Data Sheet

SSM3302

Rev. A | Page 17 of 20

Because the mono mode uses output sense circuitry attached to

the left channel outputs, run PCB traces directly from the speaker

to the left channel outputs and then extend the PCB traces to

the right channel outputs.

OUTPUT MODULATION DESCRIPTION

The SSM3302 uses three-level, Σ-Δ output modulation. Each

output can swing from PGND to PVDD and vice versa. Ideally,

when no input signal is present, the output differential voltage is

0 V because there is no need to generate a pulse. In a real-world

situation, however, there are always noise sources present.

Due to this constant presence of noise, a differential pulse is

occasionally generated in response to this stimulus. A small

amount of current flows into the inductive load when the

differential pulse is generated. However, most of the time, the

output differential voltage is 0 V. This feature ensures that the

current flowing through the inductive load is small.

When the user sends an input signal, an output pulse is generated

to follow the input voltage. The differential pulse density is

increased by raising the input signal level. Figure 39 depicts three-

level, Σ-Δ output modulation with and without input stimulus.

Figure 39. Three-Level, Σ-Δ Output Modulation With and

Without Input Stimulus

LAYOUT

As output power increases, care must be taken to lay out PCB traces

and wires properly among the amplifier, load, and power supply;

a poor layout increases voltage drops, consequently decreasing

efficiency. A good practice is to use short, wide PCB tracks to

decrease voltage drops and minimize inductance. For lowest DCR

and minimum inductance, ensure that track widths are at least

200 mil for every inch of length and use 1 oz. or 2 oz. copper. Use

large traces for the power supply inputs and amplifier outputs.

Proper grounding guidelines help to improve audio performance,

minimize crosstalk between channels, and prevent switching

noise from coupling into the audio signal.

To maintain high output swing and high peak output power,

ensure that the PCB traces that connect the output pins to the

load and supply pins are as wide as possible to maintain the

minimum trace resistances. It is also recommended that a large

ground plane be used for minimum impedances. In addition,

good PCB layout isolates critical analog paths from sources of

high interference. High frequency circuits (analog and digital)

should be separated from low frequency circuits.

Properly designed multilayer PCBs can reduce EMI emission

and increase immunity to the RF field by a factor of 10 or more

compared with double-sided boards. A multilayer board allows

a complete layer to be used for the ground plane, whereas the

ground plane side of a double-sided board is often disrupted by

signal crossover.

If the system has separate ground planes for small signal and

high power connections, there should be no overlap between

these planes. Stitch the power plane to the SSM3302 exposed pad

using multiple vias. Proper layout improves heat conduction

into the board, allowing operation at larger output power levels

without overtemperature issues.

INPUT CAPACITOR SELECTION

Input capacitors are required if the input signal is not biased within

the recommended input dc common-mode voltage range, if

high-pass filtering is needed, or if a single-ended source is used.

If high-pass filtering is needed at the input, the input capacitor

and the input resistor of the SSM3302 form a high-pass filter

with a corner frequency determined by the following equation:

The input capacitor can significantly affect the performance of

the circuit. Failure to use input capacitors degrades the output

offset of the amplifier.

BOOTSTRAP CAPACITORS

The output stage of the SSM3302 uses a high-side NMOS driver,

rather than PMOS driver. To generate the gate drive voltage for

the high-side NMOS driver, a bootstrap capacitor for each output

terminal acts as a floating power supply for the switching cycle.

Using 0.22 μF ceramic capacitors with a voltage rating of 35 V

or greater is recommended.

OUTPUT > 0V

+5V

0V

OUTR+/

OUTL+

+5V

0V

OUTR–/

OUTL–

+5V

0V

VOUT

OUTPUT < 0V

+5V

0V

OUTR+/

OUTL+

+5V

0V

OUTR–/

OUTL–

0V

–5V

VOUT

OUTPUT = 0V

OUTR+/

OUTL+

+5V

0V

+5V

0V

OUTR–/

OUTL–

+5V

–5V

0V

VOUT