive input terminal and the negative input terminal, which is fed back to the gate of the NMOS load switch through the output terminal of the transconductance amplifier, and the soft-start process of the NMOS load switch is controlled. The present invention avoids complex frequency compensation through a simple structure, and at the same time, can programmatically realize flexible soft-start of the NMOS load switch.

Description

Technical Field

[0001] The present invention relates to the field of analog circuits and semiconductor integration technology, and in particular to a programmable soft-start circuit and method suitable for an NMOS load switch. Background Art

[0002] Inrush current refers to the impact current generated by the charging of capacitive loads such as batteries or load switches in DC / DC circuits when the switch is turned on. If the inrush current is too large and exceeds the tolerance of the capacitive load, it may cause damage to the equipment. Inrush current will cause a sudden drop in input voltage, which may also affect the stability of the entire circuit system. In addition, the size of the inrush current is directly affected by the switching speed of the pass transistor.

[0003] Soft Start refers to the circuit design that eliminates the surge current when the switching power supply starts, thereby effectively preventing the above surge current. The startup of the switching power supply is controlled by "soft start" to eliminate the surge current. The design principle is as follows: remove the load at the moment of power-on while limiting the useful current.

[0004] The existing soft start circuit of NMOS load switch is as follows Figure 1 As shown, there are the following defects: the soft-start circuit is an open-loop structure, and its soft-start capacitor 102 cannot play a soft-start role if it is too small, and will occupy a large layout area in the chip if it is too large, wasting layout resources; and once the current source 101 and the soft-start capacitor 102 are fixed, the soft-start speed cannot be adjusted programmably, which is not suitable for flexible soft-start time adjustment under different loads. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a programmable soft-start circuit and method suitable for an NMOS load switch, which avoids complex frequency compensation through a simple structure and can programmably implement flexible soft-start of the NMOS load switch.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solutions: a programmable soft-start circuit suitable for an NMOS load switch, comprising: a ramp voltage reference module, a transconductance amplifier, a third current source, a feedback sampling module and an NMOS load switch;

[0007] The third current source is used to provide load current to the transconductance amplifier, the feedback sampling module and the NMOS load switch;

[0008] The ramp voltage reference module is used to generate a programmable ramp voltage reference, which is transmitted to the non-inverting input terminal of the transconductance amplifier;

[0009] The feedback sampling module is used to collect the gate voltage of the NMOS load switch, transmit it to the inverting input terminal of the transconductance amplifier, and perform negative feedback control on the transconductance amplifier;

[0010] The transconductance amplifier generates a corresponding feedback signal according to the input voltage of the non-phase input terminal and the inverting input terminal, and feeds back to the gate of the NMOS load switch through the output terminal of the transconductance amplifier to control the soft start process of the NMOS load switch.

[0011] Furthermore, the slope voltage reference module includes: a first current source, a first switch, a first capacitor and a second switch, the input end of the first current source is connected to the operating voltage VDD, the output end of the first current source is connected to one end of the first switch, the other end of the first switch, the positive end of the first capacitor, and one end of the second switch are all connected to the non-inverting input end of the transconductance amplifier, and the negative end of the first capacitor and the other end of the second switch are both grounded.

[0012] Furthermore, the soft-start speed of the NMOS load switch is controlled by programmably adjusting the slope voltage reference of the first current source or the capacitance value of the first capacitor.

[0013] Furthermore, the feedback sampling module includes: a first resistor, a second resistor and a third switch, one end of the third switch, one end of the first resistor and one end of the second resistor are all connected to the inverting input terminal of the transconductance amplifier, and the other end of the second resistor and the other end of the third switch are both grounded.

[0014] Furthermore, the other end of the first resistor is respectively connected to the output end of the transconductance amplifier, the output end of the third current source, and the gate of the NMOS load switch.

[0015] Further, the transconductance amplifier includes: a second current source, a first PMOS tube, a second PMOS tube, a first NMOS tube, a second NMOS tube, a third NMOS tube, a zero adjustment resistor and a Miller capacitor, the input end of the second current source is connected to the working voltage VDD, the output end of the second current source is respectively connected to the source of the first PMOS tube and the source of the second PMOS tube, the drain of the first PMOS tube is respectively connected to the drain of the first NMOS tube, the gate of the first NMOS tube, and the gate of the second NMOS tube, the drain of the second NMOS tube, the drain of the second PMOS tube, one end of the zero adjustment resistor and the gate of the third NMOS tube are connected, the other end of the zero adjustment resistor is connected to one end of the Miller capacitor, and the source of the first NMOS tube, the source of the second NMOS tube, and the source of the third NMOS tube are all grounded.

