Datasheet

LM48310
SNAS430D –NOVEMBER 2007–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
GENERAL AMPLIFIER FUNCTION
The LM48310 mono Class D audio power amplifier features a filterless modulation scheme that reduces external
component count, conserving board space and reducing system cost. With no signal applied, the outputs (V
OUTA
and V
OUTB
) switch between V
DD
and GND 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 LM48310 outputs changes. For increasing output
voltage, the duty cycle of V
OUTA
increases, while the duty cycle of V
OUTB
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
2
S)
The LM48310 features ’s patent-pending E
2
S system that reduces EMI, while maintaining high quality audio
reproduction and efficiency. The E
2
S system features a synchronizable oscillator with selectable spread
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
reduce RF emissions, while maximizing THD+N and efficiency performance. The overall result of the E
2
S 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.
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).
SPREAD SPECTRUM MODE (SYNC_IN = V
DD
)
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
period does not affect the audio reproduction, efficiency, or PSRR. Set SYNC_IN = V
DD
for spread spectrum
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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