LT6119-1/LT6119-2
19
611912f
For more information www.linear.com/LT6119-1
applicaTions inForMaTion
of R
OUT
and any parasitic and/or load capacitance. Note
that reducing R
IN
or increasing R
OUT
will both have the
effect of increasing the voltage gain of the circuit. If the
output capacitance is limiting the speed of the system, R
IN
and R
OUT
can be decreased together in order to maintain
the desired gain and provide more current to charge the
output capacitance.
The response time of the comparators is the sum of the
propagation delay and the fall time. The propagation de
-
lay is
a function of the overdrive voltage on the input of
the comparators. A larger overdrive will result in a lower
propagation
delay. This helps achieve a fast system re
-
sponse time to fault events. The fall time is affected by
the load on the output of the comparator as well as the
pull-up voltage.
The LT6119 amplifier has a typical response time of 500ns
and the comparators have a typical response time of 500ns.
When configured as a system, the amplifier output drives
the comparator input causing a total system response
time which is typically greater than that implied by the
individually specified response times. This is due to
the
overdrive
on the comparator input being determined by
the speed of the amplifier output.
Internal Reference and Comparators
The integrated precision reference and comparators
combined with the high precision current sense allow
for rapid and easy detection of abnormal load currents.
This is often critical in systems that require high levels of
safety and reliability. The LT6119 comparators are opti
-
mized for fault detection and are designed with latching
outputs. Latching outputs prevent faults from clearing
themselves and require a separate system or user to
reset the outputs. In applications where the compara
-
tor output can intervene and disconnect loads from the
supply,
latched outputs are required to avoid oscillation.
Latching outputs are also useful for detecting problems
that are intermittent. The comparator outputs on the
LT6119 are always latching and there is no way to disable
this feature.
Each
of
the comparators has one input available externally,
with the two versions of the part differing by the polarity
of those available inputs. The other comparator inputs are
connected internally to the 400mV precision reference.
The input threshold (the voltage which causes the output
to transition from high to low) is designed to be equal to
that of
the reference. The reference voltage is established
with respect to the device V
–
connection.
Comparator Inputs
The comparator inputs can swing from V
–
to 60V regardless
of the supply voltage used. The input current for inputs
well above the threshold is just a few pAs. With decreas
-
ing input voltage, a small bias current begins to be drawn
out of the input near the threshold, reaching 50nA max
when at ground potential. Note that this change in input
bias current can cause a small nonlinearity in the OUTA
transfer function if the comparator inputs are coupled to
the amplifier output with a voltage divider. For example,
if the maximum comparator input current is 50nA, and
the resistance seen looking out of the comparator input
is 1k, then a change in output voltage of 50µV will be
seen on the analog output when the comparator input
voltage passes through its threshold. If both compara
-
tor inputs
are connected to the output then they must
both be considered.
Setting Comparator Thresholds
The comparators have an internal precision 400mV refer
-
ence. In order to set the trip points of the LT6119-1 com-
parators, the output currents, I
OVER
and I
UNDER
, as well as
the maximum output current, I
MAX
, must be calculated:
I
OVER
=
SENSE(OVER)
R
IN
, I
UNDER
=
SENSE(UNDER)
R
IN
,
I
MAX
=
V
SENSE(MAX)
R
IN
where I
OVER
and I
UNDER
are the over and under currents
through the sense resistor which cause the comparators
to trip. I
MAX
is the maximum current through the sense
resistor.
