MAX9705
2.3W, Ultra-Low-EMI, Filterless,
Class D Audio Amplifier
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Detailed Description
The MAX9705 ultra-low-EMI, filterless, Class D audio
power amplifier features several improvements to switch-
mode amplifier technology. The MAX9705 features output
driver active emissions limiting circuitry to reduce EMI.
Zero dead time technology maintains state-of-the-art effi-
ciency and THD+N performance by allowing the output
FETs to switch simultaneously without cross-conduction.
A unique filterless modulation scheme, synchronizable
switching frequency, and spread-spectrum mode create
a compact, flexible, low-noise, efficient audio power
amplifier while occupying minimal board space. The dif-
ferential input architecture reduces common-mode noise
pickup with or without the use of input-coupling capaci-
tors. The MAX9705 can also be configured as a single-
ended input amplifier without performance degradation.
Thermal-overload and short-circuit protection prevent the
MAX9705 from being damaged during a fault condition.
The amplifier is disabled if the die temperature reaches
+125°C. The die must cool by 10°C before normal opera-
tion can continue. The output of the MAX9705 shuts down
if the output current reaches approximately 2A. Each out-
put FET has its own short-circuit protection. This protec-
tion scheme allows the amplifier to survive shorts to either
supply rail. After a thermal overload or short circuit, the
device remains disabled for a minimum of 50µs before
attempting to return to normal operation. The amplifier will
shut down immediately and wait another 50µs before turn-
ing on if the fault condition is still present. This operation
will cause the output to pulse during a persistent fault.
Comparators monitor the MAX9705 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
ON(MIN)
at the output of
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
ON(MIN)
. This
causes the net voltage across the speaker (V
OUT+
-
V
OUT-
) to change.
Operating Modes
Fixed-Frequency Modulation (FFM) Mode
The FFM mode is selected by setting SYNC = GND for a
1.1MHz switching frequency. In FFM mode, the frequen-
cy spectrum of the Class D output consists of the funda-
mental switching frequency and its associated
harmonics (see the Wideband Output Spectrum Fixed-
Frequency Mode graph in the
Typical Operating
Characteristics
).
Pin Description
for normal operation.
Frequency Select and External Clock Input.
= 1100kHz.
= external clock frequency.
Exposed Pad. Internally connected to ground. Connect to a large ground plane to
maximize thermal performance. Not intended as an electrical connection point
(TDFN only).
