Datasheet

LM48511
SNAS416G –JULY 2007–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
GENERAL AMPLIFIER FUNCTION
The LM48511 features a Class D audio power amplifier that utilizes a filterless modulation scheme, reducing
external component count, conserving board space and reducing system cost. The outputs of the device
transition from PV1 to GND with a 300kHz switching frequency. With no signal applied, the outputs (V
LS+
and
V
LS-
) 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 LM48511 outputs changes. For increasing output
voltage, the duty cycle of V
LS+
increases, while the duty cycle of V
LS-
decreases. For decreasing output voltages,
the converse occurs. The difference between the two pulse widths yields the differential output voltage.
FIXED FREQUENCY
The LM48511 features two modulations schemes, a fixed frequency mode (FF) and a spread spectrum mode
(SS). Select the fixed frequency mode by setting SS/FF = GND. In fixed frequency mode, the amplifier outputs
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
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 10% about a 330kHz 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 LM48511 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 SS/FF = V
DD
for spread spectrum mode.
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM48511 includes fully differential amplifier that features differential input and output stages. 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 in signal to noise ratio relative to differential inputs.
The LM48511 also offers the possibility of DC input coupling which eliminates the two external AC coupling, DC
blocking capacitors. The LM48511 can be used, however, as a single ended input amplifier while still retaining
it's fully differential benefits. In fact, completely unrelated signals may be placed on the input pins. The LM48511
simply amplifies the difference between the signals. A major benefit of a differential amplifier is the improved
common mode rejection ratio (CMRR) over single input amplifiers. The common-mode rejection characteristic of
the differential amplifier reduces sensitivity to ground offset related noise injection, especially important in high
noise applications.
AUDIO AMPLIFIER POWER DISSIPATION AND EFFICIENCY
The major benefit of a Class D amplifier is increased efficiency versus a Class AB. The efficiency of the
LM48511 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.
REGULATOR POWER DISSIPATION
At higher duty cycles, the increased ON-time of the switch FET means the maximum output current will be
determined by power dissipation within the LM48511 FET switch. The switch power dissipation from ON-time
conduction is calculated by:
P
D(SWITCH)
= DC x (I
INDUCTOR(AVE)
)
2
x R
DS(ON)
(W)
where
• DC is the duty cycle. (1)
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