LT1719 Linear Technology, LT1719 Datasheet - Page 8

• Manufacturer Part Number: LT1719
• Description: 4ns/ 150MHz Dual Comparator with Independent Input/Output Supplies
• Manufacturer: Linear Technology

APPLICATIONS
LT1719
Figure 2 shows a typical topside layout of the LT1719 on
such a multilayer board. Shown is the topside metal etch
including traces, pin escape vias, and the land pads for an
SO-8 LT1719 and its adjacent X7R 10nF bypass capacitors
in the 1206 case.
8
The ground trace from Pin 5 runs under the device up to
the bypass capacitor, shielding the inputs from the
outputs. Note the use of a common via for the LT1719 and
the bypass capacitors, which minimizes interference from
high frequency energy running around the ground plane or
power distribution traces.
The supply bypass should include an adjacent
10nF ceramic capacitor and a 2.2 F tantalum capacitor no
farther than 5cm away; use more capacitance on + V
driving more than 4mA loads. To prevent oscillations, it is
helpful to balance the impedance at the inverting and
noninverting inputs; source impedances should be kept
low, preferably 1k or less.
The outputs of the LT1719 are capable of very high slew
rates. To prevent overshoot, ringing and other problems
with transmission line effects, keep the output traces
shorter than 10cm, or be sure to terminate the lines to
maintain signal integrity. The LT1719 can drive DC termi-
nations of 200 or more, but lower characteristic imped-
ance traces can be used with series termination or AC
termination topologies.
Shutdown Control
The LT1719 features a shutdown control pin for reduced
quiescent current when the comparator is not needed.
During shutdown, the inputs and the outputs become high
impedances. The LT1719 is enabled when the shutdown
input is pulled low. A logic high disables the comparator.
The logic interface is based on the output power rails, + V
and GND, with a threshold roughly two diode drops less
Figure 2. Typical Topside Metal for Multilayer PCB Layouts
U
INFORMATION
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1719 F02
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S
if
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than + V
pull-up resistor should be used. Because shutdown is
active high, this resistor adds little power drain during
shutdown.
For applications that do not use the shutdown feature, it
may be helpful to tie the shutdown control to ground
through a 100 resistor rather than directly. This allows
the SHDN pin to be pulled high during debug or in-circuit
test (bed of nails) so that the output node can be wiggled
without damaging the low impedance output driver of the
LT1719.
The shutdown state is not guaranteed to be useful as a
multiplexer. Digital signals can have extremely fast edge
rates that may be enough to momentarily activate the
LT1719 output stage via internal capacitive coupling. No
damage to the LT1719 will result, but this could prove
deleterious to the intended recipient of the signal.
The LT1719 includes a FET pull-up on the shutdown
control pin (see Simplified Schematic) as well as other
internal structures to make the shutdown state current
drain <<1 A. Shutdown is guaranteed with an open circuit
on the shutdown control pin. When the shutdown control
pin is driven to + V
impedance of the pull-up FET will cause a current flow of
7 A (typ) into the +V
Currents in all other power supply terminals will be <1 A.
Power Supply Sequencing
The LT1719 is designed to tolerate any power supply
sequencing at system turn-on and power down. In any of
the previously shown power supply configurations, the
various supplies can activate in any order without exces-
sive current drain by the LT1719.
As always, the Absolute Maximum Ratings must not be
exceeded, either on the power supply terminals or the
input terminals. Power supply sequencing problems can
occur when input signals are powered from supplies that
are independent of the LT1719’s supplies. For the com-
parator inputs, the signals should be powered from the
same V
down input, the signal should be powered from the same
+V
S
as the LT1719.
CC
S
. Therefore, if driven by a standard TTL gate, a
and V
EE
supplies as the LT1719. For the shut-
S
S
– 0.5V, the 70k
pin and out the shutdown pin.
linear region