[0016] Furthermore, the gate of the second PMOS tube serves as a non-inverting input terminal of the transconductance amplifier, the gate of the first PMOS tube serves as an inverting input terminal of the transconductance amplifier, and the other end of the Miller capacitor and the drain of the third NMOS tube serve as output terminals of the transconductance amplifier.

[0017] Furthermore, the present invention also provides a soft-start method for the programmable soft-start circuit applicable to an NMOS load switch, and the specific process is as follows:

[0018] In the soft-start control stage, the first switch is closed, the second switch and the third switch are opened, the first current source in the ramp voltage reference module charges the first capacitor, and the generated ramp voltage signal is fed back to the non-inverting input terminal of the transconductance amplifier; the third current source receives current from the charge pump output voltage VCP source, the feedback sampling module samples the gate voltage of the NMOS load switch, and feeds back to the inverting input terminal of the transconductance amplifier, the voltage signal at the inverting input terminal is clamped and limited by the voltage signal at the non-inverting input terminal of the transconductance amplifier, and a pull-down current is generated through the third NMOS tube. At this time, the current generated by the third current source flows to the NMOS load switch, the feedback sampling module and the third NMOS tube respectively, and the ramp voltage reference module is adjusted according to the soft-start speed of the NMOS load switch to realize the soft-start process of the NMOS load switch;

[0019] At the end of the soft start, the voltage signal at the inverting input terminal of the transconductance amplifier is less than the voltage signal at the non-inverting input terminal, and the gate voltage of the NMOS load switch is charged and pulled up by the third current source to be equal to the charge pump output voltage VCP, and the soft start ends;

[0020] In the soft start recovery stage, there is no input current in the third current source, the first switch is disconnected, the second switch and the third switch are closed, the voltage signals at the non-inverting input and the inverting input of the transconductance amplifier are both 0, and the gate voltage of the NMOS load switch is pulled down to 0.

[0021] Furthermore, the soft start time T of the programmable soft start circuit is:

[0022]

[0023] Among them, V ngate represents the gate voltage of the NMOS load switch, C s represents the capacitance value of the first capacitor, I0 represents the output current of the first current source, and N represents the resistance ratio of the second resistor to the first resistor.

[0024] Compared with the prior art, the present invention has the following beneficial effects: the programmable soft-start circuit of the present invention applicable to the NMOS load switch is composed of a ramp voltage reference module, a transconductance amplifier, and a feedback sampling module to form a soft-start closed-loop loop. The feedback sampling module samples the gate voltage of the NMOS load switch and can achieve frequency compensation of the soft-start closed-loop loop only through simple Miller compensation, and has a simple structure. At the same time, the ramp voltage reference module in the present invention can adjust the slope of the ramp voltage reference through programming to control the soft-start speed of the NMOS load switch, thereby achieving flexible adjustment of the soft-start of the NMOS load switch. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of a soft-start circuit of an existing NMOS load switch;

[0026] Figure 2 The present invention is a schematic diagram of a programmable soft-start circuit applicable to an NMOS load switch;

[0027] Figure 3 This is a schematic diagram of the ramp voltage reference module circuit in the present invention;

[0028] Figure 4 This is a circuit diagram of the feedback sampling module in the present invention;

[0029] Figure 5 Schematic diagram of the transconductance amplifier circuit in the present invention. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further explained below in conjunction with the accompanying drawings.

[0031] like Figure 2The present invention is a schematic diagram of a programmable soft-start circuit applicable to an NMOS load switch, the programmable soft-start circuit comprising: a ramp voltage reference module 201, a transconductance amplifier 202, a third current source 203, a feedback sampling module 204 and an NMOS load switch 205; the third current source 203 receives current from a charge pump output voltage VCP source, and is used to provide a load current to the transconductance amplifier 202, the feedback sampling module 204 and the NMOS load switch 205; the ramp voltage reference module 201 is used to generate a programmable ramp voltage reference, and transmit it to the positive input terminal 21 of the transconductance amplifier 202; the feedback sampling module 204 is used to collect the gate voltage of the NMOS load switch 205, and transmit it to the negative input terminal 22 of the transconductance amplifier 202, so as to perform negative feedback control on the transconductance amplifier 202; the transconductance amplifier 202 generates a corresponding feedback signal according to the input voltages of the positive input terminal 21 and the negative input terminal 22, and feeds back to the gate of the NMOS load switch 205 through the output terminal of the transconductance amplifier 202, so as to control the soft-start process of the NMOS load switch 205. The present invention comprises a ramp voltage reference module 201, a transconductance amplifier 202 as a feedforward path, and a feedback sampling module 204 as a feedback path, which together form a soft-start closed-loop loop. The feedback sampling module 204 samples the gate voltage of the NMOS load switch 205, and can achieve frequency compensation of the soft-start closed-loop loop through simple Miller compensation. The structure is simple. At the same time, the ramp voltage reference module 201 in the present invention can adjust the slope of the ramp voltage reference through programming to control the soft-start speed of the NMOS load switch 205, thereby achieving flexible adjustment of the soft-start of the NMOS load switch 205.

