Audio playback during short-circuit conditions

Abstract

An audio system (100) includes an H-bridge (104). The audio system (100) implements one or more techniques to ensure that if a short circuit is detected on the output of the H-bridge (104), transistors within the H-bridge (104) do not conduct. Other transistors within the H-bridge (104) can conduct, and thus audio can still be played to a speaker (106).

Description

Background Art

[0001] Many audio output systems include a Class D amplifier configured to drive a speaker. The Class D amplifier includes transistors configured to alternate between coupling power and ground to an output terminal. Audio systems including Class D amplifiers are often integrated into telecommunications equipment, such as a telephone. In some examples, the telecommunications equipment is integrated into a vehicle, such as an automobile. An audio system including a Class D amplifier can be configured to output audio associated with a telecommunications conversation, such as a telephone call.

[0002] During operation of an audio system that includes a Class D amplifier, a short-circuit condition may occur, in which one of the Class D amplifier's output terminals is shorted to ground or power. In response to this condition, many audio systems are configured to disable the audio output. However, in certain situations, completely disabling audio may not be appropriate. For example, in an emergency, such as after a car accident, disabling the audio output of a telecommunications device may be undesirable. Summary of the Invention

[0003] An audio system includes an H-bridge. The audio system implements one or more techniques to ensure that if a short circuit is detected on an output terminal of the H-bridge, transistors within the H-bridge do not turn on. Other transistors within the H-bridge can turn on, and thus audio can still be played to a speaker.

[0004] In one example, an audio system includes a modulator having an input and an output, and an H-bridge having an input and an output. The output of the modulator is coupled to the input of the H-bridge. A load diagnostic circuit having an input and a first control output is included. A direct current (DC) summing circuit having an input and an output is included. The output of the DC summing circuit is coupled to the input of the modulator, and the first control output from the load diagnostic circuit is coupled to the DC summing circuit.

[0005] Another example includes an audio system including a modulator having an input and an output, and an H-bridge having an input and an output. The system includes a load diagnostic circuit having an input and a first control output. A signal shielding circuit is coupled between the output of the modulator and the input of the H-bridge. The signal shielding circuit includes a control input coupled to the first control output of the load diagnostic circuit.

[0006] In another example, an audio system includes a modulator having an input and an output, and an H-bridge having an input and an output. A short-circuit feedback circuit has an input and an output, and the input of the short-circuit feedback circuit is coupled to the output of the modulator. A first switch is coupled between the output of the short-circuit feedback circuit and the input of the modulator. A second switch is coupled between the output of the H-bridge and the input of the modulator. A load diagnostic circuit has an input coupled to the output of the H-bridge. The load diagnostic circuit is configured to control the operating states of the first and second switches. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] For a detailed description of various examples, reference will now be made to the accompanying drawings, in which:

[0008] Figure 1 An example of an audio system including a Class D amplifier is described.

[0009] Figure 2 An example illustrating an H-bridge for a Class D amplifier.

[0010] Figure 3 A timing diagram showing the signals within the modulator of a Class D amplifier.

[0011] Figure 4 An example of an audio system is described that includes a Class D amplifier and a direct current (DC) summing circuit to allow the audio system to continue playing audio in the face of a short circuit on one output terminal of an H-bridge.

[0012] Figure 5 An example of a modulator used with a Class D amplifier is shown.

[0013] Figure 6 is a timing diagram illustrating the addition of negative DC to the audio signal after certain types of short circuits are detected.

[0014] Figure 7 is a timing diagram illustrating the addition of positive DC to the audio signal after other types of short circuits are detected.

[0015] Figure 8 Implementations of audio systems including signal shielding circuits are described.

[0016] Figure 9 An example of an audio system including a virtual feedback loop is described.

[0017] Figure 10 An example of an audio system including a DC summing circuit, a signal shielding block, and a virtual feedback loop is described.

[0018] Figure 11 An example implementation of a load diagnostic circuit for detecting the presence of a short circuit at the output of an audio amplifier is shown. DETAILED DESCRIPTION

[0019] An audio device that includes a Class D amplifier and supports audio output during a short-circuit condition is described. A telecommunications device utilizing the described principles can continue to provide audio output even during a short-circuit condition. Thus, the telecommunications device can support emergency calls even when experiencing a short-circuit condition. Audio quality may be compromised by the short-circuit condition, but may be sufficient for emergency or other situations.

