Description of Operation
The FPF270X family of current limit load switches is
designed to meet the power requirements of a variety of
applications with wide input voltage range of 2.8 V to
36 V and adjustable current-limit value. The FPF270X
family offers control and protection while providing
optimum operation current for safe design practices.
The core of each switch is a typical 88 mΩ (VIN = 12 V)
N-channel MOSFET and a controller capable of
functioning over an input voltage range of 2.8 V to 36 V.
FPF270X offers adjustable current limiting, under-
voltage lockout (UVLO), power-good indicator
(PGOOD), fault flag output (FLAGB), and thermal
shutdown protection. In the event of an over-current
condition, the load switch limits the load to the current
limit value. The current limit value for each switch can
be adjusted from 400 mA to 2 A through the ISET pin.
The ON pin is active LOW for and controls the state of
the switch. Pulling the ON pin continuously to LOW
holds the switch in ON state. The switch moves into
OFF state when the ON pin is pulled HIGH. The ON pin
can be pulled HIGH to a maximum voltage of 5.5 V.
An under-voltage condition on the input voltage or a
junction temperature in excess of 140°C overrides the
ON control and turns off the switch. In addition, an over-
current condition causes the switch to turn off in the
FPF2700 and FPF2701 after the expiration of the
blanking time. The FPF2700 has an auto-restart feature
that automatically turns the switch ON again after the
auto-restart time. For the FPF2701, the ON pin must be
toggled to turn the switch on again. The FPF2702 does
not turn off in response to an over-current condition; it
remains operating in Constant-Current Mode as long as
ON is enabled and the thermal shutdown or UVLO have
not activated. The ON pin does not have internal pull-
down or pull-up resistors and should not be left floating.
Upon detection of an over-current condition, an input
UVLO, or an over-temperature condition, the FLAGB
signals the Fault Mode by activating LOW. In the event
of an over-current condition for the FPF2700 or
FPF2701, the FLAGB goes LOW at the end of the
blanking time (Figure 24 and Figure 25). FLAGB goes
LOW immediately for the FPF2702 (Figure 29). If the
over-current condition lasts longer than blanking time,
FLAGB remains LOW through the auto-restart time for
the FPF2700. For the FPF2701, FLAGB is latched LOW
and ON must be toggled to release it.
For FPF2702, FLAGB is LOW during a fault and
immediately returns HIGH at the end of the fault
condition. FLAGB is an open-drain MOSFET that
requires a pull-up resistor. The maximum pull-up voltage
is 36 V (Figure 29).
During shutdown, the pull-down on FLAGB is disabled
to reduce current draw from the supply. A 100 kΩ pull-
up resistor is recommended in the application.
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-limit
level is adjustable through an external resistor
connected between the ISET pin and GND.
The typical current limit level is adjustable from 510 mA
to 2.5 A. The minimum current limit (ILIM(MIN)) range is
from 0.4 A to 2.0 A, including 20% current-limit
tolerance. The FPF2700 and FPF2701 have a blanking
time during which the switch acts as a constant-current
source (Figure 27). If the over-current condition persists
beyond the blanking time, the FPF2700 latches off and
shuts the switch off (Figure 32). If the ON pin is kept
active, an auto-restart feature releases the switch and
turns the switch on again after the auto-restart time
(Figure 33). If the over-current condition persists beyond
the blanking time, the FPF2701 latch-off feature shuts
the switch off. The switch is kept off until the ON pin is
toggled or input power is cycled. The FPF2702 has no
current-limit blanking period, so it remains in a constant-
current state until the ON pin is deactivated or the
thermal shutdown turns off the switch.
Besides the current-limiting functionality, the switch is
protected by the thermal shutdown protection and an
independent SOA protection circuit is available.
SOA Protection Current Limit (IOUT > 12 A)
FPF270X has an SOA protection feature to protect the
load switch in response to current surges exceeding
12 A in normal operation. If a short-circuit event occurs
(IOUT>12 A), the switch is turned off in about 1µs by an
independent Safe Operating Area (SOA) protection
circuit (Figure 26, Figure 28). This feature protects the
switch in case of sudden, high-current events at the
output, such as a short to GND. The switch turns on
automatically after a turn-on delay of about 2.7 ms.
Short-Circuit Current Limit (VOUT < VSCTH = 2 V)
When the output voltage drops below the short-circuit
threshold voltage, VSCTH, the current-limit value re-
conditions itself to the short-circuit current limit value,
which is 75% of the nominal current limit (0.75 x ILIM,)
(Figure 24). This prevents early thermal shutdown by
reducing the power dissipation of the device. The VSCTH
value is set at 2 V. At about VOUT = 2.1 V, the switch is
removed from short-circuit current-limiting mode and the
current limit is set to the nominal current limit value.
Setting the Current Limit Value
FPF270X has an adjustable 0.4 A to 2.0 A minimum
current limit set through an external resistor, RSET,
connected between ISET and GND. A precision RSET
value must be used, such as 1% tolerance or lower, to
minimize the total current limit tolerance of the system.
Use the following equation to calculate the value of the
resistor for intended typical current limit value:
ILIM (TYP ) (A)
ILIM(TYP) is the typical current limit value based on a RSET.
© 2010 Fairchild Semiconductor Corporation
FPF2700 / FPF2701 / FPF2702 • Rev. 1.0.3