LT6108-1/LT6108-2
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610812fa
sense voltage produces an output voltage. With a positive
offset voltage, the output voltage is zero until the input
sense voltage exceeds the input offset voltage. Neglect-
ing V
OS
, the output circuit is not limited by saturation of
pull-down circuitry and can reach 0V.
Response Time
The LT6108 amplifier is designed to exhibit fast response
to inputs for the purpose of circuit protection or current
monitoring. This response time will be affected by the
external components in two ways, delay and speed.
If the output current is very low and an input transient
occurs, there may be an increased delay before the
output voltage begins to change. The Typical Performance
Characteristics show that this delay is short and it can
be improved by increasing the minimum output current,
either by increasing R
SENSE
or decreasing R
IN
. Note that
the Typical Performance Characteristics are labeled with
respect to the initial sense voltage.
The speed is also affected by the external components.
Using a larger R
OUT
will decrease the response time, since
V
OUT
= I
OUTA
• Z
OUT
where Z
OUT
is the parallel combination
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 comparator is the sum of the
propagation delay and the fall time. The propagation delay
is a function of the overdrive voltage on the input of the
comparator. A larger overdrive will result in a lower propaga-
tion delay. This helps achieve a fast system response 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 LT6108 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
APPLICATIONS INFORMATION
overdrive on the comparator input being determined by
the speed of the amplifier output.
Internal Reference and Comparator
The integrated precision reference and comparator com-
bined 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 LT6108-1 comparator is optimized for fault
detection and is designed with a latching output. The latch-
ing output prevents faults from clearing themselves and
requires a separate system or user to reset the output. In
applications where the comparator output can intervene
and disconnect loads from the supply, a latched output
is required to avoid oscillation. The latching output is
also useful for detecting problems that are intermittent.
The comparator output on the LT6108-2 is non-latching
and can be used in applications where a latching output
is not desired.
The comparator has one input available externally. The
other comparator input is 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 Input
The comparator input 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 input is 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.