[0020] Figure 1 At least a portion of an audio system 100 is illustrated. Audio system 100 includes a modulator 102, an H-bridge 104, and a speaker 106. Modulator 102 receives a differential audio input signal including Audio_P and Audio_N. Modulator 102 generates output pulse-width modulated signals P and N that are coupled to H-bridge 104. H-bridge 104 includes a plurality of transistor switches and includes output terminals coupled to speaker 106. The output terminals provide signals labeled OUTP and OUTM.

[0021] Figure 2 An example of an H-bridge 104 is shown. In this example, the H-bridge 104 includes transistors 202, 204, 206, and 208. Figure 2 In the example shown, transistors 202-208 include N-type metal oxide semiconductor field effect transistors (NMOS), but may be implemented as other types of transistors. The drain of transistor 202 is coupled to power supply terminal 210 (PVDD), and the source of transistor 202 is coupled to first output node 222 (OUTP) and the drain of transistor 206. The source of transistor 206 is coupled to ground 212. The gate of transistor 202 receives the P signal from modulator 102, and the gate of transistor 206 receives the inverted control signal (via inverter 205). Therefore, when P is a logic high, transistor 202 is turned on and transistor 206 is turned off, and when P is a logic low, transistor 202 is turned off and transistor 206 is turned on. The drain of transistor 204 is also coupled to power supply terminal 210 (PVDD), and the source of transistor 204 is coupled to second output node 224 (OUTN) and the drain of transistor 208. The source of transistor 208 is coupled to ground 212. The gate of transistor 204 receives the N signal from the modulator, and the gate of transistor 108 receives the logical inverse of N (via inverter 207).

[0022] A load (such as speaker 106) can be connected between first output node 222 and second output node 224 of H-bridge 104. Transistors 202-208 operate as switches (on or off). During normal operation (i.e., in the absence of a short-circuit condition), the P and N signals from modulator 102 cause transistors 202-208 to be configured in any of the following modes: transistors 202 and 208 are on and transistors 204 and 206 are off; transistors 204 and 206 are on and transistors 202 and 208 are off; or transistors 206 and 208 are on and transistors 202 and 204 are off. Both transistors on one side of the H-bridge should never be on at any time. That is, transistors 202 and 206 should never be on at the same time. Similarly, transistors 204 and 208 should never be on at the same time. If both transistors on one side of H-bridge 104 were on at the same time, supply voltage terminal 210 would effectively be shorted to ground 212.

[0023] Unfortunately, a short circuit may occur within H-bridge 104. For example, output node 222 (OUTP) may be shorted to ground 212 or power supply voltage terminal 210. Similarly, output node 224 (OUTN) may be shorted to ground 212 or power supply voltage terminal 210. If OUTP is inadvertently shorted to ground, a short circuit condition will occur if transistor 202 is on. If OUTP is shorted to power supply voltage terminal 210, a short circuit condition will occur if transistor 206 is on. Similarly, if OUTN is shorted to ground, a short circuit condition will occur if transistor 204 is on, and if OUTN is shorted to power supply voltage terminal 210, a short circuit condition will occur if transistor 208 is on.

[0024] In such a short circuit condition, some audio systems can respond by completely disabling the audio amplifier to prevent the battery (e.g., a car battery) that powers the audio system (to the supply voltage terminal 210) from being drained and to prevent damage to other devices and electronic devices that share the supply terminal 210. However, turning off the audio system will completely prevent any audio from passing through the speaker 106. In various situations (such as an emergency), stopping all audio may be undesirable. The examples described herein allow at least some audio to be played through the speaker 106 despite the short circuit on one of the H-bridge output nodes 222, 224.

[0025] Figure 3is a timing diagram illustrating the operation of modulator 102 when no short circuit is present on the output nodes of H-bridge 104. Differential audio signals are shown as Audio_P and Audio_N. Audio_P and Audio_N can be digital signals. The audio signal is encoded as the difference between Audio_P and Audio_N. Dashed line 315 represents the common-mode voltage level of Audio_P and Audio_N. Modulator 102 implements or receives ramp signal 310. The modulator also includes a comparator that generates an output signal B based on a comparison of Audio_P with ramp signal 310, and generates an output signal A based on a comparison of Audio_N with ramp signal 310. When ramp signal 310 is greater than Audio_P, B is high; otherwise, B is low. Similarly, when ramp signal 310 is greater than Audio_N, A is high; otherwise, A is low. Other logic within modulator 102 (shown and described below) generates the P and N signals. exist Figure 3 In the example of , Audio_N is more positive than Audio_P (a negative audio signal), and therefore the width of the A pulse 320 is greater than the width of the B pulse 330. When AB is greater than 0, the P signal is determined to be AB, otherwise P is 0. Therefore, when A is high and B is low, P is high, otherwise P is low. When BA is greater than 0, the N signal is determined to be BA, otherwise N is 0. Therefore, when B is high and A is low, N is high, otherwise N is low. Figure 3 Because B is not high when A is low, N remains logic low.

