LTC2995 Datasheet, PDF (14/20 Pages) Linear Technology – Temperature Sensor and Dual Voltage Monitor

LTC2995
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 10μA, 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.
V_n
R_C
LTC2995
V_Hn
–
UV_n
+
R_B
+– _0.5V
–
V_Ln
+
R_A
OV_n
2995 F07
Figure 7. 3-Resistor Positive UV/OV Monitoring
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 _• V_N
I_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 _• 5V
10μA 5.5V
≈ 45.3k
2995f
14