Datasheet

LTC2910
9
2910fc
APPLICATIONS INFORMATION
2. Choose R
B
to complete the design
Once R
A
is known, R
B
is determined by:
R
V
R
B
m
A
= −
I
m
(2)
If any of the variables V
m
, I
m
, or V
UV
change, then both
steps 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.
1. Find R
A
to set the UV trip point of the monitor.
R
V
µA
V
V
k
A
= ≈
05
10
5
45
56 2
.
•
.
.
2. Determine R
B
to complete the design.
R
V
µA
kk
B
= −≈
5
10
56 2 499.
Negative Voltage Monitor Example
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
m
and V
UV
.
1. Find R
A
to set the UV trip point of the monitor.
R
V
µA
VV
VV
k
A
=
−
−
≈
05
10
51
45 1
54 9
.
•
–
–.
.
2. Determine R
B
to complete the design.
R
VV
µA
kk
B
=
−
−≈
–
.
51
10
57 6 549
Power-Up/Down
As soon as V
CC
reaches 1V during power up, the RST output
asserts low and the RST output weakly pulls to V
CC
.
The LTC2910 is guaranteed to assert RST low and RST high
under conditions of low V
CC
, down to V
CC
= 1V. Above V
CC
= 2V (2.1V maximum) the Vn inputs take control.
Once all inputs and V
CC
become valid, an internal timer is
started. After an adjustable delay time, RST pulls low and
RST weakly pulls high.
Threshold Accuracy
Reset threshold accuracy is important in a supply sensitive
system. Ideally, such a system would reset only if supply
Figure 3. Positive Supply Monitor
2910 F03
R
B
449k
R
A
56.2k
5V ± 10%
RST
RST
SEL
V7
V
CC
5V
GND
LTC2910
Figure 4. Negative Supply Monitor
2910 F04
R
A
54.9k
R
B
549k
–5V ± 10%
RST
RST
SEL
REF
V7
V
CC
5V
GND
LTC2910