[0026] therefore, Figure 3 It shows that the P pulse is intermittent (on and off) as shown in the figure and N remains low. Figure 2 As P pulses on and off, transistor 202 is also pulsed on and off. When P is high, transistor 202 is on, and when P goes low, transistor 202 is off. Transistor 206 switches on and off inversely to transistor 202, so that transistors 202 and 206 are never on at the same time. However, when N is low, transistor 204 remains off, and transistor 208 remains on. Figure 3 An example is shown for a negative audio signal (Audio_P is less than Audio_N). For a positive audio signal (Audio_P is greater than Audio_N), the timing of the signals is similar, but the N signal pulses intermittently while P remains low.

[0027] Figure 4An example of an audio system 400 including a modulator 102, an H-bridge 104, and a speaker 106 is shown. The audio system 400 also includes a load diagnostic circuit 408 and a direct current (DC) summing circuit 402. The load diagnostic circuit 408 monitors the output nodes 222 and 224 of the H-bridge 104 to detect whether the output nodes 222 or 224 are shorted to ground or a supply voltage terminal. An output signal 409 indicates whether a short circuit condition has been detected and whether the DC summing circuit 402 is to add a DC value (positive or negative) to the audio signal. A control signal 410 is coupled to the modulator 102 and is used to control a multiplexer within the modulator, as described below. Figure 11 An example implementation of the load diagnostic circuit 408 is shown and described below.

[0028] The DC summing circuit 402 has an input terminal that receives Audio_P and Audio_N, as well as an output signal 409 from the load diagnostic circuit 408. The output signal from the DC summing circuit 402 includes Audio_DC_P and Audio_DC_N. The DC summing circuit 402 can be implemented as a digital summer to add a positive value to the audio signal (the audio signal is the difference between Audio_P and Audio_N) or a negative value to the audio signal. If a short circuit condition is not detected by the load diagnostic, neither a positive value nor a negative value is added to the audio signal, and therefore Audio_DC_P is equal to Audio_P, and Audio_DC_N is equal to Audio_DC_N. However, a positive or negative value is added to the audio signal. Whether a positive or negative value is added to the audio signal depends on the type of short circuit condition detected by the load diagnostic circuit 408. If the load diagnostic circuit 408 detects that OUTP is shorted to the power supply voltage terminal or OUTN is shorted to ground, a positive value is added. If the load diagnostic circuit 408 detects that OUTP is shorted to ground or OUTN is shorted to the supply voltage terminal, a negative value is added. Figure 5 An example implementation of the modulator 102 is provided, and Figure 6 and Figure 7 Provides example timing diagrams illustrating the addition of negative and positive values to an audio signal.

[0029] exist Figure 5 , the exemplary modulator 102 includes comparators 501 and 502, a ramp generator 509, AND gates 503 and 504, NAND gates 505 and 506, and multiplexers 507 and 508. Audio_P_DC is provided to the negative input of comparator 501, and Audio_N_DC is provided to the negative input of comparator 502. Ramp generator 509 generates a ramp signal 310, which is provided to the positive inputs of comparators 501 and 502 to generate A and B signals.

[0030] Each of AND gates 503 and 504, as well as NAND gates 505 and 506, includes a non-inverting input and an inverting input, as shown. Signal A is provided to the non-inverting inputs of AND gates 503 and NAND gate 505, and to the inverting inputs of AND gates 504 and NAND gate 506. Similarly, signal B is provided to the inverting inputs of AND gates 503 and NAND gate 505, and to the non-inverting inputs of AND gates 504 and NAND gate 506. The output of AND gate 503 is a signal labeled P_GND. P_GND is high only when A is high and B is low; otherwise, P_GND is low. The output of AND gate 504 is a signal labeled N_GND. NM_GND is high only when B is high and A is low; otherwise, N_GND is low. The output of NAND gate 505 is a signal labeled N_SUP. N_SUP is low only when A is high and B is low; otherwise, N_SUP is high. The output of NAND gate 506 is a signal labeled P_SUP. P_SUP is low only when B is high and A is low, otherwise P_SUP is high.

