JP2010087692A - Btl amplifier protection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the following problem: when a power voltage used by a BTL (Bridged Tied Load) amplifier is low, the current of an output transistor becomes small in case of grounding or power supplying, and a protecting circuit is hardly actuated to easily cause an ASO (Area of Safety Operation) breakage. <P>SOLUTION: In the BTL amplifier protection circuit, a monitor current generating circuit 32-1 generates a monitor current I<SB>MO1</SB>corresponding to a turn-on current of an output transistor Q1. A protecting operation unit 28 compares the monitor current with a threshold to determine grounding. A bias current generating circuit 34-1 supplies no bias current I<SB>MB1</SB>when the power voltage V<SB>CC</SB>is not lower than the threshold, and a non-zero current when lower than the threshold. A correction monitor current I<SB>M1</SB>generated by adding I<SB>MB1</SB>to the I<SB>MO1</SB>is to be compared with the threshold. In a state where V<SB>CC</SB>is less than the threshold, the I<SB>M1</SB>is biased upward and then a substantial criterion value in an abnormal state decreases. Consequently, a current increase of the Q1 in an abnormal state is detected irrespective of whether the V<SB>CC</SB>is high or lower to perform the protecting operation. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a protection circuit provided in a BTL amplifier in which two push-pull circuits each composed of a pair of transistors connected in series between two reference power sources are connected to each other by BTL (Bridged Tied Load or Balanced Transformerless).

Conventionally, BTL amplifiers are used as power amplifiers for driving loads (speakers) such as audio. FIG. 3 is a circuit diagram showing a configuration of a conventional BTL amplifier 2. The BTL amplifier 2 has two push-pull circuits. Each push-pull circuit 4, 6 has a pair of transistors connected in series between two power sources such as a positive voltage source _VCC and a ground potential GND. The pair of transistors Q1 and Q2 constituting the push-pull circuit 4 are driven in opposite phases, and their connection point becomes an output terminal (+ OUT). Similarly, the pair of transistors Q3 and Q4 constituting the push-pull circuit 6 are also driven in opposite phases, and their connection point becomes the output terminal (-OUT). Further, the two push-pull circuits 4 and 6 are driven in opposite phases. For example, during the positive period of the input signal, the transistors Q1 and Q4 are turned on, and current flows from the terminal (+ OUT) to the terminal (−OUT) through the speaker RL, which is the load. The transistors Q3 and Q2 are turned on, and a current flows through the speaker RL from the terminal (−OUT) to the terminal (+ OUT).

Here, the BTL amplifier 2 can be fixed on the semiconductor substrate as an integrated circuit, but normally, the BTL amplifier 2 and the speaker RL are configured separately, and they are separately connected by a signal line. Therefore, erroneous connection to the output terminal of the amplifier or a short circuit of the signal line during use can occur. For this reason, the conventional BTL amplifier 2 also takes measures against such abnormal connection. Typical cases of abnormal connection or the like, a ground fault which the output terminal is short-circuited to GND, the and battery short conceivable shorted to V _CC. In an abnormal state where a ground fault or a power fault has occurred, the output transistor Q1 to Q4 has a voltage between the two terminals (collector-emitter voltage or drain-source voltage) increased from that during normal operation. Causes an excessive conduction current when an ON voltage is applied to the control terminal (base or gate). Specifically, when the ground fault output terminal, an excessive current to the upper transistors Q1 or Q3 is connected between the output terminal and V _CC flows, also when power supply fault is the output terminal An excessive current flows through the lower transistor Q2 or Q4 connected between the transistor and GND, and there is a risk of ASO (Area of Safety Operation) destruction.

As a countermeasure, a current (monitor currents I _{M1 to} I _M4 ) that changes in conjunction with the conduction currents of the output transistors Q1 to Q4 is generated, and a ground fault occurs when the monitor current exceeds a certain reference value I _TH. And a protection operation for turning off the output transistors Q1 to Q4 is performed by determining that a power fault has occurred. The comparison between the monitor current and the reference value I _TH and the protection operation are performed by the protection operation unit 8. The reference value I _TH is set to a constant value that basically does not vary with the power supply _VCC using a constant current circuit or the like provided in the protection operation unit 8.

