JPH10160769A - Overcurrent detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an overcurrent detection circuit which functions reliably regardless of the load impedance while lowering the detection level upon short circuit of the load. SOLUTION: Resistance of a resistor 51 is set such that a transistor 53 is turned on with a current lower than a maximum load current. A voltage corresponding to the load current is generated in a load 30. Resistance of resistors 54, 56 is set such that the voltage level at a terminal 60a of a control section 60 exceeds a threshold voltage Vth when the voltage between the terminals of the load 30 (emitter voltage of the transistor 53) exceeds a voltage level Vk. Since the transistor 53 is turned on before a maximum load current flows and the voltage of the load 30 is substantially '0' when the load is short- circuited, the voltage level at the terminal 60a is lower than the voltage Vth. When the impedance of the load 30 is high, the voltage of the load 30 is higher than the voltage level Vk even if the transistor 53 is turned on and the voltage level at the terminal 60a is higher than the voltage Vth. Consequently, an overcurrent can be detected based on the voltage level at the terminal 60a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an overcurrent detection circuit. Specifically, the current detection means generates a detection signal corresponding to the load current, and based on the voltage generated at the load by the load current and the detection signal from the current detection means, whether the overcurrent has flowed in the overcurrent determination means. By discriminating whether or not the overcurrent detection level is lowered, a reliable overcurrent detection operation is obtained.

[0002]

2. Description of the Related Art In audio output devices such as stereo devices,
For example, when an output terminal is short-circuited or abnormal oscillation occurs, an excessive current may flow through the power amplifier. For this reason, an overcurrent detection circuit is provided which detects a current flowing through the power amplifier and outputs an overcurrent detection signal when a predetermined current or more flows, and a power supply for the power amplifier is provided based on the overcurrent detection signal. And the supply of the drive signal is stopped to protect the power amplifier.

FIG. 2 shows a configuration in which a conventional overcurrent detection circuit is applied to an audio output device. In FIG. 2, the audio signal SIN output from the audio signal generator 11 is shown.
Is supplied to the amplifier section 20 via the resistor 12. In the amplifier section 20, the audio signal SIN is amplified and the drive signal SD
1, SD2 is generated. The amplifier section 20 is of a two-power-supply driving system, and receives power supply voltages ± from power supply terminals 15a and 15b.
Vp is supplied.

The drive signal SD output from the amplifier unit 20
1 is an NPN transistor 21 constituting a power amplifier.
And the drive signal SD2 is supplied to the base of a PNP transistor 22 constituting a power amplifier.

[0005] The transistor 21 and the transistor 22 are complementary connected, and the emitter of the transistor 21 is connected to the emitter of the transistor 22. The collector of the transistor 21 is connected to the switch 14.
The positive power supply voltage + Vp is supplied to the power supply terminal 15a via a. The collector of the transistor 22 is connected to the power supply terminal 15b via the resistor 23 for detecting overcurrent and the switch 14b, and the negative power supply voltage -Vp is supplied. The emitter of the transistor 21 and the transistor 22
Is connected to an output terminal, and an output signal SOUT obtained by amplifying the audio signal SIN is supplied to a load 30 such as a speaker via the switch 16 from the output terminal.

The collector of the transistor 22 and the resistor 23
The connection point is connected to the base of an NPN transistor 25 via a resistor 24. This transistor 2
5 has a power supply terminal 1 via a switch 14b.
5b, and the collector is connected to the signal input terminal 40a of the protection operation control unit 40 via the resistor 26.

In the protection operation control unit 40, the signal input terminal 4
Overcurrent determination is performed based on the voltage level of 0a, and a control signal SWC is generated based on the detection result and output from the signal output terminal 40b. In addition, the power supply voltage Vqa is supplied to the protection operation control unit 40 from the power supply terminal 15c.

The connection point between the resistor 12 and the amplifier section 20 is grounded via a switch 13, and the amplifier section 20 has a power supply voltage ± V via switches 14a and 14b.
p is supplied.

The above switches 13, 14a, 14b, 1
In 6, the switch operation is controlled by the control signal SWC output from the signal output terminal 40 b of the protection operation control unit 40.

In the audio output device configured as described above,
The collector current of transistor 22 is detected as terminal voltage V23 of resistor 23. Here, this terminal voltage V
When the voltage 23 exceeds the base-emitter voltage VBE of the transistor 25, the transistor 25 is turned on, and the signal level of the signal input terminal 40a of the protection operation control unit 40 is reduced. For this reason, the protection operation control unit 40 determines that an overcurrent has flowed, and the control signal SWC switches, for example, the switch 13 from the OFF state to the ON state to perform a muting operation of the audio signal SIN, or , The switches 14a and 14b are switched from the on state to the off state, and the power supply voltage
The supply of Vp is stopped, or the switch 16 is switched from the on state to the off state, and the supply of the output signal SOUT to the load 30 is stopped, thereby preventing an overcurrent from flowing.

