LTC2912

9

2912fa

APPLICATIONS INFORMATION

When the VH input drops below its designed threshold,

the UV pin asserts low. When the input recovers above

its designed threshold, the UV output timer starts. If the

input remains above the designed threshold when the

timer finishes, the UV pin weakly pulls high. However, if

the input falls below its designed threshold during this

timeout period, the timer resets and restarts when the

input is above the designed threshold. The OV and OV

outputs behave as the UV output when LATCH is high

(LTC2912-1, LTC2912-3).

Selecting the UV/OV Timing Capacitor

is adjustable to accommodate a variety of applications.

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

only limitation is the availability of a large value capaci-

tor with low leakage. Capacitor leakage current must not

exceed the minimum TMR charging current of 1.3μA.Tying

Undervoltage Lockout

undervoltage lockout (UVLO) condition. During UVLO, UV

is asserted and pulled low while OV and OV are cleared

follows the same timing procedure as an undervoltage

condition on the VH input.

Shunt Regulator

operates as a direct supply input for voltages up to 6V. Under

this condition, the quiescent current of the device remains

6V, the device operates as a shunt regulator and should

to limit the current to no greater than 10mA.

When choosing this resistance value, select an appropriate

location on the I-V curve shown in the Typical Performance

UV, OV and OV Output Characteristics

The DC characteristics of the UV, OV and 0V pull-up and

pull-down strength are shown in the Typical Performance

Characteristics. Each pin has a weak internal pull-up to

allows these pins to have open-drain behavior while pos-

sessing 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

on. Therefore, an external pull-up resistor of no more than

100k is recommended on the OV pin if the state and pull-up

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, OV and OV outputs have strong pull-down capa-

bility. The following formula estimates the output fall time

(90% to 10%) for a particular external load capacitance

on the pin. Assuming a 150pF load capacitance, the fall

time is 16.5ns.