max9746 Maxim Integrated Products, Inc., max9746 Datasheet - Page 8

• Manufacturer Part Number: max9746
• Description: Max9746 1.2w, Low-emi, Filterless, Class D Audio Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

The MAX9746 filterless, Class D audio power amplifier
features several improvements to switch-mode amplifier
technology. The MAX9746 offers Class AB performance
with Class D efficiency, while occupying minimal board
space. A unique filterless modulation scheme, synchro-
nizable switching frequency, and SSM mode create a
compact, flexible, low-noise, efficient audio power
amplifier. The differential input architecture reduces
common-mode noise pickup, and can be used without
input-coupling capacitors. The device can also be con-
figured as a single-ended input amplifier.
Comparators monitor the MAX9746 inputs and com-
pare the complementary input voltages to the sawtooth
waveform. The comparators trip when the input magni-
tude of the sawtooth exceeds their corresponding input
voltage. Both comparators reset at a fixed time after the
rising edge of the second comparator trip point, gener-
ating a minimum-width pulse t
the second comparator (Figure 1). As the input voltage
increases or decreases, the duration of the pulse at one
output increases (the first comparator to trip) while the
other output pulse duration remains at t
causes the net voltage across the speaker (V
V
The MAX9746 features two FFM modes. The FFM modes
are selected by setting SYNC = GND for a 1.1MHz
1.2W, Low-EMI, Filterless,
Class D Audio Amplifier
8
OUT-
_______________________________________________________________________________________
BUMP
) to change.
A1
B1
C1
C2
B2
A3
B3
A4
C4
B4
Fixed-Frequency Modulation (FFM) Mode
NAME
SHDN
SYNC
PGND
OUT+
OUT-
PV
GND
V
IN+
IN-
DD
DD
Detailed Description
Analog Power Supply
Noninverting Audio Input
Inverting Audio Input
Analog Ground
Active-Low Shutdown Input. Connect to V
Frequency Select and External Clock Input.
SYNC = GND: Fixed-frequency mode with f
SYNC = Float: Fixed-frequency mode with f
SYNC = V
SYNC = Clocked: Fixed-frequency mode with f
Power Ground
Amplifier-Output Positive Phase
Amplifier-Output Negative Phase
H-Bridge Power Supply
ON(MIN)
Operating Modes
DD
at the output of
: Spread-spectrum mode with f
ON(MIN)
OUT+
. This
-
switching frequency, and SYNC = FLOAT for a 1.45MHz
switching frequency. In FFM mode, the frequency spec-
trum of the Class D output consists of the fundamental
switching frequency and its associated harmonics (see
the Wideband Output Spectrum graph in the Typical
Operating Characteristics). The MAX9746 allows the
switching frequency to be changed by +32%, should the
frequency of one or more of the harmonics fall in a sensi-
tive band. This can be done at any time and does not
affect audio reproduction.
The MAX9746 features a unique, patented spread-spec-
trum mode that flattens the wideband spectral compo-
nents, improving EMI emissions that may be radiated by
the speaker and cables by 5dB. Proprietary techniques
ensure that the cycle-to-cycle variation of the switching
period does not degrade audio reproduction or efficien-
cy (see the Typical Operating Characteristics). Select
SSM mode by setting SYNC = V
switching frequency varies randomly by ±120kHz
around the center frequency (1.22MHz). The modulation
scheme remains the same, but the period of the saw-
tooth waveform changes from cycle to cycle (Figure 2).
Instead of a large amount of spectral energy present at
multiples of the switching frequency, the energy is now
spread over a bandwidth that increases with frequency.
Above a few megahertz, the wideband spectrum looks
like white noise for EMI purposes.
DD
S
S
S
FUNCTION
for normal operation.
= 1100kHz.
= 1450kHz.
= 1220kHz ±120kHz.
S
= external clock frequency.
Spread-Spectrum Modulation (SSM) Mode
Pin Description
DD
. In SSM mode, the