MAX9879 Maxim Integrated Products, MAX9879 Datasheet - Page 19

• Manufacturer Part Number: MAX9879
• Description: Stereo Class D Audio Subsystem
• Manufacturer: Maxim Integrated Products

www.datasheet4u.com
The MAX9879 features three Volume Control registers
(see Table 4), allowing independent volume control of
speaker and headphone amplifier outputs. There is one
Speaker Volume Control register that evenly controls both
speaker outputs. Two Headphone Volume Control regis-
ters provide independent control of each headphone out-
put. Each volume control register provides 31 attenuation
steps providing 0dB to -75dB (typ) of total attenuation
and a mute function.
The MAX9879 integrates a filterless Class D amplifier
that offers much higher efficiency than Class AB with-
out the typical disadvantages.
The high efficiency of a Class D amplifier is due to the
switching 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
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 power. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9879 still exhibits 88% efficiency
under the same conditions (Figure 3).
In traditional Class D amplifiers, the high dV/dt of the
rising and falling edge transitions results in increased
EMI emissions, which requires the use of external LC
filters or shielding to meet EN55022 electromagnetic-
Figure 3. MAX9879 Efficiency vs. Class AB Efficiency
100
90
80
70
60
50
40
30
20
10
0
0
Ultra-Low EMI Filterless Output Stage
vs. IDEAL CLASS EFFICIENCY
______________________________________________________________________________________
MAX9877 EFFICIENCY
0.25
Class D Speaker Amplifier
MAX9879
V
V
Volume Control and Mute
OUTPUT POWER (W)
DD
SUPPLY
= PVDD_ = 3.7V (MAX9879)
with DirectDrive Headphone Amplifier
0.50
= 3.7V (IDEAL CLASS AB)
IDEAL CLASS AB
2
R loss of the MOSFET
0.75
Stereo Class D Audio Subsystem
1.00
interference (EMI) regulation standards. Limiting the
dV/dt normally results in decreased efficiency. Maxim’s
active emissions limiting circuitry actively limits the
dV/dt of the rising and falling edge transitions, provid-
ing reduced EMI emissions, while maintaining up to
88% efficiency.
In addition to active emission limiting, the MAX9879 fea-
tures a patented spread-spectrum modulation mode that
flattens the wideband spectral components. 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 ).
With spread-spectrum modulation, the switching fre-
quency varies randomly by ±40kHz around the center
frequency (700kHz). The effect is to reduce the peak
energy at harmonics of the switching frequency. Above
10MHz, the wideband spectrum looks like white noise for
EMI purposes (see Figure 4).
Most applications do not enter current limit unless the
output is short circuited or connected incorrectly.
When the output current of the speaker amplifier
exceeds the current limit (1.5A, typ) the MAX9879 dis-
ables the outputs for approximately 250µs. At the end of
250µs, the outputs are re-enabled, and if the fault condi-
tion still exists, the MAX9879 continues to disable and re-
enable the outputs until the fault condition is removed.
The integrated DPST analog audio switch allows the
MAX9879’s Class D amplifier to be bypassed. In bypass
mode, the Class D amplifier is automatically disabled
allowing an external amplifier to drive the speaker con-
nected between OUTL+ and OUTL- through RXIN+ and
RXIN- (see the Typical Application Circuit ).
The bypass switch is enabled at startup. The switch can
be opened or closed even when the MAX9879 is in soft-
ware shutdown (see the I
Unlike discrete solutions, the switch design reduces
coupling of Class D switching noise to the RXIN_
inputs. This eliminates the need for a costly T-switch.
The bypass switch is typically used with two 10Ω resis-
tors connected to each input. These resistors, in combi-
nation with the switch on-resistance and an 8Ω load,
approximate the 32Ω load expected by the external
amplifier. Although not required, using the resistors
optimizes THD+N.
Drive RXIN+ and RXIN- with a low-impedance source
to minimize noise on the pins. In applications that do
not require the bypass mode, leave RXIN+ and RXIN-
unconnected.
2
C Register Description section).
Speaker Current Limit
Bypass Mode
19