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LT6205 Datasheet(PDF) 10 Page - Linear Technology |
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LT6205 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LT6205/LT6206/LT6207 10 620567f APPLICATIO S I FOR ATIO Amplifier Characteristics Figure 1 shows a simplified schematic of the LT6205/ LT6206/LT6207. The input stage consists of transistors Q1 to Q8 and resistor R1. This topology allows for high slew rates at low supply voltages. The input common mode range extends from ground to typically 1.75V from VCC, and is limited by 2 VBEs plus a saturation voltage of a current source. There are back-to-back series diodes, D1 to D4, across the + and – inputs of each amplifier to limit the differential voltage to ±1.4V. RIN limits the current through these diodes if the input differential voltage ex- ceeds ±1.4V. The input stage drives the degeneration resistors of PNP and NPN current mirrors, Q9 to Q12, which convert the differential signals into a single-ended output. The complementary drive generator supplies cur- rent to the output transistors that swing from rail-to-rail. The current generated through R1, divided by the capaci- tor CM, determines the slew rate. Note that this current, and hence the slew rate, are proportional to the magnitude of the input step. The input step equals the output step divided by the closed loop gain. The highest slew rates are therefore obtained in the lowest gain configurations. The Typical Performance Characteristic Curve of Slew Rate vs Closed Loop Gain shows the details. ESD The LT6205/LT6206/LT6207 have reverse-biased ESD protection diodes on all inputs and outputs as shown in Figure 1. If these pins are forced beyond either supply unlimited current will flow through these diodes. If the current is transient, and limited to 25mA or less, no damage to the device will occur. Layout and Passive Components With a gain bandwidth product of 100MHz and a slew rate of 450V/ µs the LT6205/LT6206/LT6207 require special attention to board layout and supply bypassing. Use a ground plane, short lead lengths and RF-quality low ESR supply bypass capacitors. The positive supply pin should be bypassed with a small capacitor (typically 0.01 µF to 0.1 µF) within 0.25 inches of the pin. When driving heavy loads, an additional 4.7 µF electrolytic capacitor should be used. When using split supplies, the same is true for the negative supply pin. For optimum performance all feed- back components and bypass capacitors should be con- tained in a 0.5 inch by 0.5 inch area. This helps ensure minimal stray capacitances. The parallel combination of the feedback resistor and gain setting resistor on the inverting input can combine with the input capacitance to form a pole which can degrade stability. In general, use feedback resistors of 1k or less. Capacitive Load The LT6205/LT6206/LT6207 are optimized for wide band- width video applications. They can drive a capacitive load of 20pF in a unity-gain configuration. When driving a larger capacitive load, a resistor of 10 Ω to 50Ω should be connected between the output and the capacitive load to avoid ringing or oscillation. The feedback should still be taken from the output pin so that the resistor will isolate the capacitive load and ensure stability. The Typical Per- formance Curves show the output overshoot when driving a capacitive load with different series resistors. Video Signal Characteristics Composite video is the most commonly used signal in broadcast-grade products and includes Luma (or lumi- nance, the intensity information), Chroma (the colorim- etry information) and Sync (vertical and horizontal raster timing) elements combined into a single signal, NTSC and PAL being the common formats. Component video for entertainment systems include separate signal(s) for the Luma and Chroma (i.e. Y/C or YPbPr) with Sync generally applied to the Luma channel (Y signal). In some instances, native RGB signals (separate intensity information for each primary color: red, green, blue) will have Sync included as well. All the signal types that include Sync are electrically similar from a voltage-swing standpoint, though various timing and bandwidth relationships exist depend- ing on the applicable standard. The typical video waveforms that include Sync (including full composite) are specified to have nominal 1VP-P ampli- tude. The lower 0.3V is reserved for “sync tips” that carry timing information, and by being at a lower potential than all the other information, represents blacker-than-black intensity, thereby causing scan retrace activity to be |
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