Datasheet

Data Sheet ADM12914
Rev. D | Page 13 of 16
UV/OV OUTPUT CHARACTERISTICS
Both the
OV
and
UV
outputs have strong pull-down to ground
and weak internal pull-up to V
CC
. This permits the pins to behave
as open-drain outputs. When the rise time on the pin is not
critical, the weak pull-up removes the requirement for an
external pull-up resistor. The open-drain configuration allows
for wire-OR’ing of outputs, which is particularly useful when
more than one signal needs to pull down on the output.
At V
CC
= 1 V, a maximum V
OL
= 0.15 V at
UV
is guaranteed. At
V
CC
= 1 V, the weak pull-up current on
OV
is almost turned on.
Consequently, if the state and pull-up strength of the
OV
pin is
important at very low V
CC
, an external pull-up resistor of no more
than 100 kΩ is advised. By adding an external pull-up resistor,
the pull-up strength on the
OV
pin is greater. Therefore, if it is
connected in a wire-OR’ed configuration, the pull-down strength
of any single device must account for this additional pull-up
strength.
GLITCH IMMUNITY
The ADM12914 is immune to short transients that may occur
on the monitored voltage rails. The device contains internal
filtering circuitry that provides immunity to fast transient
glitches. Figure 9 illustrates glitch immunity performance by
showing the maximum transient duration without causing a
reset pulse. Glitch immunity makes the ADM12914 suitable for
use in noisy environments.
UNDERVOLTAGE LOCKOUT (UVLO)
The ADM12914 has an undervoltage lockout circuit that monitors
the voltage on the V
CC
pin. When the voltage on V
CC
drops below
1.94 V (minimum), the circuit activates. The
UV
output is asserted
and the
OV
output is cleared and is not allowed to assert. When
V
CC
recovers,
UV
exhibits the same timing characteristics as
though an undervoltage condition had occurred on the inputs.
SHUNT REGULATOR
The ADM12914 is powered via the V
CC
pin. The V
CC
pin can be
directly connected to a voltage rail of up to 6 V. In this mode,
the supply current of the device does not exceed 100 µA. An
internal shunt regulator allows the ADM12914 to operate at
voltage levels greater than 6V by simply placing a dropper
resistor in series between the supply rail and the V
CC
pin to limit
the input current to less than 10 mA.
Once the supply voltage, V
IN
, has been established, an
appropriate value for the dropper resistor can be calculated.
Begin by determining the maximum supply current required,
I
CCtotal
, by adding the current drawn from the reference and/or
the pull resistors between the outputs and the V
CC
pin to the
maximum specified supply current. The minimum and
maximum shunt regulator voltage specified in Table 1, V
SHUNT min
and V
SHUNT max
, are also required in the following calculations.
Calculate the maximum and minimum dropper resistor values
CCtotal
SHUNTmax
INmin
MAX
I
VV
R
−
=
μ100
SHUNTmin
INmax
MIN
VV
R
−
=
Based on these values, choose a real-world resistor value within
this range. Then, given the specified accuracy of this resistor,
calculate the minimum and maximum real resistor value
variation, R
REALmin
and R
REALmax
, respectively.
The maximum device power is calculated as follows:
( )
+






−
−
=
TOTAL
REAL
SHUNTmax
IN
SHUNTmax
DeviceMax
Icc
R
VV
VP
min
max
CCtotal
SHUNTmax
IV
To check that the calculated value of the resistor will be
acceptable, calculate the maximum device temperature rise
DeviceMax
JARISEmax
PθTemp =
Add this value to the ambient operating temperature. If the
resistor value is acceptable, the result will lie within the
specified operating temperature range of the device.
OV LATCH (ADM12914-1)
If an overvoltage condition occurs when the
LATCH
pin is
pulled low, the
OV
pin latches low. Pulling
LATCH
high clears
the latch. If an
OV
condition clears while
LATCH
is high, the
latch is bypassed and the
OV
pin behaves in the same way as the
UV
pin, with an identical timeout period. If the
LATCH
pin is
pulled low while the timeout period is active, the
OV
pin latches
low, as in normal operation.
DISABLE (ADM12914-2)
Pulling the DIS pin high disables both the
UV
and
OV
outputs,
and forces both outputs to remain weakly pulled high, regard-
less of any faults that are detected at the inputs. If a UVLO
condition is detected, the
UV
output is asserted and pulls low;
however, the timeout function is bypassed. As soon as the
UVLO condition clears, the
UV
output pulls high. To guarantee
normal operation when the pin is left unconnected, DIS has a
weak 2 µA internal pull-down current.