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AD8385 Datasheet(PDF) 8 Page - Analog Devices |
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AD8385 Datasheet(HTML) 8 Page - Analog Devices |
8 / 24 page AD8385 Rev. 0 | Page 8 of 24 OVERLOAD PROTECTION The AD8385 employs a 2-stage overload protection circuit with an enable/disable function that is programmable through the 3-wire serial interface. It consists of an output current limiter and a thermal shut down. When enabled, the maximum current at any one output of the AD8385 is, on average, internally limited to 100 mA. In the event of a momentary short circuit between a video output and a power supply rail (VCC or AGND), the output current limit is sufficiently low to provide temporary protection. The thermal shutdown debiases the output amplifier when the junction temperature reaches the internally set trip point. In the event of an extended short circuit between a video output and a power supply rail, the output amplifier current continues to switch between 0 mA and 100 mA typ, with a period deter- mined by the thermal time constant and the hysteresis of the thermal trip point. Thermal shutdown provides long-term protection by limiting the average junction temperature to a safe level. When disabled, no overload protection is present. EXPOSED PADDLE To ensure optimal thermal performance, the exposed paddle must be electrically connected to an external plane such as AVCC or GND, as described in the Applications Circuit section. MAXIMUM POWER DISSIPATION The junction temperature limits the maximum power that can be safely dissipated by the AD8385. The maximum safe junction temperature for plastic encapsulated devices, determined by the glass transition temperature of the plastic, is approximately 150°C. Exceeding this limit can cause a temporary shift in the parametric performance due to a change in the stresses exerted on the die by the package. Exceeding a junction temperature of 175°C for an extended period can result in device failure. OPERATING TEMPERATURE RANGE The maximum operating junction temperature is 150°C. The junction temperature trip point of the overload protection is 165°C. Production test guarantees a minimum junction temperature trip point of 125°C. Consequently, the maximum guaranteed operating junction temperature is 125°C with the overload protection enabled, and 150°C with the overload protection disabled. To ensure operation within the specified operating temperature range, the maximum power dissipation must be limited: ( ) ) 9 . 0 ( 3 lfm in Airflow θ T T P JA A JMAX DMAX × − − ≈ 1.5 2.5 2.0 3.0 85 80 70 75 *65 90 95 100 105 110 105 95 100 **90 115 120 125 130 AMBIENT TEMPERATURE (°C) STILL AIR 200 lfm 500 lfm 100MHz 60Hz XGA QUIESCENT *OVERLOAD PROTECTION ENABLED **OVERLOAD PROTECTION DISABLED AD8385 on a 4-layer JEDEC PCB with a thermally optimized landing pattern, as described in the Applications Circuit section. Figure 2. Maximum Power Dissipation vs. Temperature Note that the quiescent power dissipation of the AD8385 is 1.84 W when operating under the conditions specified in this data sheet. When driving a 12-channel XGA panel with an input capacitance of 150 pF, the AD8385 dissipates a total of 2.3 W when displaying 1 pixel wide alternating white and black vertical lines generated by a standard 60 Hz XGA input video. When the frequency of the pixel clock is raised to 100 MHz, the total power dissipation increases to 2.54 W. These specific power dissipations are shown in Figure 2 for reference. |
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