Rainbow Electronics MAX9768 User Manual
Page 14
Filterless Modulation/PWM Modulation
The MAX9768 features two output modulation
schemes: filterless modulation or classic PWM, selec-
table through SCLK when the device is in analog mode
(ADDR2 and ADDR1 = GND, Table 2) or through the
I
2
C interface (Table 7). Maxim’s unique, filterless modu-
lation scheme eliminates the LC filter required by tradi-
tional Class D amplifiers, reducing component count,
conserving board space and system cost. Although the
MAX9768 meets FCC and other EMI limits with a low-
cost ferrite bead filter, many applications still may want
to use a full LC-filtered output. If using a full LC filter,
the performance is best with the MAX9768 configured
for classic PWM output.
Switching between schemes while in normal operating
mode with the I
2
C interface, the output is not click-and-
pop protected. To have click-and-pop protection when
switching between output schemes, the device must
enter shutdown mode and be configured to the new out-
put scheme before the startup sequence is terminated.
The startup time for the MAX9768 is typically 220ms.
The startup time for the MAX9768B is typically 15ms.
Efficiency
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 output
stage is mostly due to the I
2
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 power. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9768 still exhibits > 80% efficiencies
under the same conditions (Figure 2).
Soft Current Limit
When the output current exceeds the soft current limit,
2A (typ), the MAX9768 enters a cycle-by-cycle current-
limit mode. In soft current-limit mode, the output is
clipped at 2A. When the output decreases so the out-
put current falls below 2A, normal operation resumes.
The effect of soft current limiting is a slight increase in
distortion. Most applications will not enter soft current-
limit mode unless the speaker or filter creates imped-
ance nulls below 8Ω.
MAX9768
10W Mono Class D Speaker
Amplifier with Volume Control
14
______________________________________________________________________________________
Table 2. Modulation Scheme Selection In Analog Mode
ADDR2
ADDR1
SDA/VOL
SCLK
FUNCTION
0
0
Analog Volume Control
0
Filterless Modulation
0
0
Analog Volume Control
1
Classic PWM (50% Duty Cycle)
EFFICIENCY vs. OUTPUT POWER
MAX9768 fig02
OUTPUT POWER (W)
EFFICIENCY (%)
8
6
4
2
10
20
30
40
50
60
70
80
90
100
0
0
10
MAX9768
CLASS AB
PV
DD
= 12V
f
IN
= 1kHz
R
L
= 8
Ω
Figure 2. MAX9768 Efficiency vs. Class AB Efficiency