Vicor PI2002-EVAL1 Active ORing With Load Disconnect Evaluation Board - Manual (Page 6)

Picor Corporation • www.picorpower.com PI2002-EVAL1 User Guide Rev 1.0 Page 6 of 11
3.0 Over Current Timer: OCT
The OCT off-time is set with the OCT capacitor, where
the specific value can be determined from Figure 5.
Every time an overcurrent condition occurs the PI2002
pulls the Gate pin low, discharges the OCT capacitor
and then starts to charge it again over the
programmed off-time. Only when the OCT capacitor
voltage reaches the OCT threshold (1.75 V) will the
Gate pin then start to charge the MOSFET gates.
4.0 Short Circuit Detect: SCD
SCD pin can be connected to the load directly or
programmed to a higher voltage with a resistor
divider. SCD function allows the user to define the
(Hard Short) voltage level expected if a non-ideal
short circuit occurs at the load. This feature enables
distinguishing between a faulted load versus
powering capacitive and low resistive loads without
entering the OCT mode. This pin can be grounded to
provide a fast gate charge or pulled to Vc for lower
gate current to drive highly capacitive loads with
resulting slow gate charge under the fault condition.
5.0 Auxiliary Power Supply (Vaux):
5.1 The PI2002 Controller has a separate input (VC) that
provides power to the control circuitry and the gate
driver. An internal voltage regulator (VC)
clamps the VC voltage to 15.5 V typically.
5.2 Connect independent power source to Vaux inputs of
PI2002-EVAL1 Evaluation Board to supply power to
the VC input. The Vaux voltage should be 5V higher
than Vin (redundant power source output voltage) to
fully enhance the MOSFET. If the MOSFET is replaced
with a different MOSFET, make sure that the Vaux
voltage is equal to Vin + 0.5 V + the required voltage
to enhance the MOSFET.
5.3 10 Ω bias resistors (Rbias, reference designators R11
and R22) are installed on the PI2002-EVAL1 between
each Vaux input and VC pin of one of the PI2002
controller.
5.4 If Vaux is higher than the Clamp voltage, 15.5 V
typical, the Rbias value has to be changed using the
following equations:
5.4.1 Select the value of Rbias using the
following equations:
Rbias =
Vaux min – VC clampMAX
IC
max
5.4.2 Calculate Rbias maximum power dissipation:
Pd Rbias =
(Vaux max – VC clampMIN ) 2
IC max
Where:
Vaux min : Vaux minimum voltage
Vaux max : Vaux maximum voltage
VC
ClampMAX
: Maximum controller clamp
voltage, 16.0 V
VC
ClampMIN
: Minimum controller clamp
voltage, 14.0 V
IC
max
: Controller maximum bias current,
use 4.2 mA
Figure 5 – OCT off time vs. OCT capacitor value