[0031] The 0 input of multiplexer 507 receives P_GND, and the 1 input of multiplexer 507 receives P_SUP. The output of multiplexer 507 is the P signal to H-bridge 104. The 0 input of multiplexer 508 receives N_GND, and the 1 input of multiplexer 508 receives N_SUP. The output of multiplexer 508 is the N signal.

[0032] The control signal to multiplexers 507 and 508 is control signal 410 from load diagnostic circuit 408. In response to the load diagnostic circuit detecting a short circuit between OUTP and ground or between OUTN and ground, control signal 410 is asserted to cause multiplexers 507 and 508 to select their 0 input terminals. In response to the load diagnostic circuit detecting a short circuit between OUTP and the power supply voltage terminal or between OUTN and the power supply voltage terminal, control signal 410 is asserted to cause multiplexers 507 and 508 to select their 1 input terminals. When no short circuit is detected, control signal 410 is asserted to cause multiplexers 507 and 508 to select their 0 input terminals.

[0033] If a short circuit occurs between OUTP and ground, the modulator 102 should not allow transistor 202 to turn on. If a short circuit occurs between OUTP and the power supply voltage terminal 210, the modulator 102 should not allow transistor 206 to turn on. To prevent NMOS transistor 202 from turning on, the modulator 102 forces the P signal to remain at a logic low level. To prevent NMOS transistor 206 from turning on, the modulator 102 forces the P signal to remain at a logic high level. Similarly, if a short circuit occurs between OUTN and ground, the modulator 102 should not allow transistor 204 to turn on. If a short circuit occurs between OUTN and the power supply voltage terminal 210, the modulator 102 should not allow transistor 208 to turn on. To prevent transistor 204 from turning on, the modulator 102 forces the N signal to remain at a logic low level. To prevent transistor 208 from turning on, the modulator 102 forces the N signal to remain at a logic high level.

[0034] Figure 6 is a timing diagram illustrating how the system reacts to a short circuit between OUTP and ground detected by load diagnostic 408. In this case, a negative DC value is added to the audio signal. Adding the negative DC value causes the difference between Audio_P and Audio_N to become more negative. Figure 6 This illustrates that Audio_N increases to a more positive level, while Audio_P becomes more negative. A sufficiently large negative DC value is added so that Audio_N_DC and Audio_P_DC do not cross each other. The common-mode level 315 of Audio_N_DC and Audio_P_DC does not change.

[0035] The resulting A and B comparator output signals are also Figure 6 . The P signal remains at logic low because AB is not greater than 0. However, the N signal pulses high and low as shown. With P forced low, transistor 202 cannot turn on, and transistor 206 remains on. Transistor 204 pulses on and off according to the N signal, and audio can be played through the speaker (albeit with reduced quality).

[0036] Figure 7 is a timing diagram illustrating how the system reacts to a short circuit between OUTN and ground detected by load diagnostic 408. In this case, a positive DC value is added to the audio signal. Adding the positive DC value causes the difference between Audio_P and Audio_N to correct. Figure 7This illustrates that Audio_P increases to a more positive level, while Audio_N becomes more negative. A sufficiently large positive DC value is applied so that Audio_P_DC and Audio_N_DC do not cross each other. The N signal remains at a logic low because BA is not greater than 0. However, the P signal pulses high and low as shown. With N forced to remain low, transistor 204 cannot turn on, and transistor 208 remains on. Transistor 202 pulses on and off according to the P signal, and audio can be played through the speaker (albeit with reduced quality).

[0037] Figure 6 and Figure 7 The response to a short circuit between OUTP and ground and a short circuit between OUTN and ground is illustrated. In the event of a short circuit between OUTP and the supply voltage terminal, the load diagnostic circuit 408 asserts a signal 409 to the DC summing circuit 402 to cause the DC summing circuit 402 to add a positive value to the audio signal and asserts a control signal 410 to cause multiplexers 507 and 508 to select their 1 input terminals. Similarly, for a short circuit between OUTN and the supply voltage terminal, signals 409 and 410 are asserted to cause the DC summing circuit 402 to add a negative DC value to the audio signal and cause the multiplexers 507 and 508 to select their 1 input terminals.