The lower reference value I _TH is safer from the viewpoint of preventing the transistor from being destroyed, but it is required to set it higher than the maximum conduction current during normal use of the output transistor from the viewpoint of avoiding malfunction. I _TH is determined in consideration of both of these viewpoints.
JP-A-8-21855

The power supply voltage supplied to the BTL amplifier 2 can vary depending on, for example, the system in which the BTL amplifier 2 is used. When the voltage difference between the two reference power supplies to which the push-pull circuits 4 and 6 are connected changes, the voltage between both terminals of the output transistors Q1 to Q4 also changes, and the conduction current between both terminals also changes. For example, in a bipolar transistor, the collector current I _CE increases with the collector-emitter voltage V _CE in the saturation region, which is called the Early effect. Also in MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), a drain and a saturated region - the drain current I _D is known to increase gradually along with the source voltage V _DS.

Figure 4 is a graph showing respective and change the maximum load current I _LMAX and the maximum collector current I _CMAX of the output transistor to a change in power supply voltage, the relationship between the collector current threshold I _CTH protection circuit operates. Here, the maximum load current I _LMAX is the maximum value of the drive current supplied to the load such as the speaker RL, and is a state where the push-pull operation is normally performed in the push-pull circuits 4 and 6 of the BTL amplifier 2, that is, This corresponds to the maximum value of the collector currents I _CE (I _{CE1 to} I _CE4 ) of the output transistors Q1 to Q4 in a state where the output terminals (+ OUT) and (−OUT) are not _{grounded / powered} . Current flowing through the load RL, expressed the voltage between the output terminals _{V OUT,} is given by _{V OUT} / RL. Since V _OUT increases according to the amplitude of the input signal and its maximum value can be approximately V _CC , the maximum load current I _LMAX is about V _CC / RL. On the other hand, the maximum collector current _{I CMAX} is the output transistor Q1~Q4 by ground-power supply fault collectors - which is the maximum value of the collector current _{I CE} in a state _{where V CC} as emitter voltage _{V CE} is applied. The collector current threshold I _{CTH at which the} protection circuit operates has a corresponding relationship with the reference value I _TH compared with the above-described monitor current I _{M1 and the} like, and the monitor current from the collector current I _CE of the output transistor in the monitor current generation circuit Expressing conversion ratio _{I M} / _{I CE} to I _M in _{α, I CTH = I TH /} α relational expression holds.

In FIG. 4, the horizontal axis is the power supply voltage V _CC and the vertical axis is the current. Figure 4, voltage range _{[V MIN,} V _MAX] Power _{V CC} of BTL amplifiers 2 maximum load current _I characteristics of _LMAX 10, the maximum collector current _{I CMAX} characteristic 12 and the current threshold for operation of the protection circuit The characteristic 14 of _ICTH is shown. Both the characteristics 10 and 12 basically increase linearly as the power supply voltage _VCC increases. The slope of the change of the maximum load current _{I LMAX} for the power supply voltage _{V CC} smaller the larger the load resistance RL, as described above, basically the maximum load current _{I LMAX} is less than the maximum collector current _{I CMAX} at any _{V CC} Become. Characteristics 14 of the current thresholds _{I CTH} is voltage range _{[V MIN,} V _MAX] To avoid malfunction of the protection circuit, the maximum load current _{I LMAX} voltage range _{[V MIN,} V _MAX] greater than or equal to the maximum value in That is, it is set to be _{equal to} or greater than I _LMAX in V _CC = V _MAX .

Conventionally, the current threshold _{I CTH} is set to a fixed value which does not depend upon the power _{V CC.} Therefore, in a case using the voltage range is wide (voltage range in Fig. _{4 [V MIN, V X]} ) power _{V CC} is lower region current threshold _{I CTH} in that that the inverted state of exceeding the maximum collector current _{I CMAX} occurs there were. For example, when the maximum collector current I _{CMAX at} V _CC = V _MIN is lower than the maximum load current I _LMAX at V _CC = V _MAX , the above inversion state is inevitably caused in the prior art.

In the inversion state, even if a ground fault or a power fault occurs, the monitor current I _M does not reach the reference value I _TH and the protection operation is not activated. For this reason, a relatively large collector current that is less than the current threshold _ICTH flows through the output transistor for a long time, raising the temperature of the output transistor and causing ASO breakdown. Here, the case where the output transistor is a bipolar transistor has been described as an example with reference to FIG. 4, but the same problem occurs when the output transistor is a MOEFET.

