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EP-3918351-B1 - ANALOG BASED SPEAKER THERMAL PROTECTION IN CLASS-D AMPLIFIERS

EP3918351B1EP 3918351 B1EP3918351 B1EP 3918351B1EP-3918351-B1

Inventors

  • CHADHA, Jasjot, Singh

Dates

Publication Date
20260513
Application Date
20200131

Claims (11)

  1. A thermal protection circuit, comprising: a sensing resistor (Rs) coupled between a first node (205) and a second node (124), wherein the sensing resistor has a resistance Rs and is coupled in series with a load (120) of a speaker; a first amplifier circuit (210) coupled in parallel to the sensing resistor (Rs) at the first and second nodes (128, 205), wherein the first amplifier circuit has a first gain factor G; a filter (150) coupled between the output of the first amplifier circuit (210) and the negative input of a second amplifier circuit (245) configured to filter the output of the first amplifier circuit (210) to a pilot tone frequency added to an audio input signal; the second amplifier circuit (245) having an output coupled to a current mirror (255) configured to generate a current, wherein the second amplifier circuit has a second gain factor (1/A), a current mirror ratio of the current mirror (255) is based on the first and second gain factors (G, 1/A), and the current is applied to a reference resistor (130) having a resistance equal to an initial resistance of the load (120); a buffer (260) having an input coupled to an output of the current mirror (255); a third amplifier circuit (310) having an input coupled to an output of the buffer (260) and configured to receive a voltage across the load (120) and a voltage on a ground node, and configured to output a voltage difference between the reference voltage, generated by the output current of the current mirror (255) and the reference resistor (130), and the voltage across the load (120); and a peak voltage detector (320) having an input coupled to an output of the third amplifier circuit (310), which is configured to output a voltage based on the peak current for the load (120) and the difference in resistance for the speaker due to temperature changes.
  2. The thermal protection circuit of claim 1, wherein the first amplifier circuit (210) comprises a first input, a second input, and a first output, wherein the first input is coupled to the first node, wherein the second input is coupled to the second node, and the second amplifier circuit (245) is coupled to the first output.
  3. The thermal protection circuit of claim 1, wherein the current mirror (255) comprises: a first transistor having a first control input and a first current terminal coupled to the second amplifier circuit, the first transistor further having a second current terminal coupled to a supply voltage node; a fifth resistor coupled to the first current terminal and the second amplifier circuit; a second transistor having a second control input coupled to the second amplifier circuit, a third current terminal coupled to the supply voltage node, and a fourth current terminal coupled to the buffer; and a sixth resistor coupled to the fourth current terminal and the buffer; and preferably, wherein the fifth resistor has a resistance A*Rs, and wherein the sixth resistor has a resistance A*Rload/G, where Rload represents an initial resistance of the load.
  4. The thermal protection circuit of claim 1, wherein the third amplifier circuit (310) comprises: a third differential amplifier having a third negative input, a third positive input, and a third output, wherein the third output is coupled to the peak voltage detector; and a seventh, an eighth, a ninth, a tenth, an eleventh, and a twelfth resistor having a third resistance, wherein: the seventh resistor is coupled to the third negative input and the second node; the eighth resistor is coupled to the third negative input and the ground node; the ninth resistor is coupled to the third positive input and the buffer; the tenth resistor is coupled to the third positive input and a third node, wherein the second and third nodes are configured to be coupled to the load; the eleventh resistor is coupled to the third negative input and the third output; and the twelfth resistor is coupled to the third positive input and a supply voltage node.
  5. A system (100), comprising: a gain control circuit (110) configured to receive an input signal and apply a gain factor; a class-D amplifier (115) coupled to the gain control circuit (110) and configured to be coupled to a load (120) of a speaker; a ΔR detection circuit (140) comprising the thermal protection circuit of claim 1 coupled to the class-D amplifier (115) and configured to determine a difference in resistance between the load (120) and the reference resistor (130); and a power limiter circuit (160) coupled to the ΔR detection circuit (140) and configured to decrease the gain factor.
  6. The thermal protection circuit of claim 2 or the system (100) of claim 5, wherein the first amplifier circuit (210) further comprises: a first differential amplifier (220) having a first negative input, a first positive input, and the first output; a first resistor (Rf) coupled between the first input and the first negative input; a second resistor (Rf) coupled between the second input and the first positive input, wherein the first and the second resistors have a first resistance Rf; a third resistor coupled between a supply voltage node and the first positive input; and a fourth resistor coupled between the first negative input and the first output, wherein the third and the fourth resistors have a second resistance G*Rf, and wherein a ratio of the first resistance to the second resistance is the first gain factor G.
  7. The thermal protection circuit of claim 1 or the system (100) of claim 5, wherein the second amplifier circuit (245) comprises a second differential amplifier (245) having: a second negative input coupled to the first amplifier circuit (210); and a second positive input and a second output coupled to the current mirror (255).
  8. The system (100) of claim 5, wherein the current mirror (255) comprises: a first transistor (MP1) having a first control input and a first current terminal coupled to the second amplifier circuit (245), the first transistor (MP1) further having a second current terminal coupled to a supply voltage node; a fifth resistor coupled to the first current terminal and the second amplifier circuit; and a second transistor (MP2) having a second control input coupled to the second amplifier circuit, a third current terminal coupled to the supply voltage node, and a fourth current terminal coupled to the buffer (260, 275) and the reference resistor (Rref); and preferably, wherein the fifth resistor has a resistance A*Rs, and wherein the reference resistor has a resistance A*Rload/G, where Rload represents the initial resistance of the load.
  9. The system (100) of claim 5, wherein the third amplifier circuit (310) comprises: a third differential amplifier having a third negative input, a third positive input, and a third output, wherein the third output is coupled to the peak voltage detector (320); and a seventh, an eighth, a ninth, a tenth, an eleventh, and a twelfth resistor (R 305A...R 305F) having a third resistance, wherein: the seventh resistor is coupled to the third negative input and the sensing resistor, wherein the seventh resistor is configured to be coupled to the load; the eighth resistor is coupled to the third negative input and the ground node; the ninth resistor is coupled to the third positive input and the buffer; the tenth resistor is coupled to the third positive input and configured to be coupled to the load; the eleventh resistor is coupled to the third negative input and the third output; and the twelfth resistor is coupled to the third positive input and a supply voltage node.
  10. The system (100) of claim 5, wherein the power limiter circuit (160, 410) comprises: a reference voltage generator coupled to the ΔR detection circuit and configured to generate a reference voltage using an output of the ΔR detection circuit and a threshold difference in resistance for the load; and a differential amplifier (430) having a positive input coupled to the reference voltage generator, a negative input coupled to the ΔR detection circuit, and an output coupled to the gain control circuit (110).
  11. A thermal protection method performed by the system of claim 5, comprising: determining a current through the load (120) of the speaker; determining the reference voltage based on the current through the load and the initial resistance of the load (120); determining the voltage difference between the reference voltage and the voltage across the load (120); comparing the voltage difference to a threshold voltage difference, wherein the threshold voltage difference corresponds to a threshold difference in resistance of the load (120); and modifying the current through the load (120) based on the comparison.

