JP2000013991A - Method and circuit for limiting overcurrent using mosfet switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To gradually limit an overcurrent without cutting off the the overcurrent immediately by switching to by-passes by outputting a flag by connecting two MOSFET switches having different 'ON' resistances in parallel with the current path of a high-side switch when the overcurrent is detected. SOLUTION: The P-channel MOSFETs of switches 1 and 2 having different 'ON' resistances are connected in parallel. A D/A output 5 which makes analog output operation is connected to the gate of the MOSFET of the switch 1 having the lower 'ON' resistance. The value of the current flowing to the switch 1 is detected with a delay time of >=10 μs. When an overcurrent detecting means 3 detects that the current flowing to the switch 1 for >=10 μs is an overcurrent, a current limiting means 4 finally sets the switch 1 to a turned-off state (infinite resistance value) by increasing the resistance of the switch 1 by gradually changing the gate voltage of the MOSFET of the switch 1 to the voltage of an input 7 from 0 V. In addition, the means 4 turns on the MOSFET of the switch 2 having the higher 'ON' resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION Two MOSFETs (MOS Fis) having different on-resistances are provided in a current path in a high-side switch.
(eld Effect Transistor) A switch is connected in parallel, and when an overcurrent is detected, a flag is output and the switch is switched to bypass to gradually limit the overcurrent without cutting off the current immediately. To a control method and a circuit thereof.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing an example of the prior art. Referring to FIG.
And a gate control means 13 for turning on / off the switch 11 and an overcurrent detection means 12 for notifying the gate control circuit of an overcurrent detection result.

When the switch 11 is turned on, the gate voltage of the switch 11 is set to 0 V, and when the switch 11 is turned off, the switch 1 is turned on.
The on / off control of the switch 11 can be performed by using the gate voltage of 1 as the voltage of the input 14.

At the time of overcurrent detection, the flag output 17 is changed from "H" level to "L" level to notify the outside.

On / off of the switch 11 is controlled by a control input signal 16.

During overcurrent detection, current limitation cannot be performed. Therefore, when a heavy load is connected to the output 15, in the worst case, the chip is destroyed due to power consumption inside the IC, and there is a disadvantage that the output is short-circuited or open.

As described above, if an excessive current continues to flow, if the period until the external control input signal 16 is received is long, the IC will exceed the junction temperature due to self-heating, causing destruction or damage. The bonding wire of the IC may be broken by a large current. Therefore, in the conventional circuit configuration, it is difficult to keep the switch on even after detecting the overcurrent.

[0008] As a prior art, Japanese Patent Publication No. 2-2607.
There is a technology disclosed in Japanese Patent Publication No. An object of the present embodiment is to provide a switch circuit capable of accurately detecting the current of a MOS transistor arranged on the high side irrespective of the fluctuation of the power supply voltage.

For this reason, a current mirror circuit of an internal circuit is disclosed, but there is no description of overcurrent limitation and there is no relevance to the present invention.

[0010] In addition to the above, the following publications are searched as prior art.

JP-A-04-326108 JP-A-05-038134 JP-A-06-097375 JP-A-07-011031

[0012]

As described above, in the conventional prior art, current cannot be limited during overcurrent detection. Therefore, when a heavy load is connected to the output 15, in the worst case, the chip is destroyed due to power consumption inside the IC, and there is a disadvantage that the output is short-circuited or open.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high-side switch having an overcurrent limiting function in which two on-resistances having different on-resistances are different.
A switch is composed of OSFETs and one MOSFE
It is an object of the present invention to propose a method and a circuit for controlling a gate voltage of T to reduce a change in a current value when a transition from a normal operation to a current limiting operation is performed, thereby preventing a malfunction of the device.

[0014]

An overcurrent limiting circuit using a MOSFET switch according to the present invention is a MOSFE.
An IC having a flag output function of detecting a current value flowing through a switch composed of T and reporting to an external device when the current value exceeds a predetermined value, and a function of turning off the switch by a control input signal terminal, which are connected in parallel. Two switches 1 and 2 having different on-resistances, a D / A output means 5 connected to the gate of the low on-resistance MOSFET of the switch 1 for performing an analog output operation, and an overcurrent for outputting a flag for external notification. Detecting means 3 and a current value of the switch 1;
When the overcurrent detecting means 3 detects the specified current value, the signal is inputted to limit the output current.
And the MOSFETs of the switches 1 and 2 by the control input 9
And a gate control means 6 for controlling the on / off of the device.

It is an embodiment of the present invention that the transistors constituting the two switches are P-channel MOSFETs.

Further, the flag output for external notification 10
Is an "H" level during the normal operation of the switch 1 on and the switch 2 off, and is an "L" level when the switch 1 detects an overcurrent state in one embodiment of the present invention.

