LTC2913 Linear Technology, LTC2913 Datasheet - Page 9

• Manufacturer Part Number: LTC2913
• Description: Dual UV/OV Voltage Monitor
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
UV/OV Timing
The LTC2913 has an adjustable timeout period (t
holds OV or UV asserted after all faults have cleared. This
assures a minimum reset pulse width allowing a settling
time delay for the monitored voltage after it has entered
the valid region of operation.
When any VH input drops below its designed threshold,
the UV pin asserts low. When all inputs recover above
their designed thresholds, the UV output timer starts. If
all inputs remain above their designed thresholds when
the timer finishes, the UV pin weakly pulls high. However,
if any input falls below its designed threshold during this
timeout period, the timer resets and restarts when all inputs
are above the designed thresholds. The OV output behaves
as the UV output when LATCH is high (LTC2913-1).
Selecting the UV/OV Timing Capacitor
The UV and OV timeout period (t
is adjustable to accommodate a variety of applications.
Connecting a capacitor, C
ground sets the timeout period. The value of capacitor
needed for a particular timeout period is:
The Reset Timeout Period vs Capacitance graph found in
the Typical Performance Characteristics shows the desired
delay time as a function of the value of the timer capacitor
that must be used. The TMR pin must have a minimum
10pF load or be tied to V
only limitation is the availability of a large value capacitor
with low leakage. Capacitor leakage current must not exceed
the minimum TMR charging current of 1.3µA. Tying the
TMR pin to V
Undervoltage Lockout
When V
undervoltage lockout (UVLO) condition. During UVLO,
UV is asserted and pulled low while OV is cleared and
blocked from asserting. When V
follows the same timing procedure as an undervoltage
condition on any input.
C
TMR
= t
CC
UOTO
falls below 2V, the LTC2913 asserts an
CC
will bypass the timeout period.
• 115 • 10
CC
TMR
. For long timeout periods, the
–9
, between the TMR pin and
[F/s]
CC
UOTO
rises above 2V, UV
) for the LTC2913
UOTO
) that
Shunt Regulator
The LTC2913 has an internal shunt regulator. The V
operates as a direct supply input for voltages up to 6V.
Under this condition, the quiescent current of the device
remains below a maximum of 80µA. For V
higher than 6V, the device operates as a shunt regulator
and must have a resistance R
V
When choosing this resistance value, select an appropriate
location on the I-V curve shown in the Typical Performance
Characteristics to accommodate any variations in V
to changes in current through R
UV and OV Output Characteristics
The DC characteristics of the UV and 0V pull-down strength
are shown in the Typical Performance Characteristics. Each
pin has a weak internal pull-up to V
down to ground. This arrangement allows these pins to
have open-drain behavior while possessing several other
beneficial characteristics. The weak pull-up eliminates the
need for an external pull-up resistor when the rise time on
the pin is not critical. On the other hand, the open-drain
configuration allows for wired-OR connections, and is
useful when more than one signal needs to pull down
on the output. V
0.15V at UV.
At V
on. Therefore, an external pull-up resistor of no more than
100k is recommended on the OV pin if the state and pull-up
strength of the OV pin is crucial at very low V
Note however, by adding an external pull-up resistor, the
pull-up strength on the OV pin is increased. Therefore, if
it is connected in a wired-OR connection, the pull-down
strength of any single device must accommodate this
additional pull-up strength.
Output Rise and Fall Time Estimation
The UV and 0V outputs have strong pull-down capabil-
ity. The following formula estimates the output fall time
(90% to 10%) for a particular external load capacitance
(C
CC
LOAD
t
FALL
CC
pin to limit the current to no greater than 10mA.
= 1V, the weak pull-up current on OV is barely turned
):
≈ 2.2 • R
CC
PD
of 1V guarantees a maximum V
• C
LOAD
Z
between the supply and the
Z
.
CC
and a strong pull-
LTC2913
CC
CC
.
voltages
CC
CC
OL
2913fa
9
due
pin
=