Datasheet

LTC2995
14
2995f
APPLICATIONS INFORMATION
4. Finally R
TC
is determined by:
R
TC
=
1.8V – VT2
I
REF
In the Temperature Monitor example discussed earlier with
thresholds at VT1 = 60°C and VT2 = 90°C and a desired
reference current of 10A, the required values for R
TA
,
R
TB
and R
TC
can be calculated as:
R
TA
=
1.332V
10µA
= 133.2k
R
TB
=
1.452V – 1.332V
10µA
= 12k
R
TC
=
1.8V – 1.452V
10µA
= 34.8k
Voltage Monitoring
In addition to temperature measurement, the LTC2995
features a low power dual voltage monitoring circuit. Each
voltage monitor has two inputs (VH1/VL1 and VH2/VL2)
for detecting undervoltage and overvoltage conditions. If
either VH1 or VH2 falls below 0.5V (typical), the LTC2995
communicates an undervoltage condition by pulling UV
low. Similar, an overvoltage condition is flagged by pulling
OV low if either VL1 or VL2 rises above 0.5V.
When configured to monitor a positive voltage Vn using
the 3-resistor circuit configuration shown in Figure 5,
V
Hn
will be connected to the high side tap of the resistive
divider and V
Ln
will be connected to the low side tap of
the resistive divider.
Voltage Monitor Design Procedure
The following 3-step design procedure selects appropriate
resistances to obtain the desired UV and OV trip points
for the voltage monitor circuit in Figure 7.
For supply monitoring, V
n
is the desired nominal operat-
ing voltage, I
n
is the desired nominal current through the
resistive divider, V
OV
is the desired overvoltage trip point,
and V
UV
is the desired undervoltage trip point.
1. R
A
is chosen to set the desired trip point for the
overvoltage monitor:
R
A
=
0.5V
I
N
•
V
N
V
OV
(1)
2. Once R
A
is known, R
B
is chosen to set the desired
trip point for the undervoltage monitor:
R
B
=
0.5V
I
N
•
V
N
V
UV
–R
A
(2)
3. Once, R
A
and R
B
are known, R
C
is determined by:
R
C
=
V
N
I
N
–R
A
–R
B
(3)
Voltage Monitor Example
A typical voltage monitor application is shown in Figure 2.
The monitored voltage is a 5V ±10% supply. Nominal
current in the resistive divider is 10µA.
1. Find R
A
to set the OV trip point of the monitor:
R
A
=
0.5V
10µA
•
5V
5.5V
≈ 45.3k
Figure 7. 3-Resistor Positive UV/OV Monitoring
–
+
–
+
+
–
0.5V
LTC2995
UV
n
V
Hn
R
C
R
B
R
A
2995 F07
V
n
V
Ln
OV
n