LT1720 Linear Technology, LT1720 Datasheet - Page 7

• Manufacturer Part Number: LT1720
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

APPLICATIONS
Input Voltage Considerations
The LT1720/LT1721 are specified for a common mode
range of –100mV to 3.8V when used with a single 5V
supply. In general the common mode range is 100mV
below ground to 1.2V below V
common mode limit is that the output still responds
correctly to a small differential input signal. Also, if one
input is within the common mode limit, the other input
signal can go outside the common mode limits, up to the
absolute maximum limits (a diode drop past either rail at
10mA input current) and the output will retain the correct
polarity.
When either input signal falls below the negative common
mode limit, the internal PN diode formed with the sub-
strate can turn on, resulting in significant current flow
through the die. An external Schottky clamp diode
between the input and the negative rail can speed up
recovery from negative overdrive by preventing the sub-
strate diode from turning on.
When both input signals are below the negative common
mode limit, phase reversal protection circuitry prevents
false output inversion to at least – 400mV common mode.
However, the offset and hysteresis in this mode will
increase dramatically, to as much as 15mV each. The input
bias currents will also increase.
When both input signals are above the positive common
mode limit, the input stage will become debiased and the
output polarity will be random. However, the internal
hysteresis will hold the output to a valid logic level, and
because the biasing of each comparator is completely
independent, there will be no impact on any other com-
parator. When at least one of the inputs returns to within
the common mode limits, recovery from this state will take
as long as 1 s.
The propagation delay does not increase significantly
when driven with large differential voltages. However, with
low levels of overdrive, an apparent increase may be seen
with large source resistances due to an RC delay caused
by the 2pF typical input capacitance.
U
INFORMATION
U
CC
. The criterion for this
W
U
Input Protection
The input stage is protected against damage from large
differential signals, up to and beyond a differential voltage
equal to the supply voltage, limited only by the absolute
maximum currents noted. External input protection cir-
cuitry is only needed if currents would otherwise exceed
these absolute maximums. The internal catch diodes can
conduct current up to these rated maximums without
latchup, even when the supply voltage is at the absolute
maximum rating.
The LT1720/LT1721 input stage has general purpose
internal ESD protection for the human body model. For use
as a line receiver, additional external protection may be
required. As with most integrated circuits, the level of
immunity to ESD is much greater when residing on a
printed circuit board where the power supply decoupling
capacitance will limit the voltage rise caused by an ESD
pulse.
Input Bias Current
Input bias current is measured with both inputs held at 1V.
As with any PNP differential input stage, the LT1720/
LT1721 bias current flows out of the device. With a
differential input voltage of even just 100mV or so, there
will be zero bias current into the higher of the two inputs,
while the current flowing out of the lower input will be
twice the measured bias current. With more than two
diode drops of differential input voltage, the LT1720/
LT1721’s input protection circuitry activates, and current
out of the lower input will increase an additional 30% and
there will be a small bias current into the higher of the two
input pins, of 4 A or less. See the Typical Performance
curve “Input Current vs Differential Input Voltage.”
High Speed Design Considerations
Application of high speed comparators is often plagued
by oscillations. The LT1720/LT1721 have 4mV of internal
hysteresis, which will prevent oscillations as long as
parasitic output to input feedback is kept below 4mV.
LT1720/LT1721
7