Rainbow Electronics MAX9671 User Manual

Page 25

MAX9670/MAX9671

Low-Power Audio/Video Switch with Audio

Volume Control for Dual SCART Connectors

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Interfacing to an RF Modulator

If the set-top box modulates CVBS and mono audio
onto an RF carrier (for example, channel 3), a simple
application circuit can provide the needed signals (see
Figure 17). 10k

Ω resistor summer circuit between

TV_OUTR and TV_OUTL creates the mono audio sig-
nal. The resistor-divider to ground on TV_Y/CVBS_OUT
creates a video signal with normal amplitude. The
unique feature of the MAX9670/MAX9671 that facilitates
this application circuit is that the audio and video out-
put amplifiers of the MAX9670/MAX9671 can drive mul-
tiple loads if V

AUD

and V

VID

are both greater than

3.135V.

Floating-Chassis Discharge Protection

and ESD

Some set-top boxes have a floating chassis problem in
which the chassis is not connected to earth ground. As
a result, the chassis can charge up to 500V. When a
SCART cable is connected to the SCART connector,
the charged chassis can discharge through a signal
pin. The equivalent circuit is a 2200pF capacitor
charged to 311V connected through less than 0.1

Ω to a

signal pin. The MAX9670/MAX9671 are soldered on the
PCB when it experiences such a discharge. Therefore,
the current spike flows through both external and inter-
nal ESD protection devices and is absorbed by the
supply bypass capacitors, which have high capaci-
tance and low ESR.

To better protect the MAX9670/MAX9671 against
excess voltages during the cable discharge condition
or ESD events, add series resistors to all inputs and
outputs to the SCART connector if series resistors are
not already present in the application circuit. Also, add
external ESD protection diodes (for example, BAV99)
on all inputs and outputs to the SCART connector.

SCART Set-Top Box

with Analog HD Outputs

In set-top boxes with SCART connectors and cinch
connectors for high-definition YPbPr outputs, a triple-
video DAC usually outputs either standard-definition
RGB signals that are routed to the MAX9670/MAX9671
or high-definition YPbPr signals that are routed through
a high-definition filter amplifier like the MAX9653 (see
Figure 19). The set-top box devices have a limited num-
ber of video DACs, and hence, one bank of triple-video
DACs switches video format depending upon whether
standard-definition RGB or high-definition YPbPr sig-
nals are required.

When RGB signals are desired, the high-definition filter
amplifier should be turned off so that the RGB signals
do not appear on the YPbPr connectors. The
MAX9653/MAX9654 are well-suited for this application
because their video inputs are in high-impedance
mode when in shutdown.

Figure 17. Application Circuit to Connect CVBS and Mono Audio from TV SCART to RF Modulator

SCART

MAX9670/MAX9671

TV_OUTR

TV_OUTL

10k

Ω

10k

Ω

MONO AUDIO

TV_Y/CVBS_OUT

Ω OR GREATER

Ω

MODULATOR