ADVANCED CURRENT LIMIT FUNCTION FOR AUDIO AMPLIFIER
A class-D audio amplifier incorporates an overcurrent protection scheme implementing two overcurrent thresholds to avoid a dynamic impedance drop. When output current reaches the first threshold as a result of an impedance drop across the speaker, the overcurrent protection circuitry limits the output current to the value of the first threshold, but does not shut down the circuit. The second threshold is used to detect an overcurrent condition to shut down the circuit. Current limiting logic of a first channel monitors the overcurrent condition of a second channel and controls the first channel output in response thereto. This permits the second channel output current to reach the second threshold if the circuit is experiencing a short-circuit condition. This scheme also allows the output current to drop below the first threshold if the overcurrent condition of the second channel is caused by an impedance drop across the output speaker.
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Pursuant to 35 U.S.C. §119(a)-(d), this application claims the priority of Chinese Patent Application Serial No. 201410436485.9, filed Aug. 29, 2014, which is hereby incorporated by reference in its entirety for all purposes.
TECHNICAL FIELDThe present disclosure generally relates to overcurrent protection for audio amplifier circuitry and, more particularly, to a system and method for providing a current limit function to provide overcurrent protection of a bridge-tied load (BTL) class D audio amplifier.
BACKGROUNDOvercurrent protection is often implemented in class-D audio amplifiers, also known as switching amplifiers, to protect the system and connected components from damage that occurs from overcurrent conditions.
When considering the dynamic behavior of a loudspeaker, however, it may be desirable to keep the circuit 100 operational to avoid an audible interruption in the output audio if the output current exceeds an output current threshold as a result of an impedance drop across the speaker 106, as opposed to a short-circuit condition. Thus, if the output current reaches or exceeds the output current threshold due to an impedance drop across the speaker 106 (see
It is often difficult to accurately distinguish between a short-circuit condition and an impedance drop using conventional overcurrent protection circuitry. Therefore, there exists a need for an overcurrent protection circuit capable of more accurately distinguishing between a short-circuit condition and an impedance drop and implementing appropriate protective measures in response thereto.
SUMMARYThe present disclosure provides a system for providing overcurrent protection in a class-D audio amplifier circuit, the class-D audio amplifier circuit having a first drive channel and a second drive channel, the system comprising: first current protection circuitry configured to monitor an output current of the second drive channel and, in response to an overcurrent condition of the second drive channel, to drive an output signal of the first drive channel to the same state as an output signal of the second drive channel; and shutdown circuitry configured to shut down the class-D audio amplifier circuit if the output current of the second drive channel reaches a shutdown threshold.
In another embodiment, the present disclosure provides a method for providing overcurrent protection in a class-D audio amplifier circuit having a first drive channel and a second drive channel, the method comprising: monitoring an output current of the second drive channel to detect an overcurrent condition of the second drive channel; driving an output signal of the first drive channel to the same state as an output signal of the second drive channel if the overcurrent condition of the second drive channel is detected; monitoring the output current of the second drive channel to detect a shutdown condition of the second drive channel; and shutting down the class-D audio amplifier circuit if the shutdown condition of the second drive channel is detected.
In yet another embodiment, the present disclosure provides a circuit comprising: a first half-bridge drive circuit configured to output a first drive signal to a load; a first current sensor configured to sense a first current of the first drive signal; a second half-bridge drive circuit configured to output a second drive signal to said load; an overcurrent protection circuit configured to compare the sensed first current to a first threshold and cause the second half-bridge drive circuit to output the second drive signal with a same logic state as the first drive signal if the first threshold is exceeded; and a shutdown circuit configured to compare the sensed first current to a second, higher threshold and cause a circuit shutdown if the second threshold is exceeded.
The foregoing and other features and advantages of the present disclosure will become further apparent from the following detailed description of the embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the disclosure, rather than limiting the scope of the invention as defined by the appended claims and equivalents thereof.
