LTC4218

12

4218fb

APPLICATIONS INFORMATION

Using a switch (connected to ground) in series with the

external resistor allows the active current limit to change

only when the switch is closed. This feature can be used

when the startup current exceeds the typical maximum

load current.

Monitor MOSFET Current

The current in the MOSFET passes through the sense

resistor. The voltage on the sense resistor is converted to

sense resistor. This output current can be converted to a

voltage using an external resistor to drive a comparator

A microcontroller with a built-in comparator can build a

simple integrating single-slope ADC by resetting a capaci-

tor that is charged with this current. When the capacitor

voltage trips the comparator and the capacitor is reset, a

timer is started. The time between resets will indicate the

MOSFET current.

Monitor OV and UV Faults

Protecting the load from an overvoltage condition is the

main function of the OV pin. In the LTC4218-12 an internal

resistive divider (driving the OV pin) connects to a compara-

the switch will be allowed to turn on immediately. In the

LTC4218, the OV pin threshold is 1.23V when rising and

1.21V when falling out of overvoltage.

The UV pin functions as an undervoltage protection pin or

as an “on” pin. In the LTC4218-12 the MOSFET turns off

rises above 9.88V for 100ms, the switch will be allowed

to turn on again. The LTC4218 UV turn on/off threshold

is 1.23V (rising) and 1.15V (falling).

In the case of an undervoltage or overvoltage, the MOSFET

turns off and there is indication on the PG status pin. When

the overvoltage is removed, the MOSFET’s gate ramps up

immediately.

Powergood Indication

In addition to setting the foldback current limit threshold,

the FB pin is used to determine a powergood condition.

The LTC4218-12 uses an internal resistive divider on

the SOURCE pin to drive the FB pin. The PG comparator

indicates logic high when SOURCE pin rises above 10.5V.

If the SOURCE pin subsequently falls below 10.3V, the

comparator toggles low. On the LTC4218 the PG compara-

tor drives high when the FB pin rises above 1.23V and low

when falls below 1.21V.

Once the PG comparator is high, the GATE pin voltage

is monitored with respect to the SOURCE pin. Once the

GATE minus SOURCE voltage exceeds 4.2V, the PG pin

goes high. This indicates to the system that it is safe to

load the Output while the MOSFET is completely turned

“on”. The PG pin goes low when the GATE is commanded

the PG comparator drives low.

12V Fixed Version

In the LTC4218-12, the UV, OV and FB pins are driven by

internal dividers which may need to be filtered to prevent

false faults. By placing a bypass capacitor on these pins

value from the electrical table for this calculation.

In cases where the fixed thresholds need a slight adjust-

or GND will adjust the threshold up or down. Likewise,

placing a resistor between FB pin to OUT or GND adjusts

table for this calculation.

An example in Figure 4 raises the UV turn-on voltage from

9.88V to 10.5V. Increasing the UV level requires adding a

can be calculated using electrical table parameters as

follows:

R

RV

VV

k

SHUNT

IN

OLD

NEW

OLD

1

18

9 88

10 5

–

•.

.– 9

988

287

.

()

=

k