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FPF2123-FPF2125 Rev. F

Description of Operation

The FPF2123, FPF2124, and FPF2125 are current limited

switches that protect systems and loads which can be damaged

or disrupted by the application of high currents. The core of

each device is a 0.125

P-channel MOSFET and a controller

capable of functioning over a wide input operating range of

1.8-5.5V. The controller protects against system malfunctions

through current limiting under-voltage lockout and thermal

shutdown. The current limit is adjustable from 150mA to 1.5A

through the selection of an external resistor.

On/Off Control

The ON pin controls the state of the switch. When ON is high,

the switch is in the on state. Activating ON continuously holds

the switch in the on state so long as there is no fault. For all

excess of 140°C overrides the ON control to turn off the switch.

In addition, excessive currents will cause the switch to turn off in

the FPF2123 and FPF2124. The FPF2123 has an Auto-Restart

feature which will automatically turn the switch on again after

160ms. For the FPF2124, the ON pin must be toggled to turn-on

the switch again. The FPF2125 does not turn off in response to

an over current condition but instead remains operating in a

constant current mode so long as ON is active and the thermal

shutdown or under-voltage lockout have not activated.

recommended operating ranges. The ON pin voltage can be

driven by a voltage level higher than the input voltage.

Current Limiting

The current limit ensures that the current through the switch

doesn't exceed a maximum value while not limiting at less than

a minimum value. The current at which the parts will limit is

adjustable through the selection of an external resistor

connected to ISET. Information for selecting the resistor is found

in the Application Info section. The FPF2123 and FPF2124

have a blanking time of 10ms, nominally, during which the

switch will act as a constant current source. At the end of the

blanking time, the switch will be turned-off. The FPF2125 has

no current limit blanking period so it will remain in a constant

current state until the ON pin is deactivated or the thermal

shutdown turns-off the switch.

Under-Voltage Lockout

The under-voltage lockout turns-off the switch if the input

voltage drops below the under-voltage lockout threshold. With

the ON pin active, the input voltage rising above the

under-voltage lockout threshold will cause a controlled turn-on

of the switch which limits current over-shoots.

Thermal Shutdown

The thermal shutdown protects the die from internally or

externally generated

excessive

temperatures. During

an

over-temperature condition the switch is turned-off. The switch

automatically turns-on again if the temperature of the die drops

below the threshold temperature.