PI2002 Datasheet, PDF (15/23 Pages) Vicor Corporation – Active ORing Controller IC with Load Disconnect Feature

The MOSFET current rating and maximum power
dissipation are closely related. Generally the lower the
MOSFET Rds(on), the higher the current capability
and the lower the resultant power dissipation. This
leads to reduced thermal management overhead, but
will ultimately be higher cost compared to higher
Rds(on) parts. It is important to understand the
primary design goal objectives for the application in
order to effectively trade off the performance of one
MOSFET versus another.
Power dissipation in active ORing circuits is derived
from the total source current and the on-state
resistance of the selected MOSFET.
MOSFET power dissipation:
Pd _MOSFET = Is ² ∗ Rds(on)
Where :
Is
: Source Current
Rds(on) : MOSFET on-state resistance
Note:
In the calculation use Rds(on) at maximum MOSFET
temperature because Rds(on) is temperature
dependent. Refer to the normalized Rds(on) curves in
the MOSFET manufacturers datasheet. Some
MOSFET Rds(on) values may increase by 50% at
125°C compared to values at 25°C.
The Junction Temperature rise is a function of power
dissipation and thermal resistance.
should be adequate for protecting MOSFETs with
maximum Vgs ratings of ±12V or greater.
OV/UV resistor selection:
The UV and OV comparator inputs are used to
monitor the input voltage and will indicate a fault
condition when this voltage is out of range. The UV
and OV pins can be configured in two different ways,
either with a divider on each pin, or with a three-
resistor divider to the same node, enabling the
elimination of one resistor. Under-Voltage is
monitored by the UV pin input and Over-Voltage is
monitored with the OV pin input.
Note: The OV pin is not available in the SO-8 package
and OV function is disabled.
The Fault pin ( FT ) will indicate a fault (active low)
when the UV pin is below the threshold or when the
OV pin is above the threshold. The UV and OV
thresholds are 0.50V typical with 25mV hysteresis and
their input current is less than ±1µA. It is important to
consider the maximum current that will flow in the
resistor divider and maximum error due to UV and OV
input current. Set the resistor current to 100µA or
higher to maintain 1% accuracy for UV and OV due to
the bias current.
The three-resistor voltage divider configuration for
both UV and OV to monitor the same voltage node is
shown in Figure 17:
Ra = ^{V (OVTH )}
I _Ra
Trise_MOSFET = Rth_JA ∗ Pd_MOSFET = Rth_JA ∗ Is² ∗ Rds(on) ,
Where:
Rth_JA : Junction-to-Ambient thermal resistance
Rds(on) and PI2002 sensing:
The PI2002 senses voltage across the ORing
MOSFETs via the SP and SN pins to determine the
status of the current through the MOSFETs. Refer
Figure 1a. When the MOSFETs are fully enhanced,
the total drop across the back to back MOSFETs (and
between SP and SN) is; VSD=Rds(on)*Is * 2.
The reverse current threshold is set for -6mV and
when the differential voltage between the SP & SN
pins is more negative than -6mV, i.e. SP-SN≤-6mV,
the PI2002 detects a reverse current fault condition
and pulls the MOSFET gate pin low to turn off the
MOSFET and prevent further reverse current. The
reverse current fault protection disconnects the power
source fault condition from the redundant bus and
allows the system to keep running.
The GATE pin output voltage is clamped to 11V
maximum with respect to the SN pin. The 11V clamp
Figure 17: UV & OV three-resistor divider
configuration.
Ra = ^{V (OVTH )}
I _Ra
Set Ra value based on system allowable current
I _{Ra Ra =} ^{V (OVTH )}
I _Ra
Rb
=
Ra
•
⎜⎜⎝⎛
V
V
(OV
(UV
)
)
−
1_{⎟⎟⎠⎞}
Rc
=
(Ra
+
Rb) •
⎜⎜⎝⎛
V (UV
V_TH
)
− 1_{⎟⎟⎠⎞}
Where:
V (UV_TH ) : UV threshold voltage
V (OV_TH ) : OV threshold voltage
V(UV) : UV voltage
Picor Corporation • picorpower.com
PI2002
Rev1.1 Page 15 of 23