  The present invention has been made to solve the above problems, and provides a protection circuit capable of suitably protecting an output transistor constituting a BTL amplifier from an ASO breakdown due to an abnormal current without depending on the voltage of a reference power supply. The purpose is to do.

  The BTL amplifier protection circuit according to the present invention is provided in a BTL amplifier in which two push-pull circuits each composed of a pair of transistors connected in series between a first reference power source and a second reference power source are connected to each other by BTL. A protection circuit for generating a monitor current corresponding to a conduction current of the transistor of the push-pull circuit; and an abnormal state caused by a magnitude of the monitor current exceeding a predetermined upper limit level. And a control unit that performs a protection operation that cuts or reduces at least the conduction current in the abnormal state, and the control unit determines a voltage difference between the first reference power source and the second reference power source. The upper limit level is changed according to the voltage difference in response to a decrease in the conduction current in the abnormal state with a decrease.

  According to the present invention, the upper limit level of the monitor current that becomes the determination threshold value of the abnormal state of the transistor is changed according to the voltage difference between the first reference power supply and the second reference power supply applied to both ends of the push-pull circuit, The upper limit level is set to be lower than the monitor current in the abnormal state regardless of the voltage difference. As a result, the output transistor can be suitably protected from ASO breakdown without depending on the voltage of the reference power supply.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a schematic configuration of a BTL amplifier 20 according to the embodiment. The BTL amplifier 20 receives input signals having opposite phases to the input terminals (+ IN) and (−IN), and drives the speaker RL connected as a load between the output terminals (+ OUT) and (−OUT). The drive current for the speaker RL is generated by two push-pull circuits 22 and 24 that are BTL-connected across the speaker RL. The push-pull circuit 22 includes push-pull connected output transistors Q1 and Q2, and the push-pull circuit 24 includes push-pull connected output transistors Q3 and Q4.

The transistors Q1 to Q4 are power transistors. In this embodiment, the transistors Q1 and Q3 are PNP type bipolar transistors, and the transistors Q2 and Q4 are NPN type bipolar transistors. The transistor Q1 has an emitter and a collector connected to a positive voltage source V _CC and an output terminal (+ OUT). The transistor Q2 has an emitter and a collector connected to the ground potential GND and the output terminal (+ OUT). Signals V _B1 and V _B2 generated based on the input signal V _IN− are applied to the bases of the transistors Q1 and Q2. The transistor Q3 has an emitter and a collector connected to the positive voltage source V _CC and the output terminal (−OUT). The transistor Q4 has an emitter and a collector connected to the ground potential GND and the output terminal (-OUT). Signals V _B3 and V _B4 generated based on the input signal V _{IN +} are applied to the bases of the transistors Q3 and Q4.

The driver circuit 26 generates V _B1 and V _B2 that are input signals to the push-pull circuit 22 based on the input signal V _IN− input to the input terminal (−IN). The input signal V _IN− is input to the base of an NPN transistor Q5. The emitter of the transistor Q5 is connected to GND, and the collector is connected to the PNP transistor Q6 via a series connection of diodes D1 and D2 connected in the forward bias direction. The transistor Q6 has its emitter connected to the power source _{V CC.} Transistor Q6 and transistor Q7 constitute a current mirror, and the same current as transistor Q7 flows through transistor Q6. The transistor Q7 is connected to the protection operation unit 28, and a constant current determined by the protection operation unit 28 flows through the transistor Q7.

The base of an NPN transistor Q8 is connected to an intermediate point between the diode D1 and the transistor Q6, the collector of the transistor Q8 is connected to the power supply _VCC via the resistor R1, and the emitter is connected to the output terminal (+ OUT). The The base of a PNP transistor Q9 is connected to the intermediate point between the diode D2 and the transistor Q5, the emitter of the transistor Q9 is connected to the output terminal (+ OUT), and the collector is connected to GND via the resistor R2. The The driver circuit 26 outputs the potentials of the collectors of these transistors Q8 and Q9 as voltage signals V _B1 and V _B2 , respectively.