[0011]

By the way, in such an overcurrent detection circuit, a current exceeding the maximum load current is supplied to the power amplifier by short-circuiting the load 30, and it is confirmed whether the overcurrent protection operation is correctly performed. Is done.

The overcurrent detection level when the load is short-circuited is set to a value sufficiently smaller than the maximum load current, and the overcurrent protection operation is performed without flowing a current exceeding the maximum load current even when the load 30 is short-circuited. Can be confirmed, for example, when a speaker having a small nominal impedance is used as the load 30, when the impedance of the speaker is reduced by the frequency, or when the output is increased, the load 3 is increased.
A large current flows because the impedance of 0 is small,
There is a risk that the overcurrent protection operation may be activated by mistake.

Accordingly, the present invention provides an overcurrent detection circuit that can reduce the level of overcurrent detection when a load is short-circuited and that can perform a reliable overcurrent detection operation regardless of the load impedance.

[0014]

An overcurrent detection circuit according to the present invention comprises a current detection means for generating a detection signal in accordance with a load current, and a detection of a voltage generated at the load by the load current and the current detection means. Overcurrent determining means for determining whether or not an overcurrent has flowed based on the signal.

In the present invention, a detection signal corresponding to the load current is generated by the current detection means. The load generates a voltage according to the load current. In the overcurrent determination means,
When the voltage generated at the load does not exceed the predetermined level and the load current exceeds the predetermined level based on the detection signal, it is determined that an overcurrent has flown.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an overcurrent detection circuit according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration in which the overcurrent detection circuit is applied to a sound output device. Note that, in FIG. 1, the portions corresponding to those in FIG. 2 are denoted by the same reference numerals.

As in the case of FIG. 2, the audio signal SIN output from the audio signal generator 11 is amplified to a predetermined level by the amplifier 20, and becomes drive signals SD1 and SD2. These drive signals SD1 and SD2 are supplied to the bases of transistors 21 and 22, respectively, constituting a power amplifier.

The collector of the transistor 21 is connected to the power supply terminal 15a and supplied with the positive power supply voltage + Vp. The collector of the transistor 22 is connected to the power supply terminal 15b.
To supply a negative power supply voltage -Vp. The transistor 21 and the transistor 22 are complementarily connected, and the emitter of the transistor 21 is connected to the emitter of the transistor 22 via a resistor 51 serving as current detecting means. From the connection point between the emitter of the transistor 22 and the resistor 51, the amplified audio signal SIN is supplied as an output signal SOUT to the load 30 such as a speaker via the switch 16.

The emitter of the transistor 21 and the resistor 51
Is connected via a resistor 52 to the base of a transistor 53 which is an overcurrent judging means. The emitter of the transistor 53 is connected to the connection point between the emitter of the transistor 22 and the resistor 51. The collector of transistor 53 is connected to the cathode of diode 55 via resistor 54. The anode of the diode 55 is connected to the power supply terminal 15 d via the resistor 56 and to the signal input terminal 60 a of the protection operation control unit 60.

The protection operation control unit 60 is constituted by using a microcomputer (hereinafter referred to as a "microcomputer"), and is supplied with a power supply voltage Vqb from a power supply terminal 15d. In the protection operation control unit 60, a signal input terminal 60a
The control signal SWC is generated by judging whether or not the voltage level exceeds the threshold voltage Vth.

The signal input terminal 6 of the protection operation control unit 60
Since the diode 55 is connected to 0a, it is possible to prevent the terminal voltage of the signal input terminal 60a from becoming higher than the power supply voltage Vqb even if the signal level of the output audio signal SOUT is high.

The signal input terminal 60a is connected to one terminal of a capacitor 57 and the cathode of a diode 58, and the other terminal of the capacitor 57 and the anode of the diode 58 are grounded. Therefore, the diode 5
8 prevents the terminal voltage of the signal input terminal 60a from being lower than the ground level by a voltage equal to or higher than the forward drop voltage of the diode 58. When the transistor 53 is turned off after being turned on, or when the voltage at the emitter of the transistor 53 is increased, a charging current flows to the capacitor 57 via the resistor 56, so that the signal input terminal It is assumed that the time required for the terminal voltage of 60a to exceed the threshold voltage Vth is long.

In this embodiment, when the transistor 53 is turned on by the voltage V51 between the terminals of the resistor 51, the current flowing through the resistor 51 has a predetermined current level smaller than the maximum load current. Is set to the resistance value of the resistor 51. When the transistor 53 is turned on and the voltage (emitter voltage of the transistor 53) generated at the load 30 is lower than the predetermined voltage level Vk, the terminal voltage of the signal input terminal 60a is changed to the threshold voltage. The resistance values of the resistors 54 and 56 are set so as to be smaller than Vth.

