MAX9724 DirectDrive Headphone Amplifier
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Introduction of the DirectDrive Technology in headphone amplifies and overview of the MAX9724 DirectDrive® Headphone Amplifier
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MAX9724 DirectDrive Headphone Amplifier
- 7. Functional Block Diagram Class AB amplifier
- 9. Lineout Amplifier and Filter Block
- 11. Typical Operating Circuits – MAX9724B
- 12. Typical Operating Circuit – MAX9724A
Editor's Notes
- Welcome to the training module on MAXIM MAX9724 DirectDrive ® Headphone Amplifier. This training module introduces the DirectDrive Technology in headphone amplifies, and overviews the MAX9724 DirectDrive ® Headphone Amplifier.
- Most traditional headphone amplifiers used in battery-powered electronics are single-supply devices operating between a positive voltage and ground. This design results in an amplifier that can pass only positive signals. Audio signals, however, inherently swing positive and negative. So before a traditional single-supply amplifier can accept the audio signal, a DC bias must be added. However, the DC bias may cause the distortion to the speaker, waster power, and unnecessarily heat the speaker. To prevent the DC bias from reaching the speaker, a DC-blocking capacitor is typically used. This capacitor combined with the mostly resistive load creates a highpass filter.
- DirectDrive headphone amplifiers eliminate the need for a DC-blocking capacitor on the output by eliminating the DC bias. The amplifier enables to pass both positive and negative signals. The negative swing is achieved through an onboard charge pump used to generate a negative supply that tracks the amplitude of the positive supply. The charge pump used in a DirectDrive design requires only two small ceramic capacitors to operate: one flying capacitor and one hold capacitor. These capacitors are typically 1μF and can be as small as 0402. This specification results in significant space savings when compared to a conventional headphone amplifier with 220μF output capacitors, and provides superior performance.
- Especially critical in compact equipment such as cell phones, PDAs, and notebook computers, the DirectDrive technology can save the cost and space of DC-blocking capacitors. By eliminating the capacitors, DirectDrive thus eliminates a major source of click and pop, which is created when the DC-blocking capacitor in traditional headphone amplifiers is charged every time the amplifier is enabled. DirectDrive technology allows the headphone amplifier to operate directly from the supply used by the system's major digital ICs. By operating from a supply lower than the battery voltage, the headphone amplifier becomes more efficient. Finally, the output capacitor used in a traditional headphone can contribute significant distortion to the audio signal at low frequencies. By eliminating the coupling capacitor, DirectDrive eliminates this source of distortion.
- The MAX9724 stereo headphone amplifiers are designed for portable equipment where board space is at a premium. These devices use the DirectDrive™ architecture to produce a ground-referenced output from a single supply, eliminating the need for large DC-blocking capacitors, saving cost, board space, and component height. The MAX9724 suppresses RF radiation received by input and supply traces acting as antennas and prevents the amplifier from demodulating the coupled noise. The MAX9724A offers an externally adjustable gain while the MAX9724B has an internally preset gain of -1.5V/V. The MAX9724 deliver up to 60mW per channel into a 32Ω load and have low 0.02% THD+N. A high power-supply rejection ratio (PSRR) allows these devices to operate from noisy digital supplies without an additional linear regulator. Comprehensive click-and-pop circuitry suppresses audible clicks and pops on startup and shutdown.These devices operate from a single 2.7V to 5.5V supply, consume only 3.5mA of supply current, feature short-circuit and thermal-overload protection
- The stereo headphone amplifier consists of two 60mW Class AB headphone amplifiers, undervoltage lockout (UVLO)/shutdown control, charge pump, and comprehensive click-and-pop suppression circuitry. The charge pump inverts the positive supply (V DD ), creating a negative supply (PV SS ). The undervoltage lockout prevents operation from an insufficient power supply and click-and-pop suppression eliminates audible transients on startup and shutdown.
- Modern audio systems are often subject to RF radiation from sources like wireless networks and cellular phone networks. Although the RF radiation is out of the audio band, many signals, in particular GSM signals, contain bursts or modulation at audible frequencies. Most analog amplifiers demodulate the low-frequency envelope, adding noise to the audio signal. The architecture of the MAX9724 addresses the problem of the RF susceptibility by rejecting RF noise and preventing it from coupling into the audio band. The figure shows the improved RF susceptibility of the MAX9724 compared with an amplifier that is susceptible to RF interference. At the critical GSM frequencies, the MAX9724 offers a minimum of 39dB of improvement over the typical headphone amplifier.
- 2V RMS is a popular audio line level, first used in CD players, but now common in DVD and set-top box (STB) interfacing standards. A 2V RMS sinusoidal signal equates to approximately 5.7V P-P , which means that the audio system designer cannot simply run the lineout stage from a 5V supply—the resulting output swing would be inadequate. A common solution to this problem is to use op amps driven from split supplies, but This can mean adding extra cost and complexity to the system. The MAX9724A, having the ability to derive 2V RMS from a 5V supply, or even 3.3V supply, can often simplify power-supply design in some systems. When the MAX9724A is used as a line driver to provide outputs that feed stereo equipment with a digital-to-analog converter (DAC) used as an audio input source (as shown in the figure), it is often desirable to eliminate any high-frequency quantization noise produced by the DAC output before it reaches the load.
- The gain of the MAX9724B amplifier is internally set to-1.5V/V. All gain-setting resistors are integrated into the device, reducing external component count. The gain of the MAX9724A amplifier is set externally as shown in the figure. When selecting the external resistors to set the gain, the selection of proper resistor value need to be taken into consideration. First, choose feedback resistor (R F ) values in the tens of kΩ range. Since the effective value of R IN is equal to the sum of the source resistance of the input device and the value of the input resistor connected to the inverting terminal of the headphone amplifier, the overall closed-loop gain of the headphone amplifier can be reduced if the input resistor is not significantly larger than the source resistance of the input device.
- The MAX9724B is able to internally set gain, so the DirectDrive headphone amplifier requires only five small capacitors to complete the amplifier circuit: two (C1 & C2) for the charge pump, two (C IN ) for audio input coupling, and one (C3) for power-supply bypassing. The input capacitor (CIN), in conjunction with the input resistor (RIN), forms a highpass filter that removes the DC bias from an incoming signal. The power-supply bypass capacitor lowers the output impedance of the power supply, and reduces the impact of the MAX9724B’s charge-pump switching transients.
- The typical operation circuit for MAX9724A is similar to the one for MAX9724B, except it needs four external resistors to set the gain of the amplifier.
- Thank you for taking the time to view this presentation on MAX9724 DirectDrive ® Headphone Amplifier . 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 simply call our sales hotline. For more technical information you may either visit the MAXIM site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.