The driver circuit 30 generates V _B3 and V _B4 that are input signals to the push-pull circuit 24 based on the input signal V _{IN +} input to the input terminal (+ IN). The driver circuit 30 has exactly the same configuration as the driver circuit 26, and its transistors Q10 to Q13, diodes D3 and D4, and resistors R3 and R4 are transistors Q5, Q6, Q8, and Q9 of the driver circuit 26 described above, and diodes. This corresponds to D1, D2 and resistors R1, R2.

The driver circuits 26 and 30 turn off the transistors Q5 and Q9, turn on the transistor Q8, and turn on the output transistor Q1 as V _B1 during a period in which a negative signal is applied as the input signal V _{IN− and a} positive signal is applied as the V _{IN +.} A voltage that turns on and outputs a voltage that turns off the output transistor Q2 as V _B2 , while a transistor Q10 and Q13 turn on and a transistor Q12 turns off, and a voltage that turns on the output transistor Q4 as V _B4 is outputted. , V _B3 is used to output a voltage for turning off the output transistor Q3. Therefore, a current corresponding to the input signal flows to the GND through the output transistor Q1, the output terminal (+ OUT), the speaker RL, the output terminal (−OUT), and the output transistor Q4.

Further, the driver circuits 26 and 30 turn on the transistors Q5 and Q9, turn off the transistor Q8, and turn off the output transistor as V _B2 in a period in which a positive signal is applied as the input signal V _{IN− and a} negative signal is applied as the V _{IN +.} outputs a voltage for turning on the _Q2, while outputting a voltage for turning off the output transistor Q1 as _{V B1,} the transistor Q10, Q13 is turned off the transistor Q12 is turned _on, outputs a voltage for turning on the output transistor Q3 as _{V B3 ,} and it outputs a voltage for turning off the output transistor Q4 as _{V B4.} Therefore, a current corresponding to the input signal flows to GND via the output transistor Q3, the output terminal (−OUT), the speaker RL, the output terminal (+ OUT), and the output transistor Q2.

The push-pull circuits 22 and 24 of the BTL amplifier 20 and the driver circuits 26 and 30 have been described above. The BTL amplifier 20 further monitors the conduction currents I _{CE1 to} I _CE4 between the emitters and the collectors of the output transistors Q1 to Q4 constituting the push-pull circuits 22 and 24, and the output transistor Q1 when the conduction current is excessive. -Q4 is turned off for protection. Specifically, in order to monitor the magnitudes of the currents I _{CE1 to} I _CE4 , the BTL amplifier 20 generates the monitor currents I _{MO1 to} I _MO4 corresponding to the currents I _{CE1 to} I _CE4. Is provided. Further, the BTL amplifier 20 includes a bias current generation circuit 34 that generates bias currents I _{MB1 to} I _MB4 having different magnitudes according to the potential difference between the two reference power supplies _VCC and GND. As will be described later, the bias current is used to bias the monitor currents I _{MO1 to} I _MO4 to generate the corrected monitor currents I _{M1 to} I _M4 . Protection operation unit 28, by one of the magnitude of the modification monitor current _I M1 _{~I M4} exceeds a predetermined reference value _{I TH,} the land for the current _I CE1 _{~I CE4} output transistor Q1~Q4 excessive Determine the occurrence of an abnormal condition such as a fault or a power fault, and execute a protective action.

For example, the monitor current generation circuit 32-1 that generates the monitor current I _M1 for the conduction current I _CE1 of the output transistor Q1 is applied based on the signal V _B1 output from the driver circuit 26 in the same manner as the output transistor Q1. The transistor Q14 operates in the same manner as the transistor Q1. Specifically, the transistor Q14 is the same PNP type and the output transistor Q1, the base is applied a signal _{V B1} through the resistor R5, the emitter is connected to the power source _{V CC,} a collector of a pair of transistors Q15, Q16 Is connected to the output terminal (+ OUT) through a current mirror circuit. The emitter of the transistor Q14 - current _{I MO1} between the collector Since the increase or decrease in the same manner as current _{I CE1} with respect to the signal _{V B1,} based on the current _{I MO1,} determination current _{I CE1} is either excessive or not the output transistor Is possible. Note that this determination is made by comparing the size with a reference value. The current _IMO1 does not have to be the same as the current _ICE1 even in the current value, and the transistor Q14 has a current capacity compared to the output transistor Q1. Can be small.

