Datasheet
LTC2914
10
2914fc
For more information www.linear.com/LTC2914
For positive supply monitoring, V
n
is the desired nomi-
nal operating voltage,
I
nd
is the desired nominal current
through the resistive divider, I
na
is the resistive divider
current calculated using the 1% resistor R
A
. V
OV
is the
desired overvoltage trip point 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
n
, V
OV
and V
UV
before using each in the following equations.
1A. Choose R
A
to obtain the desired OV trip point
R
A
is chosen to set the desired trip point for the over-
voltage monitor.
R
A
=
0.5V
I
n
•
V
n
V
OV
(1)
1B. Calculate I
na
I
na
=
0.5V
R
A
•
V
n
V
OV
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
na
•
V
n
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
na
–R
A
–R
B
(3)
If any of the variables V
n
, I
na
, I
nd
, V
UV
or V
OV
change, then
each step must be recalculated.
Positive Voltage Monitor Example
A positive voltage monitor application is shown in Figure 3.
The monitored voltage is a 5V ±10% supply. Nominal
current in the resistive divider is 10µA.
1A. Find R
A
to set the OV trip point of the monitor.
R
A
=
0.5V
10µA
•
5V
5.5V
≈ 45.3k
1B. Calculate I
na
I
na
=
0.5V
45.3k
•
5V
5.5V
=10.034µA
2. Find R
B
to set the UV trip point of the monitor.
R
B
=
0.5V
10.034µA
•
5V
4.5V
– 45.3k ≅10k
3. Determine R
C
to complete the design.
R
C
=
5V
10.034µA
– 45.3k −10k ≈442k
Negative Voltage Monitor Example 1
A negative voltage monitor application is shown in Figure 4.
The monitored voltage is a –5V ±10% supply. Nominal
current in the resistive divider is 10µA. For the negative
case, 1V is subtracted from V
n
, V
OV
and V
UV
.
1A. Find R
A
to set the OV trip point of the monitor.
R
A
=
0.5V
10µA
•
–5V –1V
–5.5V –1V
≈ 46.4k
1B. Calculate I
na
I
no
•
0.5V
46.4k
•
–5V –1V
–5.5V –1V
= 9.947µA
2. Find R
B
to set the UV trip point of the monitor.
R
B
=
0.5V
9.947µA
•
–5V –1V
–4.5V –1V
−46.4k ≅ 8.45k
Figure 3. Positive Supply Monitor
applicaTions inForMaTion
VH1
R
C
442k
R
B
10k
R
A
45.3k
V
CC
GND
LTC2914
VL1
2914 F03
OV
UV
SEL
V
CC
5V
V1
5V ±10%