LTC2995CUD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2995CUD' PDF : 20 Pages View PDF
LTC2995
APPLICATIONS INFORMATION
2. Find RB to set the UV trip point of the monitor:
RB =
0.5V • 5V
10μA 4.5V
– 453 ≅ 10k
3. Determine RC to complete the design:
RC =
5V
10μA
– 453Ω – 100Ω ≈ 442k
Power-Up and Undervoltage Lockout
As soon as VCC reaches approximately 1V during
power-up, the OV as well as TO1 and TO2 weakly pull to VCC
while the UV output asserts low indicating an undervolt-
age lockout condition. Above VCC = 2V (typical), the VH
and VL inputs take control. Once both VH inputs and VCC
are valid, an internal timer is started. After an adjustable
delay time, UV weakly pulls high.
When VCC falls below 1.9V, the LTC2995 indicates again
an undervoltage lockout (UVLO) condition by pulling low
UV while OV is cleared.
Threshold Accuracy
Reset threshold accuracy is important in a supply sensitive
system. Ideally, such a system would only reset if supply
voltages fell outside the exact threshold for a specified
margin. All LTC2995 VHn/VLn inputs have a relative
threshold accuracy of ±1.5% over the full operating
temperature range. For example, when the LTC2995 is
configured to monitor a 5V input with a 10% tolerance,
the desired UV trip point is 4.5V. Because of the ±1.5%
relative accuracy of the LTC2995, 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 RA,
RB, and RC can affect the UV and OV trip points as well.
Using the previous example, if the resistances used to set
the UV trip point have 1% accuracy, the UV trip range can
grow to between 4.354V and 4.650V. This is illustrated in
the following calculations.
The UV trip point is given as:
VUV
=
0.5V
t
⎛
⎜1+
⎝
RAR+CRB⎞⎠⎟
The two extreme conditions, with a relative accuracy of
1.5% and resistance accuracy of 1%, result in:
VUV(MIN)
= 0.5V
t 0.985
t
⎛
⎜1+
⎝
(RAR+CRtB0).9t 91.01⎞⎠⎟
and
VUV(MAX)
= 0.5V
t 1.015 t
⎛
⎜1+
⎝
(RAR+CRtB1).t001.99⎞⎠⎟
For
a
desired
trip
point
of
4.5V,
RC
RA + RB
=
8
Therefore,
VUV(MIN) = 0.5V t 0.985 t
⎛
⎜1+
⎝
8
01..9091⎞⎠⎟
= 4.3545V
and
VUV(MAX) = 0.5V t 1.015 t
⎛
⎜1+ 8
1.01
⎞
⎟
⎝ 0.99 ⎠
= 4.650V
Glitch Immunity
In any supervisory application, noise on the monitored DC
voltage can cause spurious resets. To solve this problem
without adding hysteresis tothe VH/VL comparators, which
would add error to the trip voltage, the LTC2995 lowpass
filters the output of the comparator. This filter causes the
output of the comparator to be integrated before assert-
ing the UV or OV logic. Any transient at the input of the
comparator must be of sufficient magnitude and duration
before the comparator will trigger the output logic. The
Typical Performance Characteristics section shows a graph
of the Typical Transient Duration vs Comparator Overdrive.
In temperature monitoring, the voltage at VPTAT must
exceed a threshold for five consecutive temperature up-
date intervals before the respective TO pin is pulled low.
Once the VPTAT voltage crosses back the threshold with
an additional 20mV of hysteresis, the respective TO pin
is released after a single update interval and an additional
delay adjustable by the capacitor on TMR.
2995f
15
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