Embodiments are illustrated by way of example in the accompanying figures not necessarily drawn to scale, in which like numbers indicate similar parts, and in which:
Reference is now made to
In operation, the differential amplifier 202 receives positive input signal 210A and negative input signal 210B, and produces differential output signals 211A and 211B, which are filtered by the integration circuitry 204A and 204B. Integration circuitry 204A includes an amplifier 205A which receives, at its negative input terminal, the differential output signal 211A and feedback input 212A from the output signal 235, and receives, at its positive input terminal, a reference voltage Vref. Integration circuitry 204A produces a filtered signal 207A that is representative of the positive audio input to the circuit 200. Similarly, integration circuitry 204B includes an amplifier 205B which receives, at its negative input terminal, the differential output signal 211B and feedback input 212B from the output signal 245, and receives, at its positive input terminal, a reference voltage Vref. Integration circuitry 204B produces a filtered signal 207B that is representative of the negative audio input to the circuit 200.
Drive channel output signals OUTP and OUTN are ultimately determined by a comparison of the filtered signals 207A and 207B received from respective integration circuitry 204A and 204B, respectively, and the triangle waveforms produced by the waveform generator circuitry 206A and 206B. Regarding OUTP, comparator 208A compares the filtered positive channel input signal 207A to the triangle waveform generated by 206A, and produces a pulse width modulated (PWM) signal 209A having a duty cycle that is directly proportional to the instantaneous value of the input signal 207A. PWM signal 209A is then fed into the current limiting logic 220A. Regarding OUTN, comparator 208B compares the filtered negative channel input signal 207B to the triangle waveform generated by 206B, and produces PWM signal 209B having a duty cycle that is directly proportional to the instantaneous value of the input signal 207B. PWM signal 209B is then fed into the current limiting logic 220B.
The current limiting logic 220A and 220B are now discussed with reference to
The current limiting logic 220A also includes output circuitry 330A, which receives the PWM signal 209A and current limit signal 315A, and produces an output signal LOG P for driving the switching amplifier 222A in accordance with the comparisons of the sensing current Icsp to the threshold TH1. For example, when current limit signal 315A is low, the output signal LOG P is equal to the PWM signal 209A. When the current limit signal 315A is high, the output signal OUTP is reset low. Thus, the current limiting logic 220A drives the switching amplifier 222A such that the output current 235 is limited to a value not to exceed the first threshold TH1.
Referring now to
The current limiting logic 220B also includes output circuitry 330B, which receives the PWM signal 209B and current limit signal 315B, and produces an output signal LOG N for driving the switching amplifier 222B in accordance with the comparisons of the sensing current Icsn to the threshold TH1. For example, when the current limit signal 315B is low, the output signal LOG N is equal to the PWM signal 209B. When the current limit signal 315B is high, the output signal OUTN is reset low. Thus, the current limiting logic 220B drives the switching amplifier 222B such that the output current 245 is limited to a value not to exceed the first threshold TH1.
Referring again to
The amplifier circuit 200 also includes shutdown logic 260, which compares the sensing current Icsp and Icsn to a second threshold TH2 to detect a short-circuit condition or other overcurrent condition to trigger shutdown of the circuit 200. The shutdown logic 260 is now described with reference to
Referring again to
In the embodiment illustrated in
Same-channel monitoring refers to the circuit configuration whereby current limiting logic 220A and 220B each monitor the output current of their respective drive channel, and reset low the output signal for their own channel if an overcurrent condition exists for its respective channel. In other words, current limiting logic 220A monitors the positive drive channel output current 235 and resets low the output signal OUTP if an overcurrent condition is detected. Current limiting logic 220B monitors the negative drive channel output current 245 and resets low the output signal OUTN if an overcurrent condition is detected.
Operation of the current limiting logic 220A is now discussed in greater detail with reference to representative waveforms 500 illustrated in
When the output current 235 is less than the current limiting threshold TH1, the current limiting logic 220A drives the drive logic 224A, which in turn drives the half-bridge drive circuit 226A to produce OUTP. At reference 510, the output current 235 reaches the current limiting threshold TH1, and current limit signal 315A goes high. When the current limit signal 315A is high, OUTP is reset low for the remainder of the PWM cycle (see, dotted reference 511). When OUTP is reset low, the voltage across the speaker 230 is reduced to zero, thereby reducing the output current 235 below the current limiting threshold TH1 (reference 512). Thus, when the circuit 200 experiences an overcurrent condition as a result of an impedance drop across the speaker 230, the current limiting circuitry 220A drives the drive logic 224A and, in turn, switches 226A, such that the output current 235 of OUTP is limited to the value of the current limiting threshold TH1. Although it is not illustrated, if the output current 235 reaches the shut-down threshold TH2, the shutdown signal OCSD produced by the shutdown logic 260 goes high, and triggers a shutdown of the amplifier circuit 200.
