MAX9598 Maxim Integrated Products, MAX9598 Datasheet - Page 13

• Manufacturer Part Number: MAX9598
• Description: Low-Power Audio/Video Switch
• Manufacturer: Maxim Integrated Products

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The TV audio channel incorporates a zero-crossing
detect (ZCD) circuit that minimizes click noise due to
abrupt signal level changes that occur when switching
between audio signals at an arbitrary moment.
To implement the zero-crossing function when switch-
ing audio signals, set the ZCD bit high (Audio Control
Register 00h, bit 6). Then set the mute bit high (Audio
Control Register, 00h, bit 0). Next, wait for a sufficient
period of time for the audio signal to cross zero. This
period is a function of the audio signal path’s low-fre-
quency 3dB corner (f
time period to wait for a zero-crossing detect is 1/20Hz
or 50ms.
After the wait period, select a new audio source for the
TV audio channel by writing to bits 1 and 0 of the TV
Audio Control Register (01h). Finally, clear mute (Audio
Control Register, 00h, bit 0) but leave ZCD (Audio
Control Register, 00h, bit 6) high. The MAX9598 switch-
es the signal out of mute at the next zero crossing. See
Tables 10 and 11.
The MAX9598 audio output amplifiers feature Maxim’s
patented DirectDrive architecture, thereby eliminating
the need for output-coupling capacitors required by
conventional single-supply audio line drivers. An inter-
nal charge pump inverts the positive supply (VAUD),
creating a negative supply (CPVSS). The audio output
amplifiers operate from these bipolar supplies with their
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.
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 capacitors are needed to block this DC bias.
Clicks and pops are created when the coupling capaci-
tors are charged during power-up and discharged dur-
ing power-down.
The MAX9598 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 on the video outputs generated by
turn-on and turn-off transients.
______________________________________________________________________________________
L3dB
). Thus, if f
Low-Power Audio/Video Switch for
Clickless Switching
L3dB
Audio Outputs
= 20Hz, the
Dual SCART Connectors
The SCART standard specifies 2V
for audio signals. As the audio circuits process
0.5V
MAX9598, the gain-of-4 output amplifiers restore the
audio signals to a full scale of 2V
To select which audio input source is routed to the TV
SCART connector, write to bits 1 and 0 of the TV Audio
Control Register (01h). To select which audio input
source is routed to the VCR SCART connector, write to
bits 3 and 2 of the TV Audio Control Register (01h). The
power-on default is for the TV and VCR audio outputs to
be muted (the inputs of the output amplifiers are con-
nected to audio ground). See Tables 8 and 11.
The video circuit routes different video formats between
the set-top box decoder, the TV SCART connector, and
the VCR SCART connector. It also routes slow-switch
and fast-switch control information. See Figure 3.
Figure 2. Conventional Driver Output Waveform vs. MAX9598
Output Waveform
RMS
full-scale audio signals internal to the
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
V
OUT
V
OUT
RMS
RMS
.
Video Section
V
GND
GND
as the full-scale
V
+V
-V
DD
DD
DD
DD
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