LTC2913 Datasheet, PDF (7/12 Pages) Linear Technology – Dual UV/OV Voltage Monitor

LTC2913
APPLICATIONS INFORMATION
Voltage Monitoring
The LTC2913 is a low power dual voltage monitoring
circuit with two undervoltage and two overvoltage inputs.
A timeout period that holds OV and UV asserted after all
faults have cleared is adjustable using an external capacitor
and is externally disabled.
Each voltage monitor has two inputs (VHn and VLn) for
detecting undervoltage and overvoltage conditions. When
configured to monitor a positive voltage V_n using the
3-resistor circuit configuration shown in Figure 1, VHn
is connected to the high side tap of the resistive divider
and VLn is connected to the low side tap of the resistive
divider.
3-Step Design Procedure
The following 3-step design procedure determines ap-
propriate resistances to obtain the desired UV and OV trip
points for the voltage monitor circuit in Figure 1.
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. Choose R_A to obtain the desired OV trip point
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)
V_n
R_C
LTC2913
VHn
–
+
UV_n
R_B
+– _0.5V
–
VLn
+
R_A
OV_n
2913 F01
Figure 1. 3-Resistor Positive UV/OV Monitoring Configuration
2. Choose R_B to obtain the desired UV trip point
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
• ^Vn
V_UV
– R_A
(2)
3. Choose R_C to complete the design
Once, R_A and R_B are known, R_C is determined by:
R_C
=
V_n
I_n
– R_A
– R_B
(3)
If any of the variables V_n, I_n, V_UV or V_OV change, then each
step must be recalculated.
Voltage Monitor Example
A typical voltage monitor application is shown in Figure 2.
The monitored voltage is a 5V ±10% supply. Nominal cur-
rent in the resistive divider is 10µA.
1. Find R_A to set the OV trip point of the monitor.
R_A
=
0.5V _•
10µ A
5V
5.5V
≈ 45.3k
2. Find R_B to set the UV trip point of the monitor.
R_B
=
0.5V _•
10µ A
5V
4.5V
– 45.3k
≅ 10.2k
3. Determine R_C to complete the design.
R_C
=
5V
10µ A
–
45.3k − 10.2k
≈
442k
V1
5V ±10%
R_C
442k
R_B
10.2k
R_A
45.3k
V_CC
5V
V_CC
VH1
OV
LTC2913
VL1
UV
GND
2913 F02
Figure 2. Typical Supply Monitor
2913fa
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