It should be appreciated that the current limiting logic 220B operates similar to the current limiting logic 220A discussed above. See
In the example class-D amplifier circuit embodiment illustrated in
Reference is now made to
The amplifier circuit 600 replaces the current limiting logic 220A and 220B of
Output circuitry 730A receives the PWM signal 209A, current limit signal 715A, and representative output signal OUTN′, and produces an output signal LOG P for driving the switching amplifier 222A to control the output signal OUTP in response to the current limit signal 715A. Thus, the current limiting logic 620A of the positive drive channel controls the positive channel output signal OUTP in response to the negative drive channel output current 245.
As shown in
Output circuitry 730B receives the PWM signal 209B, current limit signal 715B, and representative output signal OUTP′, and produces an output signal LOG N for driving the switching amplifier 222B to control the output signal OUTN in response to the current limit signal 715B. Thus, the current limiting logic 620B of the negative drive channel controls the negative channel output signal OUTN in response to the positive drive channel output current 235.
Referring now to
In operation, when the negative drive channel output current 245 is below the first threshold TH1 (i.e., current limit signal 715A is low), NOR latch output 815A is low, and PWM signal 209A is output as LOG P (via NOR gate 820A and inverter 825A) regardless of the state of output signal OUTN. When the output current 245 is greater than, or equal to, the first threshold TH1 (i.e., current limit signal 715A is high), the NOR latch output 815A is high. Accordingly, the NOR gate 820A outputs a low state which is then inverted by NOT gate 825A, and the output signal LOG P is driven high so as to drive OUTP to a high state. Note that, in this overcurrent condition, the negative channel output OUTN is not reset low. Rather, the positive channel output OUTP is driven high.
Referring now to
In operation, when the positive drive channel output current 235 is below the first threshold TH1 (i.e., current limit signal 715B is low), NOR latch output 815B is low, and PWM signal 209B is output as LOG N (via NOR gate 820B and inverter 825B) regardless of the state of output signal OUTP. When the output current 235 is greater than, or equal to, the first threshold TH1 (i.e., current limit signal 715B is high), the NOR latch output 815B is high. Accordingly, the NOR gate 820B outputs a low state which is then inverted by NOT gate 825B, and the output signal LOG N is driven high so as to drive OUTN to a high state. Note that, in this overcurrent condition, the positive channel output OUTP is not reset low. Rather, the negative channel output OUTN is driven high.
In the embodiment illustrated in
The current limiting logic 620A monitors the overcurrent condition of the negative channel output OUTN by comparing the current sense signal Icsn to the first threshold and controlling the positive channel output OUTP in response thereto. If the current sense signal Icsn is greater than, or equal to, the first threshold TH1, the current limit signal 715A is set logic high and the current limiting logic 620A sets the positive channel output OUTP to logic high.
When current limit signal 715A is high, an overcurrent condition exists at the negative channel output OUTN. When this occurs, OUTN remains high, and LOG P is set logic high to cause OUTP to go high as well. If OUTN is shorted (e.g., to ground), the OUTN output current 245 will quickly rise from the first threshold to the second threshold, and the device 600 will be shut down. If the current limit signal 715A is generated as the result of an impedance drop across the speaker 230, then the OUTN output current 245 will not rise because the voltage drop across the speaker 230 will be zero (OUTN−OUTP=0), and the device 600 will continue to operate.
Conversely, the current limiting logic 620B monitors the overcurrent condition of the positive channel output OUTP by comparing the current sense signal Icsp to the first threshold and controlling the negative channel output OUTN in response thereto. If the current sense signal Icsp is greater than, or equal to, the first threshold TH1, the current limit signal 715B is set logic high and the current limiting logic 620B sets the negative channel output OUTN to logic high.
