AD1994ACPZ Analog Devices Inc, AD1994ACPZ Datasheet - Page 15

• Manufacturer Part Number: AD1994ACPZ
• Description: IC AMP AUDIO PWR 25W 64LFCSP
• Manufacturer: Analog Devices Inc
• Type: Class Dr

Specifications of AD1994ACPZ

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 25W x 2 @ 6 Ohm
• Voltage - Supply: 4.5 V ~ 5.5 V
• Features: Depop, Mute, Short-Circuit and Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 64-LFCSP
• Operational Class: Class-D
• Audio Amplifier Output Configuration: 1-Channel Mono/2-Channel Stereo
• Output Power (typ): 50x1@3Ohm/25x2@6OhmW
• Audio Amplifier Function: Speaker
• Total Harmonic Distortion: 0.003@1W%
• Single Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (typ): 5/20V
• Supply Current (max): 260@12V/27@5V/7@5VmA
• Power Supply Requirement: Triple
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 65dB
• Single Supply Voltage (min): Not RequiredV
• Single Supply Voltage (max): Not RequiredV
• Dual Supply Voltage (min): 4.5/6.5V
• Dual Supply Voltage (max): 5.5/22.5V
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 64
• Package Type: LFCSP EP
• For Use With: EVAL-AD1994EBZ - BOARD EVAL FOR AD1994
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
OVERVIEW
The AD1994 is a 2-channel, high performance, switching, audio
power amplifier. Each of the two Σ-Δ modulators converts a
single-ended analog input into a 2-level pulse stream that
controls the differential, full H-bridge, power output stage. The
combination of an Σ-Δ modulator and a switching power stage
provides an inherently linear and efficient means of amplifying
the entire range of audio frequencies. The AD1994 also offers
warning and protection circuits for overcurrent and over-
temperature conditions, as well as silent turn-on and turn-off
transitions.
Σ-Δ MODULATOR
The AD1994 is a switching type, also known as a Class-D, audio
power amplifier. This class of amplifiers maximizes efficiency
by only using its power output devices in full-on or full-off
states. While most Class-D amplifiers use some variation of
pulse-width modulation (PWM), the AD1994 uses Σ-Δ
modulation to determine the switching pattern of the output
devices. This provides a number of important benefits. Σ-Δ
modulators do not produce a sharp peak with many harmonics
in the AM frequency band as pulse-width modulators (PWM)
often do. In addition, the 1-bit quantizer produces excellent
linearity across the full amplitude range.
Σ-Δ modulators require feedback to generate an error signal
with respect to the input. The feedback voltages for the AD1994
modulators come from the outputs of the power devices and
before the passive low-pass filters (see Figure 45). This compensates
for nonlinear behavior in the power stage, such as nonoverlap
time, mismatched rise and fall times, and propagation delays. It
also reduces sensitivity to both dc and transient changes of the
power supply voltage.
Σ-Δ modulators operate in discrete time. As with all time-
quantized systems, the Nyquist frequency is equal to half of
the sampling frequency and input signals above that point
aliases back into the base band. The AD1994 sampling frequency
(master clock) is equal to half the frequency of the input clock,
approximately 6 MHz, so images only alias for input frequencies
above approximately 3 MHz. This is far enough above the audio
band that bandwidth and aliasing are not a problem in real
applications.
The AD1994 implements a seventh-order, Σ-Δ modulator with
a 1-bit quantizer. Traditionally, higher-order designs such as
this are not suitable for driving a Class-D amplifier because of
stability problems at higher modulation factors. The modulator
design of the AD1994 is unusual in that it is stable to 90%
modulation. To allow the amplifier to drive even further, the
AD1994 dynamically reverts from seventh order to second
order above a fixed modulation threshold. The second-order
modulator is unconditionally stable, including during
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prolonged voltage clipping conditions, enabling stable operation
at full modulation. The dynamic-order reduction circuit uses
the high-order modulator, except during the crests of the highest
waveform peaks. During these peaks, the quantization noise
increases, but the SNR is still quite high. These modulator order
transitions are fast and smooth enough to avoid audible artifacts.
The modulator has a noise shaping effect, and SNR is increased
in the audio band by shifting the quantization noise upward in
frequency. For a nominal input clock frequency of 12.288 MHz,
the noise floor rises sharply above 20 kHz. The actual clock
frequency used in an application circuit can deviate from this
rate by as much as ±10%, and the corner frequency of the noise
scales proportionately. The frequency at which the quantization
noise dominates the output determines the amplifier’s practical
bandwidth.
The expected transition rate at the output of a typical seventh-
order, Σ-Δ modulator would be high enough to negate much of
the efficiency benefit of a switching amplifier. However, the
AD1994 incorporates a proprietary, dynamic, switching rate,
reduction scheme that lowers that average switching frequency
by approximately a factor of four. This results in slightly
increased output energy between 450 kHz and 500 kHz and
efficiency on par with other Class-D amplifiers. This low-Q
spectral boost is an artifact of the noise shaping and is in no
way related to the carrier frequency visible in the spectrum of
PWM Class-D amplifiers.
MUTE AND RESET
When power is applied and the RESET pin remains asserted,
the AD1994 is in its lowest power consumption mode. The
analog modulator is not running, and the power stage is tri-
stated. On deasserting the RESET pin, the modulator begins a
start-up sequence that includes initialization of the modulator,
the protection circuits, and other functions.
Once the start-up sequence is complete, the amplifier is in a
state in which the modulator is running, but the output stage is
not driven. When MUTE is deasserted, the output is started
using a soft-start sequence that avoids any audible pop or click
noise in the output signal.
The output power transistors do not switch while MUTE
remains asserted. Unlike the analog mute circuits found on
some amplifiers that can be limited in their attenuation by the
control logic or crosstalk, the mute attenuation on the AD1994
is greater than its dynamic range. The noise floor of the output
signal also drops while in MUTE because the output transistors
are not switching.
AD1994