[0032] like Figure 3 The ramp voltage reference module 201 includes: a first current source 301, a first switch 302, a first capacitor 303 and a second switch 304. The input end of the first current source 301 is connected to the working voltage VDD, the output end of the first current source 301 is connected to one end of the first switch 302, the other end of the first switch 302, the positive end of the first capacitor 303 and one end of the second switch 304 are all connected to the non-inverting input end 21 of the transconductance amplifier 202, and the negative end of the first capacitor 303 and the other end of the second switch 304 are all grounded. The first current source 301 is used to charge the first capacitor 303, thereby generating a ramp voltage reference. By changing the capacitance value of the first capacitor 303, the slope of the ramp voltage reference can be changed, thereby controlling the soft start speed of the NMOS load switch. Therefore, the present invention realizes the soft start function of the NMOS load switch 205 by programmably adjusting the ramp voltage reference of the first current source 301 or the capacitance value of the first capacitor 303.

[0033] like Figure 4The feedback sampling module 204 includes: a first resistor 401, a second resistor 402 and a third switch 403. One end of the third switch 403, one end of the first resistor 401 and one end of the second resistor 402 are all connected to the inverting input end of the transconductance amplifier 202, and the other end of the second resistor 402 and the other end of the third switch 403 are all grounded. The other end 23 of the first resistor 401 is respectively connected to the output end of the transconductance amplifier 202, the output end of the third current source 203 and the gate of the NMOS load switch 205. If the feedback sampling module 204 directly samples the output end VOUT of the NMOS load switch 205, since the load resistor 206 and the load capacitor 207 on the output end VOUT may have various permutations and combinations, the frequency compensation of the entire soft-start loop will be very complicated and difficult to achieve compensation. However, the present invention samples the gate voltage of the NMOS load switch 205 through the feedback sampling module 204, shields the load resistor 206 and the load capacitor 207 outside the soft-start loop, and its frequency compensation only needs simple Miller compensation to achieve.

[0034] like Figure 5The transconductance amplifier 202 includes: a second current source 501, a first PMOS tube 502, a second PMOS tube 504, a first NMOS tube 503, a second NMOS tube 505, a third NMOS tube 508, a zero adjustment resistor 506 and a Miller capacitor 507. The input end of the second current source 501 is connected to the working voltage VDD, and the output end of the second current source 501 is respectively connected to the source of the first PMOS tube 502 and the source of the second PMOS tube 504. The drain of the first PMOS tube 502 The electrodes are respectively connected to the drain of the first NMOS tube 503, the gate of the first NMOS tube 503, and the gate of the second NMOS tube 505, the drain of the second NMOS tube 505, the drain of the second PMOS tube 504, one end of the zero adjustment resistor 506, and the gate of the third NMOS tube 508, the other end of the zero adjustment resistor 506 is connected to one end of the Miller capacitor 507, and the source of the first NMOS tube 503, the source of the second NMOS tube 505, and the source of the third NMOS tube 508 are all grounded. The gate of the second PMOS tube 504 serves as the positive phase input terminal 21 of the transconductance amplifier 202, the gate of the first PMOS tube 502 serves as the negative phase input terminal 22 of the transconductance amplifier 202, and the other end of the Miller capacitor 507 and the drain of the third NMOS tube 508 serve as the output terminal of the transconductance amplifier 202. There are two poles in the bandwidth of the soft-start closed loop composed of the ramp voltage reference module 201, the transconductance amplifier 202, and the feedback sampling module 204. The main pole is at the gate of the third NMOS tube 508, and the secondary pole is at the drain of the third NMOS tube 508. Therefore, the Miller capacitor 507 is used to push the main pole to the front of the bandwidth and the secondary pole to the outside of the bandwidth. The equivalent value of the Miller capacitor 507 is greatly increased through the Miller effect, thereby achieving the purpose of changing the circuit frequency characteristics without increasing the actual capacitor size. In order to avoid the right half plane zero point generated by the Miller capacitor 507, it is offset by the zero adjustment resistor 506, thereby realizing loop compensation.