The current _IMO1 is folded back to the current path on the transistor Q16 side by a current mirror circuit composed of the transistor pair Q15 and Q16. Between the transistor Q16 and the power source _{V CC} is the current mirror circuit is provided comprising a pair of transistors Q17, Q18, the collector current of the transistor Q17 connected to the transistor Q16 is turned on transistor Q18 side. The output terminal of the bias current generating circuit 34-1 is connected to the collector of the transistor Q17 together with the transistor Q16. In the connection portion, the monitor current _IMO1 is combined so that the bias current _IMB1 is added, and the corrected monitor current _IM1 is generated. The corrected monitor current I _M1 is folded back into the current path on the transistor Q18 side and input to the protection operation unit 28.

Bias current generating circuit 34-1 includes transistors Q19～Q21, is configured to include a constant current source J1 and the above-described power supply voltage determining circuit 36-1, a power supply voltage corresponding to a potential difference between two reference power _{V CC} and GND V _CC is the size and the case is less than if the the threshold value is above a predetermined threshold to generate a different bias current.

Source voltage monitor circuit 36-1 controls the switching of the operation of the bias current generating circuit 34-1 in response to the magnitude of the supply voltage V _CC for a given threshold. The power supply voltage determination circuit 36-1 includes a Zener diode Z1 connected in a reverse bias direction between the power supply _VCC and GND, the anode of the Zener diode Z1 is connected to GND via a resistor R8, and the cathode is a resistor. It is connected to the power supply _{V CC} via R6. No current flows to the power supply voltage determining circuit 36-1 if the voltage V _CC is less than the Zener breakdown voltage V _ZD of the Zener diode Z1, the potential at the connection point between the Zener diode Z1 and resistor R7 is a GND When the voltage _VCC is equal to or higher than V _ZD , a current flows through the power supply voltage determination circuit 36-1, and the potential at the connection point between the Zener diode Z1 and the resistor R7 is the current flowing through the power supply voltage determination circuit 36-1 and the resistance value R7. A positive voltage corresponding to the product of

The anode potential of the Zener diode Z1 is applied to the base of the transistor Q19 to control the on / off of the transistor Q19. Transistor Q19 is connected between constant current source J1 and GND in parallel with transistor Q20. On the other hand, the transistor Q20 and the transistor Q21 constitute a current mirror circuit, the current flowing through the transistor Q20 is folded back to the transistor Q21 side, and the current flowing through the transistor Q21 is taken out as a bias current _IMB1 .

When the power supply voltage _{V CC} is the breakdown voltage _{V ZD} more zener diode Z1, the transistor Q19 of NPN type is applied a positive voltage to the base turned on, whereas, the power supply voltage _{V CC} is less than the breakdown voltage _{V ZD} In this case, the transistor Q19 is turned off by applying a ground potential to the base. When the transistor Q19 is on, the output current of the constant current source J1 flows to the transistor Q19 and basically does not flow to the transistor Q20. As a result, the bias current _IMB1 becomes zero. On the other hand, in the off state of the transistor Q19, the output current of the constant current source J1 flows to the transistor Q20, and as a result, a non-zero bias current I _MB1 corresponding to the constant current source J1 is generated. Incidentally, the bias current I _MB1 flows in the direction drawn to the collector of the transistor Q21. As a result, the directions of the bias current I _MB1 and the monitor current I _MO1 are both set in the direction of drawing the current from the collector of the transistor Q17, and the magnitude of the corrected monitor current I _M1 is that of each of the bias current I _MB1 and the monitor current I _MO1 . Total size.

Protection operation unit 28, the change in magnitude of the bias current _{I MB1,} i.e. using a constant reference value _{I TH} which is independent to the power source voltage _{V CC,} and compares the corrected monitoring current _{I M1} and the reference value _{I TH.} Then, when the corrected monitor current I _M1 exceeds the reference value I _TH , it is determined as an abnormal state and a protection operation is executed. For example, as a protection operation, the protection operation unit 28 stops supplying current to the transistor Q7. As a result, the transistors Q6 and Q11 constituting the current mirror circuit with the transistor Q7 are turned off, and the driver circuits 26 and 30 are stopped. Therefore, the output transistors Q1 to Q4 are turned off, and these are prevented from the ASO destruction due to the ground fault and the power fault. Protected.