[0038] Figure 8 Another embodiment of an audio system 800 is shown in which a signal masking circuit 806 is provided between the modulator 102 and the H-bridge 104. When the load diagnostic circuit 408 does not detect a short circuit, the signal masking circuit 806 allows the P signal and the N signal to be provided to the H-bridge, and based on the particular short circuit condition detected, the signal masking circuit 806 forces the P signal or the N signal to be logic low or logic high (as described above).

[0039] The signal shielding circuit 806 includes multiplexers 810 and 812 and inverters 811 and 813. In this example, each multiplexer 810, 812 includes at least four input terminals 0-3. The 0 input terminal of the multiplexer 810 is coupled to the output terminal of the modulator 102 and receives the P signal. The P signal is inverted by the inverter 811, and the output terminal of the inverter 811 is coupled to the 1 input terminal of the multiplexer 810. The 2 input terminal and the 3 input terminal of the multiplexer 810 are connected to the logic high (1) and the logic low (0), respectively. The 0 to 3 input terminals of the multiplexer 812 are similarly configured for the N signal. The N signal from the modulator 102 is provided to the 0 input terminal of the multiplexer 812. The logical inversion of the N signal is provided to the 1 input terminal via the inverter 813, and the logic high and logic low are provided to the 2 input terminal and the 3 input terminal of the multiplexer 812, respectively.

[0040] As described above, the load diagnostic circuit 408 detects four possible short circuit conditions on the output of the H-bridge 104 (OUTP shorted to the supply voltage terminal, OUTP shorted to ground, OUTM shorted to the supply voltage terminal, and OUTM shorted to ground). The control signals 830 and 832 encode the four possible short circuit conditions and serve as selection signals to the multiplexer. For example, if OUTP is shorted to ground, the control signal 830 causes the multiplexer 810 to select its 3 input terminal, which is a logic low (0). The output of the multiplexer 810 is labeled P_MASK and the output of the multiplexer 812 is labeled N_MASK. Although in Figure 4 In the example of FIG. 1 , the P signal and the N signal directly control the H bridge 104, but in FIG. Figure 8 In the example shown in FIG. 1 , the P_MASK signal and the N_MASK signal control the H-bridge 104 instead.

[0041] refer to Figure 9 , the audio system 900 is shown including a short-circuit feedback circuit 924 in the feedback loop. The audio system 900 includes the DC summing circuit 402, the modulator 102, the H-bridge 104, the speaker 106, and the load diagnostic circuit 408. The short-circuit feedback circuit 924 includes a level shifter (LS) 925 coupled to the resistor R1 provided in the P signal line. Similarly, the short-circuit feedback circuit 924 includes a level shifter (LS) 945 coupled to the resistor R2 provided in the N signal line.

[0042] In the absence of an output short-circuit condition, the output DC equals the input DC. That is, the DC level at output nodes 222 and 224 equals the DC voltage level of the input audio signal, which, in the absence of a short-circuit condition, is 0V. However, if output node 222 or 224 is shorted to supply voltage node 210 or ground 212, the output DC will no longer be approximately equal to the input DC, resulting in modulator saturation (0% or 100% duty cycle). To address this potential issue, load diagnostic circuit 408 generates control signals 950-953 to switches SW1-SW4. As shown, SW1 and SW2 are operable to couple the respective output nodes 224 and 222 to the input of modulator 102, and SW3 and SW4 are operable to couple short-circuit feedback circuit 924 (resistors R1 and R2, respectively) to the input of modulator 102, as shown. The output impedance of short-circuit feedback circuit 924 is higher (e.g., 10 to 100 times higher) than the output impedance of H-bridge 104. For example, the output impedance of the H-bridge 104 may be approximately 200 milliohms, and the output impedance of the short-circuit feedback circuit 924 may be approximately 50 ohms.

