Power-supply bypassing, Layout and grounding, Using a digital supply – Rainbow Electronics MAX9671 User Manual

Page 27: Typical operating characteristics, Is for the scart switching function. for v

MAX9670/MAX9671

Low-Power Audio/Video Switch with Audio

Volume Control for Dual SCART Connectors

______________________________________________________________________________________

Similarly, when YPbPr signals are desired, ENC_R/C_IN,
ENC_G_IN, and ENC_B_IN of the MAX9670/MAX9671
should be set to high-impedance mode by setting bit 4 in
register 08h to high if those video inputs are AC-coupled.
The high-impedance mode has higher priority whether
ENC_R/C_IN is in sync-tip clamp or bias circuit mode
(set by bit 3 in register 08h). If ENC_R/C_IN, ENC_G_IN,
and ENC_B_IN are DC-coupled, the inputs should be left
in sync-tip clamp mode. The RGB outputs of the
MAX9670 should be muted or shut down.

In either case, the inactive device should not distort the
video signals generated by the DACs.

Power-Supply Bypassing

The MAX9670/MAX9671 feature single 3.3V and 12V
supply operation and require no negative supply. The
12V supply V

is for the SCART switching function. For

, place a 0.1µF bypass capacitor as close as possi-

ble. Connect all V

AUD

pins together to 3.3V and bypass

with a 10µF electrolytic capacitor in parallel with a
0.1µF ceramic capacitor to audio ground. Bypass each
V

VID

to video ground with a 0.1µF ceramic capacitor.

Using a Digital Supply

The MAX9670/MAX9671 are designed to operate from
noisy digital supplies. The high PSRR (49dB at 100kHz)
allows the MAX9670/MAX9671 to reject the noise from
the digital power supplies (see the

Typical Operating

Characteristics

). If the digital power supply is very noisy

and stripes appear on the television screen, increase
the supply bypass capacitance. An additional, smaller
capacitor in parallel with the main bypass capacitor
can reduce digital supply noise because the smaller
capacitor has lower equivalent series resistance (ESR)
and equivalent series inductance (ESL).

Layout and Grounding

For optimal performance, use controlled-impedance
traces for video signal paths and place input termina-
tion resistors and output back-termination resistors
close to the MAX9670/MAX9671. Avoid routing video
traces parallel to high-speed data lines.

The MAX9670/MAX9671 provide separate ground con-
nections for video and audio supplies. For best perfor-
mance, use separate ground planes for each of the
ground returns and connect all ground planes together
at a single point. See the MAX9670/MAX9671 evalua-
tion kit for a proven circuit board layout example.

If the MAX9670/MAX9671 are mounted using flow sol-
dering or wave soldering, the ground via(s) for the EP
pad should have a finished hole size of at least 14mils
to insure adequate wicking of soldering onto the
exposed pad. If the MAX9670/MAX9671 are mounted
using solder mask technique, the via requirement does
not apply. In either case, a good connection between
the exposed pad and ground is required to minimize
noise from coupling onto the outputs.

Figure 19. Triple DAC is connected to both a MAX9670 and a MAX9653/MAX9654 high-definition video-filter amplifier. (A) The
MAX9670/MAX9671 are transmitting standard-definition RGB signals while the MAX9653/MAX9654 are in shutdown mode. (B) The
MAX9670/MAX9671 are not transmitting RGB signals, but the MAX9653/MAX9654 are transmitting high-definition YPbPr signals.

DAC

SET-TOP BOX

CHIP

ENC_R/C_IN

ENC_G_IN

ENC_B_IN

Ω

TV_R/C_OUT

TV_G_OUT

TV_B_OUT

MAX9653
MAX9654

SCART
CONNECTOR

OFF

YPbPr OUTPUTS

(A)

Ω

YOUT

PBOUT

PROUT

YIN

PBIN

PRIN

SHDN

DAC

3.3V

DAC