The power supply voltage _{V CC} is lower than the judgment threshold voltage _{V TH} of the power supply voltage determining circuit 36-1 provided in the Zener breakdown voltage _{V ZD,} modified monitoring current _{I M1} size obtained by adding a bias current _{I MB1} nonzero Have Comparing the added corrected monitor current I _M1 with a constant reference value I _TH is relatively equivalent to lowering the determination threshold for the monitor current I _MO1 to which the bias current I _MB1 is not added. That, BTL amplifier 20, power supply voltage _{V CC} is less than the threshold _{V TH state, V CC} activates the protection operation _{V TH} greater than or equal is state less than current _{I CE1.}

2, the maximum load current _{I LMAX} and the maximum collector current _{I CMAX} of each change in the output transistor Q1 with respect to a change in power supply voltage _{V CC,} the relationship between the collector current threshold _{I CTH} protection operation unit 28 activates the protection operation It is a graph to show. FIG. 2 is contrasted with FIG. 4 relating to the above-described prior art, and the meanings of the maximum load current I _LMAX , the maximum collector current I _CMAX , and the collector current threshold I _CTH have already been described in the description of FIG. Moreover, the working voltage range of the power supply _{V CC} of BTL amplifiers _{2 [V MIN, V MAX]} , the maximum load current _{I LMAX} characteristics 10 and maximum collector current _{I CMAX} characteristics 12, are set to the same as FIG. 4 . On the other hand, the characteristic 40 of the current threshold value _ICTH that is activated by the protection operation unit 28 shown in FIG. 2 is different from the characteristic 14 of the current threshold value _ICTH shown in FIG. 4, and in the range where V _CC is lower than V _TH , V _The current level is set to be lower than the range of _TH or higher. That is, by the operation of the bias current generation circuit 34-1 described above, when _{C CC} ≧ V _TH , I _CTH is set to a certain level I _CTH1 , and when V _CC <V _TH , I _CTH is set to a certain level I _CTH2 (<I _CTH1 ). _Here, I _{CTH1, I CTH2} and _{V TH} is voltage range _{[V MIN,} V _MAX] current threshold _{I CTH} at any _{V CC} of the maximum load current _I less than _LMAX, and a maximum collector current _{I CMAX} It is set so as to exceed the value. In the power supply voltage determination circuit 36-1, the Zener breakdown voltage V _ZD corresponds to V _TH . Also, to represent the conversion ratio _{I MO1} / _{I CE1} from the collector current _{I CE1} of the output transistor Q1 of the monitor current generating circuit 32-1 to the monitor current _{I MO1} with _{alpha, I MB1} is _I CTH1 _-I CTH2 _{= I MB1} It is set to satisfy the relational expression / α.

According to the present invention, in response to the conduction current of the output transistor in the abnormal state is reduced with a decrease in the voltage V _CC is the voltage difference between the two reference power push-pull circuit is connected, the current threshold I _{CTH is} also changed. For example, in this embodiment, as described above, magnitude generate different bias currents _{I MB1} according to the voltage _{V CC,} when the voltage _{V CC} becomes lower bias current _{I MB1} minutes, by biasing the monitor current _{I MO1} to generate a modified monitoring current _{I M1} that increases weakly monotonically with respect to a decrease in voltage _{V CC.} Comparing the corrected monitor current I _M1 added with the non-zero bias current I _MB4 to the constant reference value I _TH when _VCC is low is relatively the monitor without the bias current I _MB1 added. the determination threshold for the current _{I MO1} equivalent to be pulled when _{V CC} is low. That is, in the BTL amplifier _20, since the threshold current _{I CTH} of _{V CC} is low state _{V CC} is lower than the high state, the collector current _{I CE1} of the output transistor Q1 of the ground fault of the output terminal (+ OUT) is _{I CTH} Thus, the protection operation can be started beyond the above, and ASO destruction can be suitably prevented. On the other hand, the threshold current I _CTH at V _CC is high state V _CC is set higher than the low state, it is possible to set larger than the maximum load current I _LMAX, protected by the normal operation of the earth絡等has not occurred It is prevented that the operation unit 28 malfunctions and the operation of the BTL amplifier 20 is stopped.