When the overcurrent detection means 3 detects that the current flowing through the switch 1 is overcurrent for 10 μs or more, the current limit control means 4 gradually reduces the gate voltage of the MOSFET having a low on-resistance of the switch 1. It is a preferred embodiment of the present invention to control the switch 1 to increase the resistance value by changing the voltage from 0 V to the voltage of the input 7.

Further, the overcurrent detecting means outputs an instantaneous large current for charging a load capacitance connected to an output by 10 or more.
In order to prevent overcurrent of more than μs and prevent erroneous detection,
C. It is also a preferred embodiment of the present invention to further include an R filter.

Also, in one embodiment, the specified current value is 500 mA.

The overcurrent limiting method using the MOSFET switch of the present invention is performed by turning on the low on-resistance MOSFET of the switch 1 and turning off the high on-resistance MOSFET of the switch 2 during normal operation. An IC that operates as a voltage drop switch, which is an overcurrent limiting method when an overcurrent flows through the switch, wherein the overcurrent detecting means 3 detects an overcurrent value flowing through the switch 1; The current limiting control means 4 for limiting the output current when the overcurrent detecting means detects a specified current value.
And the current limit control means 4 changes the gate voltage of the low on-resistance MOSFET of the switch 1 to D.
A step 33 in which the on-resistance of the switch 1 is increased by gradually changing from 0 V to the voltage of the input 7 by the / A converter;
Changing the gate voltage until the set current value is reached, and turning on the high on-resistance MOSFET of the switch 2 and turning off the low on-resistance MOSFET of the switch 1 after the current value reaches the set value; Fixing the current value to a set value.

[0021]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a circuit to which an overcurrent limiting method according to the present invention is applied.

Referring to FIG. 1, two P-channel MOSFETs having different on-resistances of a switch 1 and a switch 2 are connected in parallel. Also, the low on-resistance of the switch 1 (about 1
A D / A output 5 for performing an analog output operation is connected to the gate of the MOSFET of 00 mΩ).

During normal operation, the low on-resistance MOSFET of the switch 1 is turned on, and the high on-resistance of the switch 2 (approximately 5
Ω), the MOSFET operates as a low-voltage drop switch. At this time, the flag output 10 for external notification output from the overcurrent detection means 3 is set to “H”.
Level.

The overcurrent detecting means 3 has a function of detecting the value of the current flowing through the switch 1, and the present IC uses a USB (Universal).
Used for high-side switch of power supply line for Serial Bus).

In the USB standard, the upper limit of the current supply of the power supply line is specified to be 500 mA, and when a current exceeding this value flows, the switch must be turned off.
However, since the switch receives an OFF signal from the controller IC, the switch maintains the ON state until the signal is input.
There is a case where the current is limited due to the problem of the current capacity of the device.

In practical use, an overcurrent must not be detected for an instantaneous surge current. This period is 1
It is about 0 μs. When a specified value (500 mA) is detected, a signal is sent to the current limit control means 4 to limit the output current. Further, the external output flag output 10 is set to the “L” level.

As shown in FIG. 2, a detection delay time (detection insensitive period) of 10 μs is provided for detecting the value of the current flowing through the switch 1. When the overcurrent detection means 3 detects that the current flowing through the switch 1 is overcurrent for 10 μs or more, the current limit control means 4 gradually lowers the gate voltage of the low on-resistance MOSFET of the switch 1 from 0 V to the voltage of the input 7. By changing the resistance, the resistance value of the switch 1 is increased, and finally the switch 1 is turned off (resistance value is infinite). Further, when the overcurrent detecting means 3 detects the overcurrent of the switch 1, the high on-resistance MOSFET of the switch 2 is turned on.

The gate control 6 controls on / off of a low on-resistance MOSFET of the switch 1 and a high on-resistance MOSFET of the switch 2 by a control input 9.

The operation of FIG. 1 will be described with reference to FIG.

At T1, when the input voltage rises from 0V to a predetermined voltage (5V in the case of USB) and the control input signal is set to "H" level at the same time, the switch 1 in FIG. 1 is turned on. Switch 2 remains off.

From T1 to T2, a large current flows instantaneously to charge the load capacitance connected to the output.
Although this should not be recognized as an overcurrent, the output to the current limit control means 4 and the flag output 10 flow through a CR filter (not shown) in the overcurrent detection means in FIG. Since no overcurrent is detected, no erroneous detection is performed.

At T3, when the switch 1 is turned on, the load connected to the output becomes heavy, and when the load exceeds the overcurrent detection value, the overcurrent detection means 3 detects the current value and the flag output 10
To output the detection result to the outside (flag output “H” →
"L").

From T3 to T4, the switch 1 remains on while the control input signal is "H",
The gate voltage is changed by the A output means 5, the resistance value is increased, and the current value is limited to a predetermined value (the overcurrent detection value or less). After limiting to a predetermined current value with switch 1,
Switch 1 is turned off and switch 2 is turned on to keep the resistance constant. By this operation, the resistance between the input 7 and the output 8 can be set accurately.

