LTC4232

8

4232fb

For more information www.linear.com/LTC4232

OPERATION

The Functional Diagram displays the main circuits of the

device. The LTC4232 is designed to turn a board’s supply

voltageonandoffinacontrolledmannerallowingtheboard

to be safely inserted and removed from a live backplane.

The LTC4232 includes a 25mΩ MOSFET and a 7.5mΩ cur-

rent sense resistor. During normal operation, the charge

pump and gate driver turn on the pass MOSFET’s gate to

provide power to the load. The inrush current control is

accomplished by the INRUSH circuit. This circuit limits

the GATE ramp rate to 0.3V/ms and hence controls the

voltage ramp rate of the output capacitor.

The current sense (CS) amplifier monitors the load current

usingthevoltagesensedacrossthecurrentsenseresistor.

The CS amplifier limits the current in the load by reduc-

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

is simple to adjust the current limit threshold using the

during other times such as start-up.

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 value from 5.6A to 1.5A in a linear manner as the

FB pin drops below 0.6V (see the Typical Performance

Characteristics section).

If an overcurrent condition persists, the TIMER pin ramps

up with a 100µA current source until the pin voltage

exceeds 1.235V (comparator TM2). This indicates to the

logic that it is time to turn off the pass MOSFET to prevent

overheating. At this point the TIMER pin ramps down us-

ing the 2µA current source until the voltage drops below

0.21V (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. It is suitable

for many applications to use an internal 2ms overcurrent

timer with a 100ms cooldown period. Tying the TIMER

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 also shows the monitoring blocks

of the LTC4232. The two comparators on the left side

include the UV and OV comparators. These comparators

determineiftheexternalconditionsarevalidpriortoturning

on the MOSFET. But first the undervoltage lockout circuits

UVLO1 and UVLO2 must validate the input supply and

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

external conditions remain valid for 100ms the MOSFET

is allowed to turn on.

Other features include MOSFET current and temperature

monitoring. The current monitor (CM) outputs a current

proportionaltothesenseresistorcurrent.Thiscurrentcan

drive an external resistor or other circuits for monitoring

purposes. A voltage proportional to the MOSFET tempera-

allows external circuits to predict failure and shutdown

the system.