[0035] The present invention also provides a soft-start method for a programmable soft-start circuit suitable for an NMOS load switch. The overall working process is divided into three stages: the first stage is a soft-start control stage, the second stage is a soft-start end stage, and the third stage is a soft-start recovery stage. The specific process is as follows:

[0036] In the soft-start control stage, the first switch 302 is closed, the second switch 304 and the third switch 403 are opened, the first current source 301 in the ramp voltage reference module 201 charges the first capacitor 303, and the generated ramp voltage signal is fed back to the non-inverting input terminal 21 of the transconductance amplifier 202; the third current source 203 receives current from the charge pump output voltage VCP source, the feedback sampling module 204 samples the gate voltage of the NMOS load switch 205, and feeds back to the inverting input terminal 22 of the transconductance amplifier 202, the voltage signal of the inverting input terminal 22 is clamped and limited by the voltage signal of the non-inverting input terminal 21 of the transconductance amplifier 202, and the third NMOS tube 508 is turned on and generates a pull-down current. At this time, part of the current generated by the third current source 203 charges the NMOS load switch 205, part of the current flows to the feedback sampling module 204, and the remaining part flows to the third NMOS transistor 508. The gate voltage signal in the NMOS load switch 205 is affected by the third current source 203, the third NMOS transistor 508 and the feedback sampling module 204. The ramp voltage reference module 201 is adjusted according to the soft-start speed of the NMOS load switch 205 to realize the soft-start process of the NMOS load switch 205.

[0037] At the end of the soft start, when the voltage at the output terminal VOUT of the NMOS load switch 205 rises linearly to be equal to the voltage at the input terminal VIN, the drain of the NMOS load switch 205 serves as the input terminal VIN of the NMOS load switch 205, and the voltage signal at the inverting input terminal 22 of the transconductance amplifier 202 is less than the voltage signal at the non-inverting input terminal 21. The transconductance amplifier 202 changes from an operational amplifier to a comparator, and the ramp reference signal input to the non-inverting input terminal 21 is charged and raised to the operating voltage VDD, and the level signal at the inverting input terminal 22 fed back rises at a rate less than the voltage signal at the non-inverting input terminal 21. Therefore, the third NMOS tube 508 is in the off state, and the gate voltage of the NMOS load switch 205 is charged and pulled up by the third current source 203 to be equal to the charge pump output voltage VCP, and the soft start ends;

[0038] In the soft-start recovery stage, there is no input current in the third current source 203, the first switch 302 is disconnected, the second switch 304 and the third switch 403 are closed, and the voltage signals at the positive input terminal 21 and the negative input terminal 22 of the transconductance amplifier 202 are both 0, so that the soft-start circuit is in the original recovery state, and the gate voltage of the NMOS load switch 205 is pulled down to 0.

[0039] The soft start time T of the programmable soft start circuit in the present invention is:

[0040]

[0041] Among them, V ngate represents the gate voltage of the NMOS load switch 205, Cs represents the capacitance of the first capacitor 303 , I0 represents the output current of the first current source 301 , and N represents the resistance ratio of the second resistor 402 to the first resistor 401 .