[0043] In response to OUTP being shorted to the power supply voltage node or ground, load diagnostic circuit 408 asserts control signals 951 and 952 to open SW2 and close SW3. Thus, instead of feeding OUTP from H-bridge 104 back to the input of modulator 102, a feedback signal is provided from short-circuit feedback circuit 924 (via resistor R1) to the input of the modulator. Similarly, in response to OUTM being shorted to the power supply voltage node or ground, load diagnostic circuit 408 asserts control signals 950 and 953 to open SW1 and close SW4. Thus, instead of feeding OUTM from H-bridge 104 back to the input of modulator 102, a feedback signal is provided from short-circuit feedback circuit 924 (via resistor R2) to the input of the modulator. In another embodiment, in the event of a short-circuit fault detection, both SW1 and SW2 may be opened, and SW3 and SW4 may be closed. The voltages produced by the level shifters 935, 945 and the resistance values of R1 and R2 are application specific and are set so that the DC level of the output of the short-circuit feedback circuit 924 is approximately equal to the DC level of the audio input signal.

[0044] Various aspects of the above audio systems can be combined. For example, Figure 10 An example of an audio system 1000 is depicted that includes the modulator 102, the DC summing circuit 402, the short-circuit feedback circuit 924, and the signal shielding circuit 806. Thus, in response to detecting a short circuit on the output of the H-bridge 104, the audio system 1000 can be configured to prevent specific transistors within the H-bridge 104 from turning on by adding a positive or negative DC value to the audio signal (via the DC summing circuit 402), blocking (via the signal shielding circuit 806) the output signal from the modulator (e.g., forcing P or N high or low as described above), and / or applying a feedback voltage from the short-circuit feedback circuit 924.

[0045] Figure 11 An example implementation of load diagnostic circuit 408 is shown. Load diagnostic circuit 408 includes logic circuit 1110 and current sources 1101-1104. Current sources 1101 and 1102 are coupled to output node 222 of the H-bridge, and current sources 1103 and 1104 are coupled to output node 224 of the H-bridge. Each current source can be independently turned on and off by logic circuit 1110. A short-circuit detection test is performed by logic circuit 1110 when transistors 202-208 of the H-bridge are turned off. The short-circuit detection test can be performed at system startup or during operation when no audio is playing through the speakers.

[0046] Speaker 106 has Figure 11The resistor is designated as Rspkr in Figure 1. Based on the magnitude of Rspkr, the current forced through the speaker will generate a certain voltage across the speaker terminals. In this example, the current generated by each of the four current sources 1101-1104 is I. In one example, the short-circuit detection test is performed in two steps. First, current sources 1101 and 1104 are turned on, and current sources 1102 and 1103 are turned off. Then, current sources 1102 and 1103 are turned on, and current sources 1101 and 1104 are turned off.

[0047] With current sources 1101 and 1104 turned on, a current I will flow from current source 1101 through Rspkr and to ground through current source 1104. In the absence of a short circuit on either output node 222 or 224, the current through Rspkr will be I. Therefore, the differential voltage (Vdiff) across Rspkr will be I*Rspkr. Both I and Rspkr are known a priori, and therefore, in the absence of a short circuit condition, Vdiff will be within a predicted voltage range. The common-mode voltage (VCM) between output nodes 222 and 224 will be (V222+V224) / 2, where V222 is the voltage on output node 222 relative to ground, and V224 is the voltage on output node 224 relative to ground. In the absence of a short circuit, VCM is PVDD / 2.

[0048] However, still with current sources 1101 and 1104 turned on, if output node 222 is shorted to ground, V222 will be 0V (due to the short circuit) and V224 will be 0V because current source 1104 is turned on and there is a relatively small voltage drop from ground to output node 224. At this stage (output node 222 is shorted to ground), both Vdiff and VCM will be approximately equal to 0V. If output node 224 is shorted to PVDD, rather than output node 222 being shorted to ground, V224 will be equal to PVDD due to the short circuit, and V222 will be approximately equal to PVDD due to current source 1101 being turned on. In the latter case (a short circuit between output node 224 and PVDD), Vdiff will be equal to 0V, and VCM will be equal to PVDD. Therefore, with current sources 1101 and 1104 turned on, a short circuit from output node 222 to ground or from output node 224 to PVDD can be detected.

[0049] Another short-circuit condition is when output node 222 is shorted to PVDD and output node 224 is shorted to ground. If output node 222 is shorted to PVDD and current sources 1101 and 1104 are turned on, the voltage at output node 222 will be PVDD due to the short circuit. Current I will flow through Rspkr, and therefore Vdiff will be equal to I*Rspkr. The voltage on output node 224 will be equal to PVDD-I*Rspkr, and therefore VCM will be equal to (PVDD+PVDD-I*Rspkr) / 2, which is equal to PVDD-I*Rspkr / 2. If output node 224 is shorted to ground, current I will flow through Rspkr, and Vdiff will be equal to I*Rspkr, and VCM will be equal to I*Rsprk / 2.

