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 (V_OUT) 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 V_OUT rises from 0 to 16 V, M1 remains on until TP_C goes low. The ESD-protection diode, connected internally from V_CC 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.

Reprints

Printer-Friendly

Email this Article

RSS

Submit PlanetEE del.icio.us Digg Reddit Newsvine StumbleUpon Netscape Yahoo MyWeb Live Bookmarks Fark  Submit

Font Size

What's This?

Front-End Supplies Take Digital Control Up A Notch Power-Entry Modules Now Offer Vertical Flange Mounting Bluetooth Modules Scrub For Medical Apps Laser Diode Performs At 1,400 nm To 2,000 nm Development Platform Unleashes GPU Power Handheld Spectrum Analyzers Bask In The Field

Engineers Rely On Internet For Product Info Rochester Electronics Establishes New Design and Technology Group Custom Sources Light Way To 22-nm IC Lithography In EDA, A Year Of Mergers, Failed And Otherwise Software Turns Scopes Into Vector RF Signal Analyzers Couple’s $15 Million Gift Advances Rice Engineering Education November 7, 2008 Startup Sets Sail For Speedier Spice Simulation

1) Ten Top Design Skills For Tough Times (221 views today) 2) Build A Smart Battery Charger Using A Single-Transistor Circuit (220 views today) 3) Easily Convert Decimal Numbers To Their Binary And BCD Formats (117 views today) 4) DC-AC inverter targets electroluminescent applications (77 views today) 5) Precision DC motor speed controller (75 views today) ALL TOP 20

POST YOUR COMMENTS HERE Name: Email:
Your Comments: Enter the text from the image below Please refresh the page if you have trouble reading this text.