Therefore, from T4 to T5, the output current is in an overcurrent state, and it is possible to keep flowing a constant current value not exceeding the overcurrent detection value.

At T5, the switch 2 is turned off by the external control input 9, and the output is completely turned off.

[0036]

As described above, the present invention has the following effects as compared with the prior art.

The first effect is that the current flowing at the moment when a load is connected to the switch flows about 5 μs. By setting the overcurrent detection delay time to 10 μs, a flag is output at the moment of connection. The switch can be prevented from being turned off.

The second effect is that the switch 1 has a low on-resistance M
By controlling the gate voltage of the OSFET, the on-resistance of the MOSFET can be increased. The total resistance is determined by the parallel resistance value of the switch 2 with the high on-resistance MOSFET. As described above, by gradually changing the gate voltage control by the D / A output 5 through the current limit control means 4, the on-resistance of the MOSFET can be gradually increased, and the current value can be gradually reduced. Therefore, it is possible to suppress a sudden change in the current value and the voltage of the output 8 when an overcurrent is detected, and to prevent malfunction or failure of the device.

The third effect is that it is difficult to set the on-resistance to an appropriate value (about 5Ω) with the configuration using only the switch 1. Therefore, by additionally connecting a high on-resistance MOSFET of the switch 2 in parallel. The accuracy of the current value to be limited can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a circuit to which an overcurrent limiting method using a MOSFET switch according to the present invention is applied.

FIG. 2 is a diagram showing a timing chart according to the present invention.

FIG. 3 is a flowchart of a method for limiting overcurrent performed using a MOSFET switch according to the present invention.

FIG. 4 is a block diagram showing an embodiment of the prior art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Low ON resistance switch 2 High ON resistance switch 3 Overcurrent detection means 4 Current limit control means 5 D / A output means 6 Gate control means 7 Input 8 Output 9 Control input signal 10 Flag output 11 Low ON resistance switch 12 Overcurrent detection Means 13 Gate control means 14 Input 15 Output 16 Control input signal 17 Flag output

Claims (7)

[Claims] An IC having a flag output function of detecting a current value flowing through a switch constituted by a MOSFET and reporting the current to an external device when the current value exceeds a specified value, and a function of turning off the switch by a control input signal terminal. And two switches (1) and (2) having different on-resistances connected in parallel, and D / A output means (5) connected to the gate of the low on-resistance MOSFET of the switch (1) and performing an analog output operation. )
An overcurrent detecting means (3) for detecting a current value of the switch (1) and outputting a flag (10) for external notification when the current value is equal to or more than a specified current value; Is detected, the current limit control means (4) for inputting the signal to limit the output current, and the MO of the switches (1) and (2) are controlled by the control input (9).
A gate control means (6) for controlling ON / OFF of the SFET; 2. The overcurrent limiting circuit according to claim 1, wherein the transistors forming the two switches are P-channel MOSFETs. 3. An external notification flag output (10).
3. The switch according to claim 1, wherein the switch is at an "H" level during normal operation of the switch (1) on and the switch (2) off, and is at an "L" level when the switch (1) detects an overcurrent state. Overcurrent limit circuit. When the overcurrent detection means detects that the current flowing through the switch is an overcurrent of 10 μs or more, the current limit control means detects a low on-resistance of the switch. 4. The control according to claim 1, wherein the gate voltage of the MOSFET is gradually changed from 0 V to the voltage of the input (7) so as to increase the resistance value of the switch (1). Overcurrent limit circuit. 5. The overcurrent detecting means detects an instantaneous large current for charging a load capacitance connected to an output for 10 μs.
In order to prevent the above-described overcurrent and prevent erroneous detection, C.I. R
5. The overcurrent limiting circuit according to claim 1, further comprising a filter. 6. The overcurrent limiting circuit according to claim 1, wherein the specified current value is 500 mA. 7. An IC that operates as a low voltage drop switch by turning on a low on-resistance MOSFET of a switch 1 and turning off a high on-resistance MOSFET of a switch 2 during a normal operation. A method for limiting overcurrent when a current flows, wherein the overcurrent detection means 3 detects an overcurrent value flowing through the switch 1 (30, 31); and when the overcurrent detection means detects a prescribed current value. (32) sending a signal to the current limit control means 4 to perform the output current limit;
The date voltage of the OSFET is set to 0 V by the D / A converter.
(3) gradually increasing the on-resistance of the switch 1 by gradually changing the voltage from the input to the voltage of the input 7; and (34) changing the gate voltage until the current value reaches the set current value. After that, the high on-resistance MOSFET of the switch 2 is turned on, and the low on-resistance M of the switch 1 is turned on.
Turning off the OSFET and fixing the current value to a set value (35).