LM48310

SNAS430D – NOVEMBER 2007 – REVISED MAY 2013

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APPLICATION INFORMATION

GENERAL AMPLIFIER FUNCTION

The LM48310 mono Class D audio power amplifier features a filterless modulation scheme that reduces external

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 LM48310 outputs changes. For increasing output

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

ENHANCED EMISSIONS SUPPRESSION SYSTEM (E

spectrum, and advanced edge rate control (ERC). The LM48310 ERC greatly reduces the high frequency

components of the output square waves by controlling the output rise and fall times, slowing the transitions to

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.

FIXED FREQUENCY MODE (SYNC_IN = GND)

The LM48310 features two modulations schemes, a fixed frequency mode and a spread spectrum mode. Select

the fixed frequency mode by setting SYNC_IN = GND. In fixed frequency mode, the amplifier output switch at a

constant 300kHz. In fixed frequency mode, the output spectrum consists of the fundamental and its associated

harmonics (see TYPICAL PERFORMANCE CHARACTERISTICS).

The logic selectable spread spectrum mode eliminates the need for output filters, ferrite beads or chokes. In

spread spectrum mode, 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 LM48310 spreads that energy over a larger

bandwidth (See TYPICAL PERFORMANCE CHARACTERISTICS). The cycle-to-cycle variation of the switching

mode.

EXTERNAL CLOCK MODE (SYNC_IN = CLOCK)

Connecting a clock signal to SYNC_IN synchronizes the LM48310 oscillator to an external clock, moving the

output spectral components out of a sensitive frequency band, and minimizing audible beat frequencies when

multiple LM48310s are used in a single system. The LM48310 accepts an external clock frequency between

200kHz and 1MHz. The LM48310 can be synchronized to a spread spectrum clock, allowing multiple LM48310s

to be synchronized in spread spectrum mode (see TYPICAL PERFORMANCE CHARACTERISTICS).

SYNC_OUT

SYNC_OUT is a clock output for synchronizing external devices. The SYNC_OUT signal is identical in frequency

and duty cycle of the amplifier’s switching frequency. When the LM48310 is in fixed frequency mode,

SYNC_OUT is a fixed, 300kHz clock. When the LM48310 is in spread spectrum mode, SYNC_OUT is an

identical spread spectrum clock. When the LM48310 is driven by an external clock, SYNC_OUT is identical to

the external clock. If unused, leave SYNC_OUT floating.

Multiple LM48310s can be synchronized to a single clock. In Figure 27, device U1 is the master, providing a

spread spectrum clock to the slave device (U2). This configuration synchronizes the switching frequencies of the

two devices, eliminating any audible beat frequencies. Because SYNC_OUT has no audio content, there is

minimal THD+N degredation or crosstalk between the devices, Figure 28 - Figure 30.

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