ad1990xcpzrl Analog Devices, Inc., ad1990xcpzrl Datasheet - Page 11

• Manufacturer Part Number: ad1990xcpzrl
• Description: Audio Switching Amplifier
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
OVERVIEW
The AD1990 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 AD1990 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 AD1990 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 AD1990 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 AD1990
modulators come from the outputs of the power devices and
before the passive low-pass filters (see Figure 23). 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 AD1990 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 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.
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MUTE AND RESET
When power is applied and the RESET pin remains asserted,
the AD1990 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 AD1990
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.
Power-Up Sequencing
Careful power-up is necessary when using the AD1990 to
ensure correct operation and to avoid possible latch-up issues.
The AD1990 should be powered up with RESET and MUTE
held low until all the power supplies have stabilized. Once the
supplies have stabilized, bring the AD1990 out of RESET by
bringing RESET high.
Begin the soft unmute sequence by bringing MUTE high at
least 1 sec after the RESET rising edge. The amplifier produces
audio using a shorter start-up sequence (as shown in Table 7),
but the amplifier can produce an audible pop or click noise as
the output starts switching. This is because the ac coupling
capacitors at the analog input have a long time constant. If
MUTE is deasserted substantially less than 1 sec after deasserting
RESET , then these capacitors may not have charged to a steady
state. They need ample time to settle at a bias voltage of V
the reference voltage for the single-ended inputs, or the
amplifier starts with a slight dc offset.
GAIN STRUCTURE
Analog Input Levels
The AD1990 has single-ended inputs for the left and right
channels. The analog input section uses an internal amplifier to
bias the input signal to the reference level, V
equal to AV
prevents this bias voltage from affecting the signal source. In
combination with the nominal 20 kΩ input impedance, the value
of this capacitor should be large enough to produce a flat
frequency response at the lowest input frequency of interest.
DD
/2. A dc-blocking capacitor, as shown in Figure 22,
REF
, which is nominally
AD1990
REF
,