ADM2914

Rev. B | Page 12 of 16

UV AND OV RISE AND FALL TIMES

The UV and OV output rise times (from 10% to 90%) can be

approximated using the following formula:

)

)(

(

2

.

2

LOAD

UP

PULL

R

C

R

t

−

≈

where:

When a fault occurs, the UV or OV output fall time can be

expressed as

)

)(

(

2

.

2

LOAD

DOWN

PULL

F

C

R

t

−

≈

approximately 50 Ω. Assuming a load capacitance of 150 pF, the

fall time is 16.5 ns.

UV/OV OUTPUT CHARACTERISTICS

Both the OV and UV outputs have a strong pull-down to

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.

Consequently, if the state and pull-up strength of the OV pin are

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 ADM2914 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 ADM2914 suitable for

use in noisy environments.

UNDERVOLTAGE LOCKOUT (UVLO)

The ADM2914 has an undervoltage lockout circuit that monitors

below 1.9 V (minimum), the circuit is activated. The UV output

is asserted and the OV output is cleared and not allowed to

characteristics as if an undervoltage condition had occurred

on the inputs.

SHUNT REGULATOR

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 ADM2914 to

operate at voltage levels greater than 6 V by simply placing a

pin to limit the input current to less than 10 mA.

appropriate value for the dropper resistor can be calculated.

Begin by determining the maximum supply current

reference and/or the pull resistors between the outputs and

minimum and maximum shunt regulator voltage specified

following calculations.

Calculate the maximum and minimum dropper resistor

values

CCtotal

SHUNT

IN

MAX

I

V

V

R

max

=

μ

100

min

max

SHUNT

IN

MIN

V

V

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

The maximum device power is calculated as follows:

(

)

+

⎥

⎦

⎤

⎢

⎣

⎡

−

−

=

TOTAL

REAL

SHUNTmax

IN

SHUNTmax

DeviceMax

Icc

R

V

V

V

P

min

max

CCtotal

V

To check that the calculated value of the resistor will be

acceptable, calculate the maximum device temperature rise;

DeviceMax

JA

RISEmax

P

θ

Temp

=

Add this value to the ambient operating temperature. If the

resistor value is acceptable, the result will lie within the

to +85°C.