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FPF1320 / FPF1321 — IntelliMAX™ Dual-Input Single-Output A dvanced Pow er Switch
Operation and Application D e scription
The FPF1320 and FPF13 21 are dual-input single-output
power multiplexer sw itches w ith controlled turn-on and
seamless power s ourc e trans ition. Th e c ore is a 50 mΩ
P-channel MOSFET and controller capable of
functioning over a w ide input operating range of 1.5 V to
5.5 V per channel. The EN and SEL pins are active-
HIGH, GPIO/CMOS-compatible input. They control the
state of the switch and input power source selection,
respectively. TRCB functionality blocks unw anted
reverse current during both ON and OFF states w hen
hi gher VOUT than VINA or VINB is applied. FPF1321 has a
65 Ω output discharge path during off.
Input Capacito r
To limit t he volt age dro p on the input su pply c aused by
trans ient inrus h curr ent when the switch turns on into a
discharged load capac itor; a capacitor must be placed
between the VINA or VIN B pins to the GND pin. At least
1 µF c era mic c apaci tor, CIN, placed clos e to th e pins, is
usually sufficient. Higher-value CIN can be used to
reduce more the v oltage drop.
Inrush C urr en t
Inrush current occurs when the device is turned on.
Inr ush c urr ent is dep end ent on output capac itance and
slew rate control capability, as expressed by:
LOAD
R
INITIALIN
OUTINRUSH
I
tVV
CI +
−
×=
(1)
where:
COUT: Output capacitance;
tR: Slew rate or rise time at VOUT;
VIN: Input voltage, VINA or VINB;
VINITIAL: Initial voltage at COUT, us ually GND; and
ILOAD: Load cur rent.
Higher inrush c urrent c auses h igher in put v o ltage drop,
depen din g on the distrib uted in put res is ta nce an d i nput
capacitance. High inrush current can cause problems.
FPF1320/1 has a 130 µs of slew rate capability under
3.3 VIN at 1 µF of COUT and 150 Ω of R
L so inrush
current and input v oltage drop can be minimized.
Power Source Selec tio n
Input pow er source selection can be controlled by the
SEL pin. When SEL is LOW, output is pow ered from
VINA w hile SEL is HIGH, VINB is pow ering output. The
SEL signal is ignored during device OFF.
Output V oltage Dro p dur ing T ransitio n
Output voltage drop usually occurs during input pow er
source transition period from low voltage to high
voltage. The drop is highly dependent on output
capac itanc e and load cur rent.
FPF1320/1 adopts an advanced break-before-make
control, w hich can result in minimized output voltage
drop during the transition time.
Output Capacito r
Capacitor COUT of at least 1 µF is hi ghly rec o mmen ded
between the VOUT and G ND pins to achieve minimized
output voltage drop during input power source transition.
This capac itor also prevents paras itic board inductanc e.
True Reverse-Cu r rent Blocki ng
The true reverse-current blocking feature protects the
input source against current flow from output to input
regardless of whether the load switc h is on or of f.
Board L ay out
For bes t p erf or manc e, a ll trac es s hou ld be as s hort as
possible. To be most effective, the input and output
capacitors should be placed close to the device to
minimize the effect that parasitic trace inductance on
normal and shor t-circuit operat ion . Wide trac es or large
copper planes for power pins (VINA, VIN B, VOUT and
GND) minimize the parasitic electrical effects and the
thermal impedance.