LTC2912
9
2912fb
For more information www.linear.com/LTC2912
APPLICATIONS INFORMATION
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 LTC2912 is guaranteed to assert UV low, OV high
(LTC2912‑1, LTC2912‑2) and OV low (LTC2912‑3) 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 the VH input and V
CC
become valid an internal timer
is started. After an adjustable delay time, UV weakly pulls
high.
Threshold Accuracy
Reset threshold accuracy is important in a supply‑sensitive
system. Ideally, such a system resets only if supply voltages
fall outside the exact thresholds for a specified margin.
Both LTC2912 inputs have a relative threshold accuracy
of ±1.5% over the full operating temperature range.
For example, when the LTC2912 is programmed to moni
‑
tor a 5
V input with a 10% tolerance, the desired UV trip
point is 4.5V. Because of the ±1.5% relative accuracy of
the LTC2912, the UV trip point can be anywhere between
4.433V and 4.567V which is 4.5V ±1.5%.
Likewise, the accuracy of the resistances chosen for R
A
,
R
B
and R
C
can affect the UV and OV trip points as well.
Using the example just given, if the resistances used to
set the UV trip point have 1% accuracy, the UV trip range
is between 4.354V and 4.650V. This is illustrated in the
following calculations.
The UV trip point is given as:
V
UV
= 0.5V 1+
R
C
R
A
+ R
B
⎛
⎝
⎜
⎞
⎠
⎟
The two extreme conditions, with a relative accuracy of
1.5% and resistance accuracy of 1%, result in:
V
UV(MIN)
= 0.5V • 0.985 • 1+
R
C
• 0.99
R
A
+ R
B
( )
• 1.01
⎛
⎝
⎜
⎞
⎠
⎟
and
V
UV(MAX)
= 0.5V • 1.015 • 1+
R
C
• 1.01
R
A
+ R
B
( )
• 0.99
⎛
⎝
⎜
⎞
⎠
⎟
For a desired trip point of 4.5V,
R
C
R
A
+ R
B
= 8
Therefore,
V
UV(MIN)
= 0.5V • 0.985 • 1+ 8
0.99
1.01
⎛
⎝
⎞
⎠
= 4.354V
and
V
UV(MAX)
= 0.5V • 1.015 • 1+ 8
1.01
⎛
⎝
⎞
⎠
= 4.650V
Glitch Immunity
In any supervisory application, noise riding on the moni‑
tored DC
voltage causes spurious resets. To
solve this
problem without adding hysteresis, which causes a new
error term in the trip voltage, the LTC2912 lowpass filters
the output of the first stage comparator at each input. This
filter integrates the output of the comparator before as
‑
serting the
UV or OV logic. A transient at the input of the
comparator
of sufficient magnitude and duration triggers
the output logic. The Typical Performance Characteristics
show a graph of the Transient Duration vs Comparator
Overdrive.
UV/OV Timing
The LTC2912 has an adjustable timeout period (t
UOTO
) that
holds OV, OV or UV asserted after each fault has cleared.
This delay assures a minimum reset pulse width allowing
settling time for the monitored voltage after it has entered
the “valid” region of operation.
