LTC2911
11
2911f
LTC2911-2, LTC2911-3 and LTC2911-4 respectively. V2
of the LTC2911-5 option is a high impedance input with
a nominal 0.5V threshold.
Input Noise Filtering for RST
TheV1,V2andADJcomparatorshavearesponsetimethat
is inversely proportional to overdrive. This characteristic
is illustrated in the Typical Performance Characteristics
as the graph Allowable Glitch Duration versus Overdrive.
The ADJ and the LTC2911-5’s V2 pin may be bypassed
withacapacitortoincreasethefilteringinapplicationsthat
demand it. The resultant RC lowpass filter at the inputs will
further reject high frequency components, at the cost of
slowing the monitor’s response to fault conditions.
Resistor Selection for ADJ
The threshold of the supply monitored by the ADJ pin is
configured with an external resistive divider (R2 and R1)
connected between the supply and ground. The tap point
for the divider is connected to the adjustable input (ADJ)
which has a 0.5V threshold. (See Figure 2)
Normally, the user selects a trip voltage based on the sup-
ply and acceptable tolerances, and a value of R1 based on
current drawn. For a given current, I, R1 is given by:
R1
=
0.5V
I
TominimizeerrorsarisingfromtheADJinputbiascurrent,
a value of less than 100k is recommended for R1.
R2 is then chosen by:
R2
=R1
VTRIP_ ADJ
0.5V
– 1
where, VTRIP_ADJ is the supply threshold when the ADJ
pin falls below its 0.5V threshold.
For accurate monitoring, the resistor tolerance should be
as small as possible. Resistor tolerance of 0.1% or some
trimming of components should be considered for R2/R1
in applications that require an accurate trip point.
Resistor Selection for PFI
Anexternalresistivedivider(R3andR4)connectedbetween
the supply and ground configures the threshold of the
supply monitored by the power-fail comparator. The tap
point for the divider is connected to the PFI input which
has a 0.5V threshold. (See Figure 3a)
Resistor selection follows a process similar to that for
the ADJ pin.
R3 is given by:
R3
=
0.5V
I
APPLICATIONS INFORMATION
–
+
+–
0.5V
2911 F02
ADJ
LTC2911
R2
R1
VTRIP
Figure 2. Setting the Adjustable (ADJ) Trip Point