LM48312

SNAS494D – JANUARY 2010 – REVISED MAY 2013

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APPLICATION INFORMATION

GENERAL AMPLIFIER FUNCTION

The LM48312 mono Class D audio power amplifier features a filterless modulation scheme that reduces external

component count, conserving board space and reducing system cost. The outputs of the device transition from

VDD to GND with a 300kHz switching frequency. With no signal applied, the outputs (VOUTA and VOUTB) switch

with a 50% duty cycle, in phase, causing the two outputs to cancel. This cancellation results in no net voltage

across the speaker, thus there is no current to the load in the idle state.

With the input signal applied, the duty cycle (pulse width) of the LM48312 outputs changes. For increasing output

voltage, the duty cycle of VOUTA increases, while the duty cycle of VOUTB decreases. For decreasing output

voltages, the converse occurs. The difference between the two pulse widths yields the differential output voltage.

ENHANCED EMISSIONS SUPPRESSION SYSTEM (E

2S)

The LM48312 features TI’s patented E2S system that reduces EMI, while maintaining high quality audio

reproduction and efficiency. The E2S system features spread spectrum and advanced edge rate control (ERC).

The LM48312 ERC greatly reduces the high frequency components of the output square waves by controlling the

output rise and fall times, slowing the transitions to reduce RF emissions, while maximizing THD+N and

efficiency performance. The overall result of the E2S system is a filterless Class D amplifier that passes FCC

Class B radiated emissions standards with 20in of twisted pair cable, with excellent 0.03% THD+N and high 88%

efficiency.

SPREAD SPECTRUM

The spread spectrum modulation reduces the need for output filters, ferrite beads or chokes. The switching

frequency varies randomly by 30% about a 300kHz center frequency, reducing the wideband spectral contend,

improving EMI emissions radiated by the speaker and associated cables and traces. Where a fixed frequency

class D exhibits large amounts of spectral energy at multiples of the switching frequency, the spread spectrum

architecture of the LM48312 spreads that energy over a larger bandwidth (See Typical Performance

Characteristics). The cycle-to-cycle variation of the switching period does not affect the audio reproduction,

efficiency, or PSRR.

DIFFERENTIAL AMPLIFIER EXPLANATION

As logic supplies continue to shrink, system designers are increasingly turning to differential analog signal

handling to preserve signal to noise ratios with restricted voltage signs. The LM48312 features a fully differential

speaker amplifier. A differential amplifier amplifies the difference between the two input signals. Traditional audio

power amplifiers have typically offered only single-ended inputs resulting in a 6dB reduction of SNR relative to

differential inputs. The LM48312 also offers the possibility of DC input coupling which eliminates the input

coupling capacitors. A major benefit of the fully differential amplifier is the improved common mode rejection ratio

(CMRR) over single ended input amplifiers. The increased CMRR of the differential amplifier reduces sensitivity

to ground offset related noise injection, especially important in noisy systems.

POWER DISSIPATION AND EFFICIENCY

The major benefit of a Class D amplifier is increased efficiency versus a Class AB. The efficiency of the

LM48312 is attributed to the region of operation of the transistors in the output stage. The Class D output stage

acts as current steering switches, consuming negligible amounts of power compared to their Class AB

counterparts. Most of the power loss associated with the output stage is due to the IR loss of the MOSFET on-

resistance, along with switching losses due to gate charge.

GAIN SETTING

The LM48312 features five internally configured gain settings, 0, 3, 6, 9, and 12dB. The device gain is selected

through a single pin (GAIN). The gain settings are shown in Table 1. The gain of the LM48312 is determined at

startup. When the LM48312 is powered up or brought out of shutdown, the device checks the state of GAIN, and

sets the amplifier gain accordingly. Once the gain is set, the state of GAIN is ignored and the device gain cannot

be changed until the device is either shutdown or powered down.

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