Datasheet
LTC2914
11
2914fc
For more information www.linear.com/LTC2914
Figure 4. Negative Supply Monitor
REF
R
A
46.4k
R
B
8.45k
R
C
549k
V
CC
GND
V3
–5V ±10%
LTC2914
VH3
VL3
2914 F04
OV
UV
SEL
V
CC
5V
3. Determine R
C
to complete the design.
R
C
=
–5V –1V
9.947µA
−46.4k −8.45k ≈549k
2. Calculate R
CUV
based on the desired undervoltage trip
point of –6V.
R
CUV
=(R
AUV
+R
BUV
)•
V
UV
V
REF
=
(49.9k +49.9k)•
–6V –1V
1V
≈698k
3. Calculate R
AOV
and I
na
R
AOV
=
V
REF
– 0.5V
I
nd
=
1V – 0.5V
10µA
≈ 49.9k
I
na
≈10.020µA
4. Calculate R
BOV
for a desired overvoltage trip point of
–30V.
R
BOV
=
0.5V – V
OV
I
na
=
0.5V –30–1
10.020µA
≈3.01M
applicaTions inForMaTion
Negative Voltage Monitoring Example 2
Negative voltage monitoring applications with wide operat-
ing voltage ranges such that:
2• V
UV
<
V
OV
create situations where the VH or VL pins exceeds the
–0.3V absolute maximum voltage ratings. To ensure that
the LTC2914 operates within its design specifications,
utilize the equations shown below to determine proper
resistor sizing for the circuit in Figure 5. In the following
example, the undervoltage trip point is –6V. The overvolt
-
age trip point is –30V.
1A. Find R
AUV
and R
BUV
and let node A be a virtual ground,
which ensures that the diode current will not affect the
voltage monitor threshold accuracy. Let the resistive
divider current I
nd
= 10µA.
R
AUV
=
V
REF
– 0.5V
I
nd
=
1V – 0.5V
10µA
≈ 49.9k
R
AUV
=R
BUV
1B. Calculate I
na
using the resistor values chosen above.
I
na
=
V
REF
– 0.5V
R
AUV
=
1V – 0.5V
R
AUV
≈10.020µA
REF
R
AOV
49.9k
R
AUV
49.9k
R
BUV
49.9k
R
BOV
3.01M
A
V
CC
GND
D1
LTC2914
VH3
VL3
2914 F05
OV
UV
SEL
V
CC
5V
R
CUV
698k
V3
Figure 5. Negative Supply Monitor for Wide Operating Range
Power-Up/Power-Down
As soon as V
CC
reaches 1V during power-up, the UV output
asserts low and the OV output weakly pulls to V
CC
.
The LTC2914 is guaranteed to assert UV low and OV high
under conditions of low V
CC
, down to V
CC
= 1V. Above V
CC
= 2V (2.1V maximum) the VH and VL inputs take control.
Once all VH inputs and V
CC
become valid an internal timer
is started. After an adjustable delay time, UV weakly pulls
high.