LTC4218

9

4218fb

OPERATION

The Functional Diagram displays the main circuits of the

device. The LTC4218 is designed to turn a board’s sup-

ply voltage on and off in a controlled manner, allowing

the board to be safely inserted and removed from a live

backplane. During normal operation, the charge pump and

gate driver turn on the external N-channel pass FET’s gate

to provide power to the load.

The current sense (CS) amplifier monitors the load current

using the voltage sensed across the current sense resistor.

The CS amplifier limits the current in the load by reducing

the GATE-to-SOURCE voltage in an active control loop. It

is simple to adjust the current limit threshold using the

during other times such as startup.

A short circuit on the output to ground causes significant

power dissipation during active current limiting. To limit

this power, the foldback amplifier reduces the current limit

below 0.6V (see Typical Performance Characteristics).

If an overcurrent condition persists, the TIMER pin ramps

up with a 100μA current source until the pin voltage exceeds

1.2V (comparator TM2). This indicates to the logic that it

is time to turn off the MOSFET to prevent overheating. At

this point the TIMER pin ramps down using the 2μA current

source until the voltage drops below 0.2V (Comparator

TM1) which tells the logic to start an internal 100ms timer.

At this point, the pass transistor has cooled and it is safe

to turn it on again.

The fixed 12V version, LTC4218-12, uses two separate

This version also features a divider from the SOURCE pin

to drive the FB pin. The LTC4218-12 is available in a DFN

package while the LTC4218 (adjustable version) is in a

SSOP package.

The output voltage is monitored using the FB pin and the

PG comparator to determine if the power is available for

the load. The power good condition is signaled by the PG

pin using an open-drain pull-down transistor.

The Functional Diagram shows the monitoring blocks of

the LTC4218. The comparators on the left side include

the UV and OV comparators. These comparators are used

to determine if the external conditions are valid prior to

turning on the MOSFET. But first, the undervoltage lockout

circuits (UVLO1 and UVLO2) must validate the input supply

ate the power up initialization to the logic circuits. If the

external conditions remain valid for 100ms the MOSFET

is allowed to turn on.

The current monitor (CM) outputs a current proportional to

the sense resistor current. This current can drive an external

resistor or other circuits for monitoring purposes.