Rainbow Electronics MAX9597 - Manual (Page 13)

MAX9597
Low-Power Audio/Video Interface
for Single SCART Connectors
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The full-scale output of the independent operational
amplifiers is 0.5V
RMS . The closed-loop gain of the oper-
ational amplifier circuit should be designed such that
the resulting full-scale output is 0.5V
RMS . The fixed,
gain-of-4 amplifiers that follow the independent opera-
tional amplifiers amplify the 0.5V
RMS
to 2V
RMS
, which
complies with the SCART standard.
An integrated charge pump inverts the +3.3V supply
(V
AUD ) to create a -3.3V supply (CPVSS), enabling the
audio circuit to operate from bipolar supplies. The
audio signal from the beginning to the end of the signal
path is always biased at ground.
Clickless Muting and Unmuting
The TV audio channel incorporates a zero-crossing
detect (ZCD) circuit that minimizes click noise due to
abrupt signal level changes that occur when entering
or coming out of a mute condition at an arbitrary
moment.
To implement the zero-crossing function when switch-
ing audio signals, set ZCD (register 00h, bit 6) high.
The MAX9597 switches the signal in or out of mute at
the next zero crossing after the mute or unmute request
occurs. See Table 8.
Audio Outputs
The MAX9597 audio output amplifiers feature Maxim’s
patented
† DirectDrive architecture, eliminating the need
for output-coupling capacitors required by conventional
single-supply audio line drivers. Conventional single-
supply audio line drivers have their outputs biased
about a nominal DC voltage (typically half the supply)
for maximum dynamic range. Large coupling capaci-
tors are needed to block this DC bias. Clicks and pops
are created when the coupling capacitors are charged
during power-up and discharged during power-down.
An internal charge pump inverts the positive supply
(V
AUD ), creating a negative supply (CPVSS). The audio
output amplifiers operate from the bipolar supplies with
the outputs biased about audio ground (Figure 2). The
benefit of this audio ground bias is that the amplifier
outputs do not have a DC component. The DC-blocking
capacitors required with conventional audio line drivers
are unnecessary, conserving board space, reducing
system cost, and improving frequency response.
The MAX9597 features a low-noise charge pump that
requires only two small ceramic capacitors. The
580kHz switching frequency is well beyond the audio
range and does not interfere with audio signals. The
switch drivers feature a controlled switching speed that
minimizes noise generated by turn-on and turn-off tran-
sients. The di/dt noise caused by the parasitic bond
wire and trace inductance is minimized by limiting the
switching speed of the charge pump.
The SCART standard specifies 2V RMS as the full-scale
for audio signals. As the audio circuits process
0.5V
RMS full-scale audio signals internal to the
MAX9597, the gain-of-4 output amplifiers restore the
audio signals to a full scale of 2V
RMS .
Video Section
The video circuit routes different video formats between
the set-top box decoder and the TV SCART connector.
It also routes slow-switch and fast-switch control infor-
mation as shown in Figure 3.
Video Inputs
Whether the incoming video input signal is AC-coupled
or DC-coupled into the MAX9597 depends upon the ori-
gin, format, and voltage range of the video signal. Table
1 below shows the recommended connections. Always
AC-couple an external video signal through a 0.1μF
capacitor because its voltage range is not well defined
(see the
Typical Application Circuit ). For example, the
+V DD
-V DD
GND
V OUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
V DD /2
V DD
GND
V OUT
Figure 2. Conventional Driver Output Waveform vs. MAX9597
Output Waveform
† U.S. Patent #7,061,327