LT6205CS5 LINER [Linear Technology], LT6205CS5 Datasheet - Page 10

• Manufacturer Part Number: LT6205CS5
• Description: Single/Dual/Quad Single Supply 3V, 100MHz Video Op Amps
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT6205/LT6206/LT6207
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
V
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. R
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/ s 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 F to
0.1 F) within 0.25 inches of the pin. When driving heavy
loads, an additional 4.7 F electrolytic capacitor should be
used. When using split supplies, the same is true for the
10
CC
, and is limited by 2 VBEs plus a saturation voltage of
U
U
W
IN
limits the current
U
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 to 50 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 1V
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
P-P
ampli-
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