Simple protection from overvoltage automotive video driver circuit protection

Simple protection from overvoltage automotive video driver circuit protection

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Abstract: This circuit prevents accidental contact with the car's video amplifier to the car's battery voltage (maximum 16V), by inducing a voltage fault and quickly opening the connection between the amplifier and the battery.

In a typical automotive video application, the video DAC (from the rear camera or DVD player, for example) is followed by a low-pass reconstruction filter and amplifier that transmits video signals to the LCD display. The amplifier and all similar automotive circuits must be protected from the battery voltage directly connected to the car. Since the car battery voltage ranges from 12V to 16V, the minimum protection is 16V.

Figure 1. Simple circuit-protected video filter amplifier for the output to the battery (used by the MAX9502). The MAX9502 uses the following video DAC component cost and board space required to save the passive low-pass reconstruction filter that eliminates the Legislation. In most applications, the video DAC's ground reference current output, and the RX value (75Ω to 300Ω, depending on the characteristics of the video DAC) are set at 1VP-P video amplitude.

figure 1. The circuit is powered by an overvoltage auto-protected video driver at VOUT (used in the MAX9502) by turning off the transistor supply M1.

When the output voltage (VOUT) of the circuit exceeds 7V, the inverter output goes low, and the money supply M1 turns off the transistor, thus protecting its output from the MAX9502. When the return of VOUT is lower than 7V, the inverter outputs a high level, causing the money supply M1 to open and re-establish the video channel.

Figure 2 shows the transient response of the circuit of Figure 1. Since the VOUT voltage rises from 0V to 16V, the transistor's money supply M1 keeps TP_C low. The ESD protection diode is connected from the VCC internal MAX9502 to the output, which is turned on during this period, between the clip output (TP_D) and 3.3V. This is because for a short time, the MAX9502 is not damaged and the power supply is connected to its VCC pin unaffected.

figure 2. Transient response waveform Figure 1 circuit.

Table 1 shows an appropriate termination resistor of 75Ω (Rx), with Tektronix VM700 video setup measurements taking overall video performance. The performance is good, and the effect of a video quality with the transistor supply (M1) is negligible.

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