LT6207IGN datasheet(12/18 Pages) LINER

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LT6205/LT6206/LT6207

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APPLICATIONS INFORMATION

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 exceeds

±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 current to the

output transistors that swing from rail-to-rail.

The current generated through R1, divided by the capacitor

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 Characteristics 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 dam-

age 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 0.01μF to

0.1μF) within 0.25 inches of the pin. When driving heavy

loads, an additional 4.7μF electrolytic capacitor should be

used. When using split supplies, the same is true for the

negative supply pin. For optimum performance all feedback

components and bypass capacitors should be contained

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 10Ω 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 Typi-

cal Performance Characteristics 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 colorimetry

information) and sync (vertical and horizontal raster tim-

ing) 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 depending on

the applicable standard.

The typical video waveforms that include sync (includ-

ing full composite) are speciﬁed to have nominal 1VP-P

amplitude. 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-

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