Description

BACKGROUND Speakers are often overdriven in order to obtain a desired loudness for an audio output signal. However, overdriving a speaker can cause the speaker to overheat and cause permanent damage to the speaker. Overheating a speaker can cause a change in the shape of the speaker's diaphragm, which distorts audio signals output by the speaker. Overheating the speaker can also melt components within the speaker, including glue holding a voice coil to a speaker driver, solder connecting an amplifier to the speaker, and insulation on the voice coil. Melting the insulation on the voice coil causes the voice coil to short and limit the loudness of the speaker, lowering the resistance of the speaker and further increasing the speaker temperature. These mechanical failures within a speaker due to temperature increases can cause the speaker to fail. In order to reduce the likelihood of these mechanical failures, the power applied to the speaker is controlled to reduce overheating. Some audio amplifier systems detect speaker temperature using current and voltage sensors, which detect the current through and voltage across a connected speaker. The sensed current and voltage are provided to a digital signal processor via a sigma-delta analog-to-digital converter (ADC), which determines the speaker resistance and converts the determined resistance into a speaker temperature based on the speaker's thermal characteristics. However, the ADCs, current and voltage sensors, and digital signal processors are area intensive on a semiconductor die. Some audio amplifier systems rely on feed-forward thermal protection for connected speakers. The audio output signal peak values are detected. In response to the signal level crossing a pre-defined threshold, a multi-stage automatic gain control circuit reduces the output signal power in a defined sequence, which ensures the speaker is able to play high power signals for a short period of time but not so long as to heat the speaker. However, the multi-stage automatic gain control circuit implements conservative thresholds for the audio output signal because the actual speaker temperature is not sensed, and the audio amplifier system relies on feed-forward protection for the speaker. The conservative thresholds lead to lower maximum allowed power for the audio amplifier system. US 2018/358943 A1 discloses an audio playback device that includes an audio receiver module, a loudspeaker module and an audio control circuit. SUMMARY The invention is defined by the features of the appended claims. A circuit comprises a sensing resistor with a resistance Rs, a first amplifier circuit with a first gain factor G, a second amplifier circuit with a second gain factor (1/A), a third amplifier circuit, a current mirror, a buffer, and a peak voltage detector. The first amplifier circuit is coupled to the sensing resistor at a first node and a second node and to the second amplifier circuit, which is further coupled to the current mirror. The buffer is coupled to the current mirror and to the third amplifier circuit, which is further coupled to the peak voltage detector and configured to receive a voltage across a load and a voltage on a ground node. The load is a speaker and a filter is coupled between the first and the second amplifier circuits. In some examples, the first amplifier circuit comprises a first input coupled to the first node, a second input coupled to the second node, and a first output coupled to the second amplifier circuit. The first amplifier circuit includes a first differential amplifier having a first negative input, a first positive input, and the first output. A first and a second resistor have a first resistance. The first resistor is coupled between the first input and the first negative input. The second resistor is coupled between the second input and the first positive input. A third and a fourth resistor have a second resistance, such that a ratio of the first resistance to the second resistance is the first gain factor G. The third resistor is coupled between a supply voltage node and the first positive input. The fourth resistor is coupled between the first negative input and the first output. In some examples, the second amplifier circuit includes a second differential amplifier having a second negative input coupled to the first amplifier circuit and a second positive input and a second output coupled to the current mirror. The current mirror includes a first and a second transistor and a fifth and a sixth resistor. The first transistor has a first control input and a first current terminal coupled to the second amplifier circuit, and a second current terminal coupled to a supply voltage node. The fifth resistor is coupled to the first current terminal and the second amplifier circuit. The second transistor has a second control input coupled to the second amplifier circuit, a third current terminal coupled to the supply voltage node, and a fourth current termina