Electronic Design

Headphone Amps Shed External Blocking Capacitors

A family of three ground-referenced stereo headphone amplifiers from National Semiconductor eliminates external blocking capacitors for portable music players, mobile phones, CD players, PDAs, media players, notebook computers, and other portable applications.

The amplifiers’ circuit technology uses a low-noise, inverting charge pump to generate a negative supply voltage. This allows the outputs to be referenced at ground instead of a nominal dc voltage like traditional headphone amplifiers, eliminating the output-coupling capacitors typically required to drive single-ended loads. Using this circuit technology reduces component count, system cost, and board space while improving the lower frequency response of the headphone outputs.

The LM4920 is an output capacitor-less (OCL), fixed-gain headphone amplifier in a 2- by 2-mm micro surface-mounted device (SMD) package—a 67% reduction in footprint size from previous-generation leadless lead-frame packages (LLPs). It includes low logic-level capability to interface easily with low-voltage digital integrated circuits.

Also, the LM4920 delivers 80 mW of continuous average power into a 16-Ω, single-ended load or 40 mW per channel into a 16-Ω load, with less than 1% total harmonic distortion plus noise (THD+N) from a 3-V power supply. The typical gain of 1.5 V is fixed internally, eliminating the need for gain- setting external resistors.

Its typical power-supply rejection ratio (PSRR) is 70 db at 217 Hz, and it features an internal thermal shutdown protection mechanism. Its low-power-consumption shutdown mode is individually selectable for either channel by driving the shutdown channel pins with logic low. The LM4920 also offers fixed logic levels, easing its use with low-voltage basebands and microprocessors.

Building on National’s LM4920 core, the LM4981 adds an up/down volume control that eliminates the need for complex software controls. The LM4982 offers a digitally controlled I2C-compatible volume adjustment with headphone sense, a technique that detects whether a mono or stereo headphone plug has been inserted into the output jack.

The LM4981 delivers 83 mW of continuous power into a 16-Ω load with less than 1% THD+N while operating from a 3-V power supply. It operates from a single 2- to 4.2-V supply and features up-down volume control that sets the gain of the amplifier between –33 and +12 dB in 16 discrete steps using a two-wire interface. A selectable (active high/low) low-power shutdown mode provides flexible shutdown control. It has a typical PSRR of 67 db at 217 Hz.

The LM4982 is a variable-gain amplifier capable of delivering 80 mW of continuous average power into a 16-Ω, single-ended load or 40 mW per channel into a 16-V load, with less than 1% THD+N from a 3-V power supply. It has a typical PSRR of 66 dB at 1 kHz. Its I2C-compatible volume control allows 18- to –76-db gain settings.

All the chips contain pop-and-click circuitry, which eliminates noises that would otherwise occur during turn-on and turn-off transitions.

Available now, the LM4920 is offered in 14-bump micro SMD package and costs $0.89. The LM4981 is offered in a 16-pin LLP package and costs $1.15, while the LM4982 is available in a 16-bump micro SMD package and costs $1.50. All prices are for 1000-unit quantities. More information, sample ordering, and evaluation boards are available at:


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