Datasheet

LTC2910
8
2910fc
APPLICATIONS INFORMATION
Voltage Monitoring
The LTC2910 is a low power octal voltage monitoring
circuit with eight individual undervoltage monitor inputs.
A timeout period that holds a reset after all faults have
cleared is adjustable using an external capacitor and is
disabled, by tying TMR to V
CC
.
Each voltage monitor is compared to a fi xed 0.5V reference
for detecting undervoltage conditions. When confi gured
to monitor a positive voltage V
m
, the application is con-
nected as shown in Figure 1. For negative inputs V
m
is
connected as shown in Figure 2. R
A
is now connected to
the REF pin and R
B
remains connected to the monitored
voltage V
m
.
Using the confi gurations in Figures 1 and 2, a UV condi-
tion will result when the magnitude of the voltage at V
m
is less than its designed threshold.
Polarity Selection
The three-state polarity-select pin (SEL) selects one of three
possible polarity combinations for the input thresholds,
as described in Table 1. When an input is confi gured for
negative supply monitoring, a reset condition occurs when
the supply voltage is less negative than the confi gured
threshold.
The three-state input pin SEL is connected to GND, V
CC
, or
left unconnected during normal operation. When the pin
is left unconnected, the maximum leakage allowed from
the pin is ±10μA to ensure it remains in the open state.
Table 1 shows the three possible selections of polarity
based on the SEL pin connection.
2-Step Design Procedure
The following 2-step design procedure allows selecting
appropriate resistances to obtain the desired UV trip point
for the positive voltage monitor circuit in Figure 1 and the
negative voltage monitor circuit in Figure 2.
For positive supply monitoring, V
m
is the desired nominal
operating voltage, I
m
is the desired nominal current through
the resistive divider, and V
UV
is the desired undervoltage
trip point.
For negative supply monitoring, to compensate for the 1V
reference, 1V must be subtracted from V
m
and V
UV
before
using each in the following equations.
1. Choose R
A
to obtain the desired UV trip point
R
A
is chosen to set the desired trip point for the
undervoltage monitor.
R
V
V
V
A
m
UV
=
05.
I
•
m
(1)
Figure 1. Positive UV Monitoring Confi guration Figure 2. Negative UV Monitoring Confi guration
Table 1. Voltage Polarity Programming (V
RT
= 0.5V Typical)
SEL V7 INPUT V8 INPUT
V
CC
Positive
V7 < V
RT
→ UV
Positive
V8 < V
RT
→ UV
Open Positive
V7 < V
RT
→ UV
Negative
V8 > V
RT
→ UV
GND Negative
V7 > V
RT
→ UV
Negative
V8 > V
RT
→ UV
2910 F01
R
B
R
A
V
m
UVn
LTC2910
+
–
–
+
Vn
0.5V
2910 F02
R
A
R
B
REF
Vn
V
m
0.5V
UVn
+
–
–
+
–
+
1V
+
–
LTC2910