AD1992

Rev. 0 | Page 13 of 20

THEORY OF OPERATION

OVERVIEW

The AD1992 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 AD1992 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 AD1992 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 AD1992 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 AD1992

modulators come from the outputs of the power devices and

before the passive low-pass filters (see Figure 35). 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 AD1992 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.

MUTE AND RESET

When power is applied and the RESET pin remains asserted,

the AD1992 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 AD1992

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 AD1992 to

ensure correct operation and to avoid possible latch-up issues.

The AD1992 should be powered up with RESET and MUTE

held low until all the power supplies have stabilized. Once the

supplies have stabilized, bring the AD1992 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

the reference voltage for the single-ended inputs, or the

amplifier starts with a slight dc offset.

GAIN STRUCTURE

Analog Input Levels

The AD1992 has single-ended inputs for the left and right

channels. The analog input section uses an internal amplifier to

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.