Next, the operation will be described. When the load 30 is short-circuited, the current flowing through the transistor 21 increases, and the inter-terminal voltage V51 of the resistor 51 as the current detecting means increases.
Here, transistor 53 is turned on at a predetermined current level at which the current flowing through the power amplifier is smaller than the maximum load current. Further, when the short circuit occurs, the impedance of the load 30 is very small, so that the voltage generated at the load 30 is lower than a predetermined voltage level Vk. Therefore, the terminal voltage of the signal input terminal 60a of the protection operation control unit 60 is set to a value smaller than the threshold voltage Vth. Therefore, the protection operation control unit 60 determines that an overcurrent has flowed due to the short circuit of the load 30. , The switch 13 is switched from the off state to the on state based on the control signal SWC,
Alternatively, the switches 14a and 14b and the switch 16 are switched from the ON state to the OFF state, and the overcurrent protection operation, that is, the current flowing to the power amplifier is limited.

Further, when a speaker having a small nominal impedance is used as the load 30 or when the impedance of the speaker is reduced depending on the frequency, when the output is increased, a large current flows because the impedance of the load 30 is small. For this reason, the voltage drop at the resistor 51 increases, and when the load current flowing through the power amplifier exceeds a predetermined current level, the transistor 53 is turned on as in the case where the load 30 is short-circuited. At this time, since the impedance of the load 30 is higher than that in the case where the load 30 is short-circuited or the like, the voltage generated at the load 30 is higher than a predetermined voltage level Vk. As described above, since the voltage generated at the load 30, that is, the voltage higher than the predetermined voltage level Vk is applied to the emitter of the transistor 53, even if the transistor 53 is turned on, the signal input terminal 60a The terminal voltage is set to a value larger than the threshold voltage Vth. Therefore, the protection operation control unit 60 erroneously loads the load 30 during operation.
It is possible to prevent the overcurrent protection operation from being activated due to the determination that an overcurrent has flowed due to the short circuit.

Further, when the voltage generated at the load 30 becomes higher than a predetermined voltage level Vk after the terminal voltage of the signal input terminal 60a is made lower than the threshold voltage Vth due to the short circuit of the load 30, , The capacitor 57 is charged via the resistor 56, so that the terminal voltage of the signal input terminal 60a becomes the threshold voltage Vth
It is assumed that a longer time is required until the size is increased. Therefore, the protection operation control unit 60 monitors the voltage of the signal input terminal 60a while continuing the other processing, so that it is possible to easily detect the overcurrent due to the short circuit of the load 30 without always monitoring the voltage. it can.

Thus, according to the above-described embodiment,
An overcurrent due to a load short circuit can be detected and an overcurrent protection operation can be started without flowing a current exceeding the maximum load current to the power amplifier. In addition, when a speaker with low impedance is used or when the impedance of the speaker is reduced depending on the frequency, even if the output is increased, it is not erroneously determined that an overcurrent has flowed, and the overcurrent should be reliably detected. Can be.

[0029]

According to the present invention, since the detection signal corresponding to the load current is generated by the current detection means, the detection signal is used to short-circuit the load without flowing a current exceeding the maximum load current. Overcurrent can be detected.
Since the load generates a voltage according to the load current, when a speaker with a small impedance is used as the load or when the impedance of the speaker is reduced by the frequency, the output is generated by using the voltage generated by the load. Even if the value is increased, it is not erroneously determined that an overcurrent has flowed, and the overcurrent can be reliably detected.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of an overcurrent detection circuit according to the present invention.

FIG. 2 is a diagram illustrating a configuration of a conventional overcurrent detection circuit.

[Explanation of symbols]

 11 audio signal generator 15a, 15b, 15c, 15d power supply terminal 20 amplifier unit 30 load 40, 60 protection operation control unit SIN audio signal SD1, SD2 drive signal SOUT output signal

Claims (3)

[Claims] A current detecting means for generating a detection signal in accordance with a load current; and determining whether an overcurrent has flowed based on a voltage generated in the load by the load current and a detection signal from the current detecting means. An overcurrent detection circuit, comprising overcurrent determination means for determining. 2. The overcurrent judging means according to claim 1, wherein when the voltage generated at the load by said load current is lower than a predetermined voltage level and said load current exceeds a predetermined current level based on said detection signal, 2. The overcurrent detection circuit according to claim 1, wherein it is determined that a current has flowed. 3. The overcurrent detection circuit according to claim 2, wherein said predetermined current level is set to a current level smaller than a maximum load current.