When current limit signal 715B is high, an overcurrent condition exists at the positive channel output OUTP. When current limit signal 715B is high, OUTP is also high, and LOG N is set high to cause OUTN to go high as well. If OUTP is shorted (e.g., to ground), the OUTP output current 235 will immediately rise from the first threshold TH1 to the second threshold TH2, and the device 600 will be shut down. If the current limit signal 715B is generated as the result of an impedance drop across the speaker 230, then the OUTP output current 235 will not rise because the voltage drop across the speaker 230 will be zero (OUTP−OUTN=0), and the device 600 will continue to operate.
This overcurrent protection scheme whereby the current limiting logic of a first channel monitors the overcurrent condition of the second channel and controls the first channel output in response thereto serves, in effect, to permit the output current of the second channel to spike beyond the first threshold TH1 to the second threshold TH2 if the circuit 600 is experiencing a short-circuit condition. Furthermore, this overcurrent protection scheme also allows the output current of the second channel to drop below the first threshold TH1 if the overcurrent condition of the second channel is caused by an impedance drop across the output speaker 230, so that the device 600 can continue to operate.
Reference is now made to
The voltage across the speaker 230 equals OUTP−OUTN, which, in normal conditions, is zero. But, because OUTP is shorted to ground and OUTP remains in a logic high state, the output current 235 continues to rise until it reaches the second threshold TH2. At this point (reference 907), the shutdown signal OCSD is set at reference 915, and the amplifier circuit 600 is shut down to protect against the short-circuit, overcurrent condition.
Reference is now made to
The proposed overcurrent protection scheme is capable of accurately distinguishing between a short-circuit condition and an impedance drop and implementing appropriate protective measures to permit the class-D audio amplifier to either continue operation if the overcurrent condition is the result of an impedance drop, or to shut down if the overcurrent condition is a result of a short-circuit condition.
The disclosed overcurrent protection circuitry is generally discussed in connection with short-circuit conditions occurring as a short-circuit to ground. However, it should be appreciated that the disclosed overcurrent protection circuitry may also be used to protect from an overcurrent condition occurring as a result of a short-circuit to Vcc.
The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of one or more exemplary embodiments of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims.
Claims
1. A system for providing overcurrent protection in a class-D audio amplifier circuit, the class-D audio amplifier circuit having a first drive channel and a second drive channel, the system comprising:
- first current protection circuitry configured to monitor an output current of the second drive channel and, in response to an overcurrent condition of the second drive channel, to drive an output signal of the first drive channel to the same state as an output signal of the second drive channel; and
- shutdown circuitry configured to shut down the class-D audio amplifier circuit if the output current of the second drive channel reaches a shutdown threshold.
2. The system of claim 1, wherein the first current protection circuitry comprises:
- first overcurrent detection circuitry operable to detect an overcurrent condition by comparing the output current of the second drive channel to a first overcurrent threshold and generating a first current limit signal if the output current of the second drive channel reaches the first overcurrent threshold; and
- first output circuitry operable, in response to the first current limit signal, to generate a first drive signal for driving the output signal of the first drive channel to the same state as the output signal of the second drive channel.
3. The system of claim 1, wherein the shutdown circuitry comprises:
- first comparator circuitry operable to compare the output current of the second drive channel to the shutdown threshold and to generate a shutdown signal if the output current of the second drive channel reaches the shutdown threshold.
4. The system of claim 1, wherein the first drive channel further comprises first filtering circuitry, a first comparator operable to generate a pulse width modulated signal indicative of a filtered first input signal, and first switching amplifier circuitry operable to control the first channel output signal in response to a first drive signal generated by the first current protection circuitry.
5. The system of claim 4, wherein the first switching amplifier circuitry includes a first half-bridge drive circuit.
6. The system of claim 1, further comprising:
- second current protection circuitry configured to monitor an output current of the first drive channel and, in response to an overcurrent condition of the first drive channel, to drive the output signal of the second drive channel to the same state as the output signal of the first drive channel,
- wherein the shutdown circuitry is further configured to shut down the class-D audio amplifier circuit if the output current of the first drive channel reaches the shutdown threshold.
7. The system of claim 6, wherein the second current protection circuitry comprises:
- second overcurrent detection circuitry operable to detect an overcurrent condition by comparing the output current of the first drive channel to a second overcurrent threshold and generating a second current limit signal if the output current of the first drive channel reaches the second overcurrent threshold; and
- second output circuitry operable, in response to the second current limit signal, to generate a second drive signal for driving the output signal of the second drive channel to the same state as the output signal of the first drive channel.
