Functional Description:
The PI2121 integrated Cool-ORing product takes
advantage of two different technologies combining a
1.5mΩ on-state resistance (Rds(on)) single N-
channel MOSFET with high density control circuitry.
This combination provides superior density,
minimizing PCB space to achieve an ideal ORing
diode function, significantly reducing power
dissipation and eliminating the need for heat sinking,
while minimizing design complexity.
Reverse Comparator: RVS
The reverse comparator is the most critical
comparator. It looks for negative voltage caused by
reverse current. When the SN pin is 6mV higher
than the SP pin, the reverse comparator will enable
the BK current source to charge an internal 2pF
capacitor. The blanking timer provides noise filtering
for typical switching power conversion that might
cause premature reverse current detection. Once
the voltage across the capacitor reaches the timer
threshold voltage (1.25V) the MOSFET will be
turned off. The shortest blanking time is 50ns when
BK is connected to ground. The Blanking time will
be added to the controller delay time. The Electrical
Specifications in the DIFFERENTIAL AMPLIFIER
AND COMPARATOR section for Reverse Fault to
Slave Low Delay Time “tRVS-MS or tRVS-SL” is the
controller delay time plus the blanking time.
The PI2121’s 1.5mΩ on-state resistance MOSFET
used in the conduction path enables a dramatic
reduction in power dissipation versus the
performance of a diode used in conventional ORing
applications due to its high forward voltage drop.
This can allow for the elimination of complex heat
sinking
and
other
thermal
management
requirements. Due to the inherent characteristics of
the integrated MOSFET, while the gate remains
enhanced above the gate threshold voltage it will
allow current to flow in the forward and reverse
direction. Ideal ORing applications do not allow for
reverse current flow, so the integrated controller has
to be capable of very fast and accurate detection of
reverse current caused by input power source
failures, and turn off the gate of the MOSFET as
quickly as possible. Once the gate voltage falls
below the gate threshold, the MOSFET is off and the
body diode will be reverse biased preventing reverse
current flow and subsequent excessive voltage
droop on the redundant bus. During forward over-
current conditions caused by load faults, the
controller maintains gate drive to the MOSFET to
keep power dissipation as low as possible,
otherwise the inherent body diode of the MOSFET
would conduct, which has higher effective forward
drop. Conventional ORing solutions using diodes
offer no protection against forward over-current
conditions. During the forward over-current
condition, the PI2121 will provide an active-low fault
flag to the system via the fault pin. The fault flag is
also issued during the reverse current condition,
light load conditions, VC under-voltage, Input Under-
Voltage and Over-Voltage and Over-Temperature
conditions.
Reverse Blanking Timer: BK
Connecting an external resistor ( BK ) between the
R
BK pin and ground will increase the blanking time as
shown in the following chart.
Where:
RBK ≤ 200KΩ
If BK is connected to VC for slave mode operation,
then the blanking time will be about 320ns typically,
and total delay time will be 430ns.
The reverse comparator has 3mV of hysteresis
referenced to SP-SN.
If the conditions are met for a reverse current fault,
then the active-low fault flag output will also indicate
a fault to the system after the 40µs fault delay time.
Differential Amplifier:
The PI2121 integrates a high-speed, low offset
voltage differential amplifier to sense the difference
between the Sense Positive (SP) pin voltage and
Sense Negative (SN) pin voltage with high accuracy.
The amplifier output is connected to three
comparators:
Reverse
comparator,
Forward
comparator, and Forward over-current comparator.
Picor Corporation • picorpower.com
PI2121
Rev. 1.0 Page 6 of 21
VICOR [ VICOR CORPORATION ]
