The FPF2108 , FPF2109 , and FPF2110 are a series of load switches which provide systems and loads which may adequately protect against high current conditions. The device contains a 0.125 Ω current-limiting P-channel MOSFET that operates over an input voltage range of 1.8-5.5V . Internally, back current blocking prevents current flow when the MOSFET turns off and the output voltage is higher than the input voltage. Switch control is through a logic input (ON) that can interface directly with low voltage control signals. Each part includes thermal shutdown protection which turns off the switch to prevent damage to the part when continuous overcurrent conditions can cause overheating. When the switch current reaches the current limit value, the part works in constant current mode to prevent damage caused by excessive current. For the FPF2108, if the constant current condition is still present after 10ms, the part will turn off the switch and pull the fault signal pin (FLAGB) low. The switch will remain closed until the ON pin is cycled. For the FPF2109 and FPF2110, the current limit condition will immediately pull the fault signal pin low and the device will remain in constant current mode until the switch current falls below the current limit. The minimum current limit is 200mA for the FPF2109 and 400mA for the FPF2108 and FPF2110. These devices are available in a space-saving 5-pin SOT23 package.

Typical circuit

Operation Description

FPF2108, FPF2109, FPF2110 is a current limiting switch protection system and capable of applying high currents by damaged or destroyed loads. The heart of the device is a 0.125Ω P-channel MOSFET and is capable of operating over a wide input operating range of 1.8-5.5V from a controller. The controller protects against current limit, undervoltage system fault undervoltage lockout and thermal shutdown. The current limit is preset to either 200mA or 400mA.

Undervoltage Lockout The undervoltage lockout turns on and off if the input switch voltage is below the undervoltage lockout threshold. With the ON pin active the input voltage rises above the undervoltage lockout threshold will cause the switch in a controlled turn which limits current flow through the branch. Thermal Shutdown Thermal shutdown protects parts from excessively high temperatures generated internally or externally. During an over-temperature condition the FLAGB is activated and the switch is turned off. The switch automatically opens again, if the temperature of the mold falls below the threshold temperature. On/OffControl

The ON pin controls the state of the switch. Active HI and LO versions are available. Active pairs remain on the switch continuously as long as there are no failures. For all versions, an undervoltage at VIN or an excess junction temperature of 150°C will override the ON control to close the switch. For the FPF2108, the ON pin must be toggled to open the switch again. The FPF2109 and FPF2110 do not respond to shutdown into an overcurrent condition, but instead remain in a constant current mode of operation as long as there is active and thermal shutdown or undervoltage lockout not activated. When the MOSFET is turned off, the body diode is disabled, so no current can flow through it.

error report

When an overcurrent is detected, an undervoltage input, or an overtemperature condition, the FLAGB signal fails mode by activating the LO. For FPF2108, the FLAGB de-LO at the end of the simultaneous FLAGB de-LO blanking time is immediately ate for FPF2109 and FPF2110. For FPF2108, FLAGB is locked LO and ON must be switched to release it. With FPF2109 and FPF2110, FLAGB is faulted during LO and immediately returns to HI at the end of the fault state. FLAGB is an open-drain MOSFET that requires a pull-up resistor between VIN and FLAGB. During shutdown, the pull-down FLAGB is disabled to reduce current consumption from the supply.

Limiting

The current limit ensures that the current through the switch does not exceed rather than limit to a minimum value that is less than the maximum value. For the FPF2109 the minimum current is 200mA and the maximum current is 400mA for the FPF2108 and the FPF2110 the minimum current is 400mA and the maximum current is 800mA. The FPF2108 has 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 and the FLAGB pin will activate to indicate that current limit has occurred. The FPF2109 and FPF2110 have no current limit blanking period so that FLAGB is activated immediately at the current limit condition. These parts will remain in a constant current state until the ON pin is disabled or thermal shutdown when the switch is turned off.

undervoltage lockout

The undervoltage lockout turns on and off if the input switch voltage falls below the undervoltage lockout threshold. With the ON pin active the input voltage rises above the undervoltage lockout threshold will cause the switch in a controlled turn which limits current flow through the branch.

thermal shutdown thermal

Shutdown protection is partially generated from internal or external excessive temperature. During an over-temperature condition the FLAGB is activated and the switch is turned off. The switch automatically opens again, if the temperature of the mold falls below the threshold temperature.

input capacitance

To limit transient inrush currents caused by voltage drops on the input supply, when the switch turns on into a load discharge capacitor or short circuit, the capacitor needs to be placed between V IN and GND. A 4.7µF ceramic capacitor, CIN, must be placed close to the VIN pin. The higher the value of C, IN can be used to further reduce the voltage drop experienced by the switch being turned on to a large capacitive load.

output capacitor

A 0.1µF capacitor COUT should be placed between VOUT and GND. This capacitor prevents parasitic inductance from forcing VOUT below GND when the switch is turned off.

Power consumption

During normal operation as a switch, the power dissipation is small and has little effect on the operating temperature. The part with the higher current limit will dissipate most of the power and will only typically be the maximum power dissipation when the current limit will occur when the output is shorted to ground. A brief output value will cause the part to operate in a constant current state until thermal shutdown is activated. It will then cycle into and out of thermal shutdown as long as the ON pin is activated, short or not.

circuit board

For best performance, all traces should be as short as possible. To be most effective, the input and output capacitors should be placed close to the device to minimize the effect of parasitic inductances that may have on normal and short-circuit operation. Using wide leads VIN, VOUT and GND will help minimize parasitic electrical effects and minimize case-to-ambient thermal resistance.