An Overview of theCS42L55 stereo CODEC w/Class H Headphone Amp
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Introduction of the internal functional circuits of CS42L55 and its basic operation
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An Overview of theCS42L55 stereo CODEC w/Class H Headphone Amp
- 6. Analog Inputs Pseudo-Differential Inputs
Notas del editor
- Welcome to the training module on a n overview study of CS42L55 stereo codec w/class H headphone amp. This training module introduces the internal functional circuits of CS42L55 and its basic operation.
- The CS42L55 is a highly integrated, 24-bit, ultra-low power stereo CODEC based on multi-bit delta-sigma modulation. Both the ADC and DAC offer many features suitable for low power portable applications. The chip has independent control of a variety of features of each analog channel input path. The Programmable Gain Amplifier (PGA) provides programmable analog gain with zero cross transitions to shut down without clicking. The ADC path includes a digital volume attenuator with soft ramp transitions and a programmable Automatic Level Control (ALC) and noise gate for noise suppression.
- The key features make the CS42L55 the ideal solution for high end portable audio applications which require extremely low power consumption in a minimal amount of space. The broad applications include PDA, Portable media player, digital voice recorder, Digital Cameras, smart phone, and others.
- The CS42L55 is a highly integrated, ultra-low power, 24-bit audio CODEC comprising stereo A/D and D/A converters with pseudo-differential stereo input and output amplifiers. The ADC and DAC are designed using multi-bit delta-sigma techniques; both converters operate at a low oversampling ratio of 64xFs, maximizing power savings while maintaining high performance. The beep generator delivers tones at select frequencies across nearly two octaves. The CODEC operates in one of three sample rate speed modes: Quarter, Half and Single. It accepts and is capable of generating serial audio clocks (SCLK, LRCK) derived from a 12 or 6 MHz input Master Clock (MCLK). Designed with a very low voltage digital core and low voltage Class H amplifiers (powered from an integrated low-dropout regulator and a step-down/inverting charge pump, respectively), the CS42L55 provides significant reduction in overall power consumption.
- The analog input portion of the CODEC allows selection from two stereo line-level sources into a Programmable Gain Amplifier (PGA). The CS42L55 implements a pseudo-differential input stage. The optional pseudo-differential configuration provides noise-rejection for single-ended inputs. When enabled, the Automatic Level Control (ALC) monitors the analog input signal after the digital attenuator. The ALC then detects when peak levels exceed the maximum threshold settings and first lowers the PGA gain settings and then increases the digital attenuation levels at a programmable attack rate and maintains the resulting level below the maximum threshold. When input signal levels fall below the minimum threshold, digital attenuation levels are decreased first and the PGA gain is then increased at a programmable release rate and maintains the resulting level above the minimum threshold.
- The CS42L55 accommodates analog routing of the analog input signal directly to the headphone and line out amplifiers. This feature is useful in applications that utilize an FM tuner where audio recovered over-the-air must be transmitted to the headphone amplifier without digital conversion in the ADC and DAC. This analog passthrough path reduces power consumption and is immune to modulator switching noise that could interfere with some tuners. This path is selected using the Line or HP mux bits and powering down the ADC.
- The analog output portion of the CODEC includes separate pseudo-differential headphone and line out Class H amplifiers. An on-chip step-down/inverting charge pump creates a positive and negative voltage equal to the input or one-half the input supply for the amplifiers, allowing an adaptable, full-scale output swing centered around ground. The inverting architecture eliminates the need for large DC-blocking capacitors and allows the amplifier to deliver more power to headphone loads at lower supply voltages. The step-down architecture allows the amplifier’s power supply to adapt to the required output signal. This adaptive power supply scheme converts traditional Class AB amplifiers into more power-efficient Class H amplifiers.
- The CS42L55 headphone and line output amplifiers use a patented Cirrus Logic Bi-Modal Class H technology. This technology maximizes operating efficiency of the typical Class AB amplifier while maintaining high performance. In a Class H amplifier design, the rail voltages supplied to the amplifier vary with the needs of the music passage that is being amplified. This prevents unnecessarily wasting energy during low power passages of program material or when the program material is played back at a low volume level.
- The power curves for the two modes of operation are shown in the figure. This graph details the power supplied to a load versus the power drawn from the supply for each of the three use cases. When the rail voltages are set to VCP, the amplifiers will operate in their least efficient mode. When the rail voltages are held at ±VCP/2, the amplifiers will operate in their most efficient mode, but will be will clipped if required to amplify a full-scale signal.
- The Beep is generated before the limiter and may affect desired limiting performance. If the limiter function is used, it may be necessary to set the beep volume sufficiently below the threshold to prevent the peak detect from triggering. When enabled, the limiter monitors the digital input signal before the DAC modulators, detects when levels exceed the maximum threshold settings and lowers the master volume at a programmable attack rate below the maximum threshold.
- The serial port operates in the I 2 S digital interface formats with varying bit depths up to 24 into the DAC and a fixed depth of 24 out the ADC. Data is clocked out of the ADC on an internally delayed version of the rising SCLK edge. This provides more setup time for capturing data on the rising edge of SCLK. Data is clocked into the DAC on the rising edge of SCLK.
- The control port is used to access the registers allowing the CODEC to be configured for the desired operational modes and formats. The operation of the control port may be completely asynchronous with respect to the audio sample rates. However, to avoid potential interference problems, the control port pins should remain static if no operation is required. The control port operates using an I²C interface with the CODEC acting as a slave device.
- As with any high-resolution converter, the CS42L55 requires careful attention to power supply and grounding arrangements if its full potential performance is to be realized. The CS42L55 comes in a compact QFN package. The under side of the QFN package reveals a large metal pad that serves as a thermal relief to provide for maximum heat dissipation. This pad must mate with an equally dimensioned copper pad on the PCB and must be electrically connected to ground.
- This diagram shows typical connections of any application. The headphone amplifier’s output power and distortion are rated using the nominal capacitance shown. Larger capacitance reduces the ripple on the internal amplifiers’ supplies and in turn reduces the amplifier’s distortion at high output power levels. Smaller capacitance may not sufficiently reduce ripple to achieve the rated output power and distortion. Additional bulk capacitance may be added to improve PSRR at low frequencies.
- Thank you for taking the time to view this presentation on “ an Overview Study Of CS42L55 Stereo CODEC with Class H Headphone Amp”. If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simple call our sales hotline. For more technical information you may either visit the Cirrus Logic site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.