Above, the configuration of the protection circuit of the BTL amplifier 20 corresponding to the difference of the voltage V _CC, has been described with attention to the part concerning the output transistor Q1, the output transistor Q3 is upper transistors of the same push-pull circuit and the output transistor Q1 Is similarly configured.

The same applies to the output transistors Q2 and Q4 on the lower side of the push-pull circuit. For example, in order to protect the output transistor Q2 from ASO breakdown in the event of a power failure at the output terminal (+ OUT), a corrected monitor current I _M2 is generated using the monitor current generation circuit 32-2 and the bias current generation circuit 34-2. Then, the protection operation unit 28 compares the corrected monitor current I _M2 with the reference value I _TH and controls detection of an abnormal state and activation of the protection operation. The configurations of the monitor current generation circuit 32-2 and the bias current generation circuit 34-2 are similar to those of the monitor current generation circuit 32-1 and the bias current generation circuit 34-1, and basically perform the same operations as those.

Incidentally, the bias current generation circuit 34-2 shares the bias current generation circuit 34-1 and the power supply voltage determination circuit 36-1. Here, the potential of the cathode of the Zener diode Z1 corresponds to the fact that the bias current generation circuit 34-2 has a configuration in which the bias current generation circuit 34-1 is inverted up and down in relation to the power sources V _CC and GND. Is applied to the base of the PNP transistor Q22 of the bias current generating circuit 34-2.

The base potential of the transistor Q22 when the voltage V _CC is less than the Zener breakdown voltage V _ZD becomes common to the emitter potential, the transistor Q22 is turned off and the transistor Q23 provided in parallel to the transistor Q22 in place is turned on The current of the constant current source J2 is output from the bias current generation circuit 34-2 as the bias current _IMB2 through the current mirror circuit including the transistors Q24 and Q25. The bias current I _MB2 flows toward the collector of the transistor Q26 of the monitor current generating circuit 32-2. Thus, the directions of the bias current I _MB2 and the monitor current I _MO2 are both set in the direction from the collector of the transistor Q26 toward the protection operation unit 28, and the magnitude of the corrected monitor current I _M2 is set to the bias current I _MB2 and the monitor current I. _{This is} the total size of each _MO2 .

On the other hand, the base potential of the transistor Q22 becomes lower than _{V CC} when the voltage _{V CC} is Zener breakdown voltage _{V ZD} above, the transistor Q22 is turned on, the bias current _{I MB2} of the bias current generating circuit 34-2 is outputted Basically 0.

  The monitor current generation circuit 32-4 and the bias current generation circuit 34-4 provided corresponding to the output transistor Q4 are configured similarly to the monitor current generation circuit 32-2 and the bias current generation circuit 34-2 described above.

  In FIG. 1, a power supply voltage determination circuit 36-2 including resistors R8 and R9 and a Zener diode Z2 is separated from the power supply voltage determination circuit 36-1 shared by the bias current generation circuits 34-1 and 34-2. The power supply voltage determination circuit 36-2 is shared by the bias current generation circuits 34-3 and 34-4, and the power supply voltage determination circuits 36-1 and 36-2 are integrated into one. be able to. For example, the power supply voltage determination circuit 36-1 can be shared by the bias current generation circuits 34-1 to 34-4 corresponding to the output transistors Q1 to Q4, and the power supply voltage determination circuit 36-2 can be omitted.

As described above, the monitor current generation circuit 32-2 protects the transistors Q2 to Q4 from the ASO breakdown in the same manner as the output transistor Q1 when the output terminals (+ OUT) and (−OUT) have a power fault or a ground fault. To 32-4 and bias current generating circuits 34-2 to 34-4 are provided. Then, the corrected monitor currents I _{M2 to} I _M4 are generated using them, and the protection operation unit 28 compares the corrected monitor currents I _{M2 to} I _M4 with the reference value I _TH to detect an abnormal state and start the protection operation. To control.

  Note that the specific circuit configurations of the monitor current generation circuit 32, the bias current generation circuit 34, and the power supply voltage determination circuit 36 described above are merely examples, and other circuit configurations having the same function may be employed.