8. The system of claim 6, wherein the shutdown circuitry further comprises:
- second comparator circuitry operable to compare the output current of the first drive channel to the shutdown threshold and to generate a second shutdown signal if the output current of the first drive channel reaches the shutdown threshold.
9. The system of claim 6, wherein the first current protection circuitry comprises a portion of the first drive channel, and the second current protection circuitry comprises a portion of the second drive channel.
10. The system of claim 6, wherein the second drive channel further comprises second filtering circuitry, a second comparator operable to generate a pulse width modulated signal indicative of a filtered second input signal, and second switching amplifier circuitry operable to control the second channel output signal in response to a second drive signal generated by the second current protection circuitry.
11. The system of claim 10, wherein the second switching amplifier circuitry includes a second half-bridge drive circuit.
12. A method for providing overcurrent protection in a class-D audio amplifier circuit having a first drive channel and a second drive channel, the method comprising:
- monitoring an output current of the second drive channel to detect an overcurrent condition of the second drive channel;
- driving an output signal of the first drive channel to the same state as an output signal of the second drive channel if the overcurrent condition of the second drive channel is detected;
- monitoring the output current of the second drive channel to detect a shutdown condition of the second drive channel; and
- shutting down the class-D audio amplifier circuit if the shutdown condition of the second drive channel is detected.
13. The method of claim 12, wherein monitoring the output current of the second drive channel to detect the overcurrent condition of the second drive channel comprises:
- comparing the output current of the second drive channel to a first overcurrent threshold; and
- generating a first current limit signal if the output current of the second drive channel reaches the first overcurrent threshold.
14. The method of claim 12, wherein monitoring the output current of the second drive channel to detect the shutdown condition of the second drive channel comprises:
- comparing the output current of the second drive channel to a shutdown threshold; and
- generating a shutdown signal if the output current of the second drive channel reaches the shutdown threshold.
15. The method of claim 12, further comprising controlling the first channel output signal in response to a first drive signal generated by first current protection circuitry, wherein the first current protection circuitry is operable to monitor the output current of the second drive channel.
16. The method of claim 12, further comprising:
- monitoring an output current of the first drive channel to detect an overcurrent condition of the first drive channel;
- driving the output signal of the second drive channel to the same state as the output signal of the first drive channel if the overcurrent condition of the first drive channel is detected;
- monitoring the output current of the first drive channel to detect a shutdown condition of the first drive channel; and
- shutting down the class-D audio amplifier circuit if the shutdown condition of the first drive channel is detected.
17. The method of claim 16, wherein monitoring the output current of the first drive channel to detect the overcurrent condition of the first drive channel comprises:
- comparing the output current of the first drive channel to a second overcurrent threshold; and
- generating a second current limit signal if the output current of the first drive channel reaches the second overcurrent threshold.
18. The method of claim 16, wherein monitoring the output current of the first drive channel to detect the shutdown condition of the first drive channel comprises:
- comparing the output current of the first drive channel to the shutdown threshold; and
- generating a second shutdown signal if the output current of the first drive channel reaches the shutdown threshold.
19. The method of claim 16, further comprising controlling the second channel output signal in response to a second drive signal generated by second current protection circuitry, wherein the second current protection circuitry is operable to monitor the output current of the first drive channel.
20. A circuit, the circuit comprising:
- a first half-bridge drive circuit configured to output a first drive signal to a load;
- a first current sensor configured to sense a first current of the first drive signal;
- a second half-bridge drive circuit configured to output a second drive signal to said load;
- an overcurrent protection circuit configured to compare the sensed first current to a first threshold and cause the second half-bridge drive circuit to output the second drive signal with a same logic state as the first drive signal if the first threshold is exceeded;
- a shutdown circuit configured to compare the sensed first current to a second, higher threshold and cause a circuit shutdown if the second threshold is exceeded.
Type: Application
Filed: Sep 16, 2014
Publication Date: Mar 3, 2016
Applicant: STMicroelectronics (Shenzhen) R&D Co. Ltd (Shenzhen)
Inventors: Qi Yu Liu (Shenzhen), Ru Feng Du (Shenzhen)
Application Number: 14/487,313