[0050] A similar analysis can be performed if current sources 1102 and 1103 are on (and current sources 1101 and 1104 are off).Table I lists various short circuit conditions and the resulting differential and common mode voltages based on which pair of current sources is on.

[0051] Table I

[0052]

[0053] When current sources 1101 and 1104 are turned on, and when current sources 1102 and 1103 are turned on, logic circuit 1110 monitors the voltages on output nodes 222 and 224. Based on the voltages and as described above, logic circuit 1110 can detect whether a short circuit exists and the type of short circuit (output node 222 shorted to PVDD or shorted to ground; output node 224 shorted to PVDD or shorted to ground).

[0054] Any of the examples described herein can be implemented on an integrated circuit. For example, Figure 1 (Except speakers), Figure 4 、 Figure 5 、 Figure 8 、 Figure 9 and Figure 10 Each of the examples shown in can be fabricated as an integrated circuit.

[0055] The term "coupled" is used throughout this specification. This term encompasses any connection, communication, or signal path that enables a functional relationship consistent with the description of this specification. For example, if device A generates a signal to control device B to perform an action, then in the first example, device A is coupled to device B. Alternatively, in the second example, if the intermediate component C does not substantially change the functional relationship between device A and device B, then device A is coupled to device B through the intermediate component C such that device B is controlled by device A via the control signal generated by device A.

[0056] The described embodiments may be modified, and other embodiments are possible, within the scope of the claims.

Claims

1. An audio system comprising: a modulator having an input and an output; an H-bridge having an input and an output, the output of the modulator being coupled to the input of the H-bridge; a load diagnostic circuit having an input and a first control output, wherein the load diagnostic circuit is configured to detect a short circuit condition on an output of the H-bridge; as well as A direct current adding circuit, ie, a DC adding circuit, has an input terminal and an output terminal, the output terminal of the DC adding circuit is coupled to the input terminal of the modulator, and the first control output terminal from the load diagnostic circuit is coupled to the DC adding circuit.

2. The audio system of claim 1 , wherein the DC adding circuit is configured to receive an audio input signal at its input and selectively add DC to the audio input signal in response to a control signal at the first control output from the load diagnostic circuit. 3 . The audio system of claim 2 , wherein the load diagnostic circuit asserts the control signal to the DC summing circuit in response to detecting a short circuit condition.

4. The audio system of claim 2, wherein: The output end of the H-bridge includes a first output terminal and a second output terminal; and The load diagnostic circuit is configured to selectively detect a short circuit type on the output end of the H-bridge, the type including the first output terminal being shorted to a power supply voltage terminal, the first output terminal being shorted to ground, the second output terminal being shorted to the power supply voltage terminal, or the second output terminal being shorted to ground.

5. The audio system of claim 4 , wherein the load diagnostic circuit is configured to assert the control signal based on a detected short circuit type, and the DC adding circuit is configured to selectively add a positive DC value to the audio input signal in response to a first state of the control signal or to add a negative DC value to the audio input signal in response to a second state of the control signal.

6. The audio system of claim 1, further comprising a signal shielding circuit coupled between the modulator and the H-bridge.

7. The audio system of claim 6, wherein: The load diagnostic circuit includes a second control output terminal; The signal shielding circuit includes a control input terminal coupled to the second control output terminal; The input end of the H-bridge includes a first H-bridge input end and a second H-bridge input end; The signal masking circuit is configured to force a signal on one of the first H-bridge input and the second H-bridge input to a fixed logic state in response to a control signal on a second control output of the load diagnostic circuit.

