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

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

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

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

______________________________________________________________________________________

Table 2. Modulation Scheme Selection In Analog Mode

ADDR2

ADDR1

SDA/VOL

SCLK

FUNCTION

Analog Volume Control

Filterless Modulation

Analog Volume Control

Classic PWM (50% Duty Cycle)

EFFICIENCY vs. OUTPUT POWER

MAX9768 fig02

OUTPUT POWER (W)

EFFICIENCY (%)

100

MAX9768

CLASS AB

= 12V

= 1kHz

= 8

Figure 2. MAX9768 Efficiency vs. Class AB Efficiency