Further, in the above-described configuration, the bias currents I _{MB1 to} I _MB4 generated by the bias current generation circuit 34 are added to the monitor currents I _{MO1 to} I _MO4 generated by the monitor current generation circuit 32 so that V _CC is in a low range. The monitor current has been corrected to increase. In _contrast, even when pulled towards the reference value _{I TH} at _{V CC} is lower range, voltage range _{[V MIN,} V _MAX] preferred ASO destruction in the abnormal state at any supply _{V CC} in Prevention and prevention of malfunction during normal operation can be realized. An example of the configuration will be described by paying attention to the output transistor Q1. For example, in the protection operation unit 28, the monitor current in the circuit configuration for comparing the magnitude of _{I MO1} and the reference current _{I TH,} the reference current I using a bias current _{I MB1} generated by the bias current generating circuit 34-1 Make corrections to reduce _TH .

_Further, the characteristics of the threshold voltage _{I CTH} to weakly monotonically decreases relative decrease in _{V CC} is voltage range _{[V MIN,} V _MAX] may have two or more threshold voltage _{V TH} at. Further, the way of reduction of the threshold voltage I _CTH for reduction of V _CC may be a narrow sense monotonically decreasing. An example of this narrow sense monotonically decreasing, the threshold voltage I _CTH can be employed linearly decreasing characteristic with a decrease on V _CC.

In the above-described embodiment, the bias current generation circuit 34 is provided to generate the bias currents I _{MB1 to} I _MB4 that change according to V _CC , and the change in the threshold current _ICTH according to V _CC is determined as the bias current. Use to realize. However, the change of the threshold current _{I CTH,} may also be implemented in other forms without using a bias current _I MB1 _{~I MB4.} For example, to allow a change in reference level by switching the configuration of a circuit for generating a reference current or a reference voltage used for size determination of the monitor current in the switching element, it switches the switching element on the basis of a detection signal of the level of V _CC forms It can also be.

1 is a circuit diagram showing a schematic configuration of a BTL amplifier according to an embodiment of the present invention. FIG. 6 is a graph showing the relationship between changes in the maximum load current and the maximum collector current of the output transistor with respect to changes in the power supply voltage and the collector current threshold at which the protection operation unit is activated in the embodiment of the present invention. It is a circuit diagram which shows the structure of the conventional BTL amplifier. It is a graph which shows the relationship between each change of the maximum load current of the output transistor with respect to the change of a power supply voltage, and the maximum collector current with respect to the change of a power supply voltage, and the collector current threshold value which a protection operation | movement part starts in the conventional BTL amplifier.

Explanation of symbols

  20 BTL amplifier, 22, 24 Push-pull circuit, 26, 30 Driver circuit, 28 Protection operation unit, 32 Monitor current generation circuit, 34 Bias current generation circuit, 36 Power supply voltage determination circuit, Q1-Q4 Output transistor.

Claims (3)

A protection circuit provided in a BTL amplifier in which two push-pull circuits each composed of a pair of transistors connected in series between a first reference power source and a second reference power source are BTL-connected to each other,
Monitor current generating means for generating a monitor current according to the conduction current of the transistor of the push-pull circuit;
Control means for determining the occurrence of an abnormal state when the magnitude of the monitor current exceeds a predetermined upper limit level, and performing a protective operation for interrupting or reducing at least the conduction current in the abnormal state;
Have
The control means corresponds to the decrease in the conduction current in the abnormal state as the voltage difference between the first reference power supply and the second reference power supply decreases, and the upper limit according to the voltage difference. Changing the level,
A BTL amplifier protection circuit. The BTL amplifier protection circuit according to claim 1,
Bias current generating means for generating a bias current having a different magnitude according to the voltage difference;
The control means includes
A monitor current biasing means for generating a corrected monitor current in which the monitor current is combined with the bias current and biased, whereby a current value broadly monotonically increases with respect to a decrease in the voltage difference;
Comparison determination means for determining occurrence of the abnormal state when the magnitude of the corrected monitor current exceeds a certain reference level not dependent on the voltage difference;
And the upper limit level is changed by a difference in bias amount according to the voltage difference with respect to the monitor current. The BTL amplifier protection circuit according to claim 2,
The bias current generating means switches the magnitude of the bias current between when the voltage difference is in a first range equal to or greater than a predetermined threshold and when the voltage difference is in a second range less than the threshold. A BTL amplifier protection circuit.

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