MAX9770ETI+ Maxim Integrated Products, MAX9770ETI+ Datasheet - Page 12

• Manufacturer Part Number: MAX9770ETI+
• Description: IC AMP AUDIO 1.2W MONO D 28TQFN
• Manufacturer: Maxim Integrated Products
• Type: Class Dr

Specifications of MAX9770ETI+

• Output Type: 1-Channel (Mono) with Stereo Headphones
• Max Output Power X Channels @ Load: 1.2W x 1 @ 8 Ohm; 80mW x 2 @ 16 Ohm
• Voltage - Supply: 2.5 V ~ 5.5 V
• Features: Depop, Input Multiplexer, Mute, Short-Circuit and Thermal Protection, Shutdown
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The switching frequency of the charge pump is 1/2 the
switching frequency of the Class D amplifier, regard-
less of the operating mode. When SYNC is driven exter-
nally, the charge pump switches at 1/2 f
SYNC = V
spectrum pattern.
The MAX9770 features two FFM modes. The FFM
modes are selected by setting SYNC = GND for a
1.1MHz switching frequency, and SYNC = unconnect-
ed for a 1.45MHz switching frequency. In FFM mode,
the frequency spectrum of the Class D output consists
of the fundamental switching frequency and its associ-
ated harmonics (see the Wideband Output Spectrum
(Speaker Mode) graph in the Typical Operating
Characteristics ). The MAX9770 allows the switching fre-
quency to be changed by +32% should the frequency
of one or more harmonics fall in a sensitive band. This
can be done during operation and does not affect
audio reproduction.
The MAX9770 features a unique spread-spectrum
mode that flattens the wideband spectral components,
improving EMI emissions 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 efficiency (see the
Typical Operating Characteristics ). Select SSM mode
by setting SYNC = V
frequency varies randomly by ±120kHz around the cen-
ter frequency (1.22MHz). The modulation scheme
remains the same, but the period of the sawtooth wave-
form 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 MHz, the wideband spectrum looks like white
noise for EMI purposes (Figure 3).
The SYNC input allows the MAX9770 to be synchro-
nized to a system clock (allowing a fully synchronous
system), or allocating the spectral components of the
switching harmonics to insensitive frequency bands.
1.2W, Low-EMI, Filterless, Mono Class D Amplifier
with Stereo DirectDrive Headphone Amplifiers
12
______________________________________________________________________________________
DD
Spread-Spectrum Modulation (SSM) Mode
Fixed-Frequency Modulation (FFM) Mode
, the charge pump switches with a spread-
DD
. In SSM mode, the switching
Operating Modes
External Clock Mode
SYNC
. When
Applying an external clock of 800kHz to 2MHz to SYNC
synchronizes the switching frequency of both the Class
D and charge pump. The period of the SYNC clock can
be randomized, enabling the MAX9770 to be synchro-
nized to another spread-spectrum Class D amplifier
operating in SSM mode.
The MAX9770 uses Maxim’s unique modulation scheme
that eliminates the LC filter required by traditional Class D
amplifiers, improving efficiency, reducing component
count, conserving board space and system cost.
Conventional Class D amplifiers output a 50% duty cycle
square wave when no signal is present. With no filter, the
square wave appears across the load as a DC voltage,
resulting in finite load current, increasing power con-
sumption. When no signal is present at the device input,
the outputs switch as shown in Figure 4. Because the
MAX9770 drives the speaker differentially, the two out-
puts cancel each other, resulting in no net idle mode volt-
age across the speaker, minimizing power consumption.
The efficiency of a Class D amplifier is attributed to the
region of operation of the output stage transistors. In a
Class D amplifier, the output transistors act as current-
steering switches and consume negligible additional
power. Any power loss associated with the Class D out-
put stage is mostly due to the I*R loss of the MOSFET
on-resistance, and quiescent current overhead.
The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9770 still exhibits > 80% efficiencies
under the same conditions (Figure 5).
Traditional single-supply headphone drivers have their
outputs biased about a nominal DC voltage (typically
half the supply) for maximum dynamic range. Large
coupling capacitors are needed to block this DC bias
from the headphone. Without these capacitors, a signif-
icant amount of DC current flows to the headphone,
resulting in unnecessary power dissipation and possi-
ble damage to both headphone and headphone driver.
Filterless Modulation/Common-Mode Idle
DirectDrive
Efficiency