In a typical automotive video application,
the video digital-to-analog converter
(DAC)—from a rear camera or
DVD player, for example—is followed
by a low-pass reconstruction filter and
an amplifier that transmits a video
signal to the LCD. This amplifier, and
all such similar automotive circuits, must
be protected from direct connection
to the car’s battery voltage. Since these
voltages range from 12 to 16 V, the minimum
protection required is 16 V.
A simple circuit employing an inverter
and a FET can provide this protection against shorts to the battery
(Fig. 1). The MAX9502 video filter amplifier used in this
example eliminates the requirement for the passive low-pass LC
reconstruction filter. In most applications, the video DAC has a
ground-referenced current output, and the value of Rx (75 to 300
Ω, depending on characteristics of the video DAC) sets the video
amplitude at 1 V p-p.
When the circuit’s output voltage (VOUT) exceeds 7 V, the output
of the inverter (74AHCT1G04GW) goes low and turns off
transistor M1, thus protecting the MAX9502 by isolating its output.
When VOUT drops below 7 V, the inverter output goes high,
allowing M1 to turn on and re-establish the video channel.
Figure 2 shows the circuit’s transient response. As VOUT
rises from 0 to 16 V, M1 remains on until TP_C goes low. The
ESD-protection diode, connected internally from VCC to the
MAX9502 output, turns on during this period and clamps the
output (TP_D) to about 3.3 V. Since this ON period is very
short, the MAX9502 isn’t damaged and the power supply connected
to its VCC pin is unaffected.
The table shows the overall video performance with a 75-Ω termination
resistor (Rx), as measured with a Tektronix VM700 video test
set. Performance is good, and the transistor has a negligible effect on
the video quality.