[0042] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A programmable soft-start circuit suitable for an NMOS load switch, characterized in that: include: A ramp voltage reference module (201), a transconductance amplifier (202), a third current source (203), a feedback sampling module (204) and an NMOS load switch (205); The third current source (203) is used to provide a load current to the transconductance amplifier (202), the feedback sampling module (204) and the NMOS load switch (205); The slope voltage reference module (201) is used to generate a programmable slope voltage reference, which is transmitted to the non-inverting input terminal (21) of the transconductance amplifier (202); The feedback sampling module (204) is used to collect the gate voltage of the NMOS load switch (205), transmit it to the inverting input terminal (22) of the transconductance amplifier (202), and perform negative feedback control on the transconductance amplifier (202); The transconductance amplifier (202) generates a corresponding feedback signal according to the input voltages of the non-phase input terminal (21) and the inverting input terminal (22), and feeds back the feedback signal to the gate of the NMOS load switch (205) through the output terminal of the transconductance amplifier (202), thereby controlling the soft-start process of the NMOS load switch (205); The ramp voltage reference module (201) comprises: a first current source (301), a first switch (302), a first capacitor (303) and a second switch (304); the input end of the first current source (301) is connected to an operating voltage VDD, the output end of the first current source (301) is connected to one end of the first switch (302), the other end of the first switch (302), the positive end of the first capacitor (303) and one end of the second switch (304) are all connected to a non-inverting input end (21) of a transconductance amplifier (202), and the negative end of the first capacitor (303) and the other end of the second switch (304) are all grounded; Controlling the soft start speed of the NMOS load switch (205) by programmably adjusting the ramp voltage reference of the first current source (301) or the capacitance value of the first capacitor (303); The feedback sampling module (204) comprises: a first resistor (401), a second resistor (402) and a third switch (403), one end of the third switch (403), one end of the first resistor (401) and one end of the second resistor (402) are all connected to the inverting input end of the transconductance amplifier (202), and the other end of the second resistor (402) and the other end of the third switch (403) are both grounded; The other end (23) of the first resistor (401) is respectively connected to the output end of the transconductance amplifier (202), the output end of the third current source (203), and the gate of the NMOS load switch (205); The transconductance amplifier (202) comprises: a second current source (501), a first PMOS tube (502), a second PMOS tube (504), a first NMOS tube (503), a second NMOS tube (505), a third NMOS tube (508), a zero adjustment resistor (506) and a Miller capacitor (507); an input end of the second current source (501) is connected to an operating voltage VDD; an output end of the second current source (501) is respectively connected to a source of the first PMOS tube (502) and a source of the second PMOS tube (504); and the first PMOS tube (502) is connected to a source of the second PMOS tube (504). The drain of the first NMOS tube (503), the gate of the first NMOS tube (503), and the gate of the second NMOS tube (505) are respectively connected; the drain of the second NMOS tube (505), the drain of the second PMOS tube (504), one end of the zero adjustment resistor (506), and the gate of the third NMOS tube (508) are connected; the other end of the zero adjustment resistor (506) is connected to one end of the Miller capacitor (507); the source of the first NMOS tube (503), the source of the second NMOS tube (505), and the source of the third NMOS tube (508) are all grounded; The gate of the second PMOS tube (504) serves as a positive input terminal (21) of the transconductance amplifier (202), the gate of the first PMOS tube (502) serves as a negative input terminal (22) of the transconductance amplifier (202), and the other end of the Miller capacitor (507) and the drain of the third NMOS tube (508) serve as output terminals of the transconductance amplifier (202).

2. A soft-start method for a programmable soft-start circuit suitable for an NMOS load switch according to claim 1, characterized in that: The specific process is as follows: In the soft start control stage, the first switch (302) is closed, the second switch (304) and the third switch (403) are opened, the first current source (301) in the slope voltage reference module (201) charges the first capacitor (303), and the generated slope voltage signal is fed back to the positive phase input terminal (21) of the transconductance amplifier (202); the third current source (203) receives current from the charge pump output voltage VCP source, the feedback sampling module (204) samples the gate voltage of the NMOS load switch (205), and feeds back to the negative phase input terminal (21) of the transconductance amplifier (202). The voltage signal of the input terminal (22), the inverting input terminal (22) is clamped and limited by the voltage signal of the non-inverting input terminal (21) through the transconductance amplifier (202), and a pull-down current is generated through the third NMOS tube (508). At this time, the current generated by the third current source (203) flows to the NMOS load switch (205), the feedback sampling module (204) and the third NMOS tube (508) respectively, and the ramp voltage reference module (201) is adjusted according to the soft start speed of the NMOS load switch (205), so as to realize the soft start process of the NMOS load switch (205); At the end of the soft start, the voltage signal at the inverting input terminal (22) of the transconductance amplifier (202) is smaller than the voltage signal at the non-inverting input terminal (21), the gate voltage of the NMOS load switch (205) is charged and pulled up by the third current source (203) to be equal to the charge pump output voltage VCP, and the soft start ends; In the soft start recovery phase, no current is sourced into the third current source (203), the first switch (302) is disconnected, the second switch (304) and the third switch (403) are closed, the voltage signals at the non-inverting input terminal (21) and the inverting input terminal (22) of the transconductance amplifier (202) are both 0, and the gate voltage of the NMOS load switch (205) is pulled down to 0.

3. The soft-start method of the programmable soft-start circuit applicable to the NMOS load switch according to claim 2, characterized in that: The soft start time T of the programmable soft start circuit is: ， in, represents the gate voltage of the NMOS load switch (205), represents the capacitance value of the first capacitor (303), represents the output current of the first current source (301), and N represents the resistance ratio of the second resistor (402) to the first resistor (401).

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