8. An audio system comprising: a modulator having an input and an output; An H-bridge having an input terminal and an output terminal; a load diagnostic circuit having an input terminal and a first control output terminal; as well as a signal shielding circuit coupled between the output of the modulator and the input of the H-bridge, the signal shielding circuit including a control input coupled to the first control output of the load diagnostic circuit; wherein the input end of the H-bridge comprises a first H-bridge input end and a second H-bridge input end; and The signal shielding circuit is configured to force a signal on one of the first H-bridge input terminal and the second H-bridge input terminal to a fixed logic state in response to a control signal on the first control output terminal of the load diagnostic circuit; wherein the output end of the modulator comprises a first modulator output end and a second modulator output end, and wherein the signal shielding circuit comprises: a first multiplexer having first to third inputs and a first select input, the first input of the first multiplexer being coupled to the first modulator output, the second input of the first multiplexer being configured to receive a fixed logic high, the third input of the first multiplexer being configured to receive a fixed logic low, and the first select input being coupled to the first control output of the load diagnostic circuit; as well as a second multiplexer having first to third inputs and a second select input, the first input of the second multiplexer being coupled to the second modulator output, the second input of the second multiplexer being configured to receive a fixed logic high, the third input of the second multiplexer being configured to receive a fixed logic low, and the second select input being coupled to the first control output of the load diagnostic circuit; The input terminals of the H-bridge include a first H-bridge input terminal and a second H-bridge input terminal.

9. The audio system of claim 8, wherein the inputs of the H-bridge include a first H-bridge input and a second H-bridge input, and wherein the output of the first multiplexer is coupled to the first H-bridge input and the output of the second multiplexer is coupled to the second H-bridge input.

10. An audio system comprising: a modulator having an input and an output; An H-bridge having an input terminal and an output terminal; a load diagnostic circuit having an input terminal and a first control output terminal; a signal shielding circuit coupled between the output of the modulator and the input of the H-bridge, the signal shielding circuit including a control input coupled to the first control output of the load diagnostic circuit; as well as a direct current adding circuit, i.e., a DC adding circuit, having an input and an output, the output of the DC adding circuit being coupled to the input of the modulator, and a second control output from the load diagnostic circuit being coupled to the DC adding circuit, the DC adding circuit being configured to add a DC value to an input signal at the input of the DC adding circuit in response to a signal at the second control output from the load diagnostic circuit; Wherein the load diagnostic circuit is configured to detect a short circuit condition on the output of the H-bridge and, in response, assert control signals on the first control output and the second control output.

11. An audio system comprising: a modulator having an input and an output; An H-bridge having an input terminal and an output terminal; a load diagnostic circuit having an input terminal and a first control output terminal; a signal shielding circuit coupled between the output of the modulator and the input of the H-bridge, the signal shielding circuit including a control input coupled to the first control output of the load diagnostic circuit; as well as a short-circuit feedback circuit having an input and an output, wherein the input of the short-circuit feedback circuit is coupled to the output of the modulator; a first switch coupled between the output terminal of the short-circuit feedback circuit and the input terminal of the modulator; as well as a second switch coupled between the output of the H-bridge and the input of the modulator; The first switch and the second switch are configured to change their operating states based on a control signal from the load diagnostic circuit.

12. The audio system of claim 11, wherein the H-bridge has an output impedance and the short-circuit feedback circuit has an output impedance, and the output impedance of the short-circuit feedback circuit is greater than the output impedance of the H-bridge.

13. An audio system comprising: a modulator having an input and an output; An H-bridge having an input terminal and an output terminal; a short-circuit feedback circuit having an input and an output, wherein the input of the short-circuit feedback circuit is coupled to the output of the modulator; as well as a first switch coupled between the output terminal of the short-circuit feedback circuit and the input terminal of the modulator; a second switch coupled between the output of the H-bridge and the input of the modulator; as well as A load diagnostic circuit has an input coupled to the output of the H-bridge, the load diagnostic circuit being configured to control operating states of the first switch and the second switch. 14 . The audio system of claim 13 , wherein the load diagnostic circuit is configured to detect a short circuit on the output of the H-bridge and, in response to the detected short circuit, open the second switch and close the first switch.

15. The audio system of claim 13, further comprising a signal masking circuit coupled between the output of the modulator and the input of the H-bridge, the signal masking circuit comprising a control input coupled to a control output of the load diagnostic circuit.

16. The audio system of claim 13, further comprising: a direct current adding circuit, i.e., a DC adding circuit, having an input and an output, the output of the DC adding circuit being coupled to the input of the modulator, and the control output from the load diagnostic circuit being coupled to the DC adding circuit, the DC adding circuit being configured to add a DC value to the input signal at the input of the DC adding circuit in response to a signal at the control output from the load diagnostic circuit.

17. The audio system of claim 16, wherein the DC summing circuit is configured to selectively add a positive DC value or a negative DC value to the input signal at the input of the DC summing circuit based on the signal at the control output from the load diagnostic circuit.

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