PI2125-00-LGIZ datasheet(14/20 Pages) VICOR

14 / 20 page

Blanking Timer:

Connect the blanking timer pin (BK) to GND to

program the device for the fastest reverse

comparator response time of 160ns typical.

increase the blanking time, connect the BK pin to

GND via a resistor to avoid the fault response to

short reverse current pulses. Refer to the plot in the

reverse

comparator

functional

description

for

resistor values versus the reverse blanking time.

Auxiliary Power Supply (Vaux):

Vaux is an independent power source required to

supply power to the VC input. The Vaux voltage

should be 6V higher than Vin (redundant power

source output voltage) to fully enhance the internal

MOSFET.

A bias resistor (Rbias) is required if Vaux is higher

than 15V. Rbias should be connected between the

VC pin and Vaux.

Minimize the resistor value for low Vaux voltage

levels to avoid a voltage drop that may reduce the

VC voltage lower than required to drive the gate of

the internal MOSFET.

Select the value of Rbias using the following

equations:

max

min

Vaux

Rbias

clamp

−

Rbias maximum power dissipation:

Rbias

Vaux

clamp

Rbias

max

)

(

−

Where:

min

Vaux

: Vaux minimum voltage

max

Vaux

: Vaux maximum voltage

Clamp

: Controller clamp voltage, 15.5V

max

: Controller maximum bias current, use

4.2mA

Example: Vaux 20V to 30V

−

Vaux

Rbias

clamp

max

min

Rbias

Vaux

clamp

Rbias

196

)

(

)

(

max

−

Internal N-Channel MOSFET BVdss:

The

PI2125’s

internal

N-Channel

MOSFET

breakdown voltage (BVdss) is rated for 30V at 25°C

and will degrade at -40°C to 28V, refer to Figure 10.

In an application when the MOSFET is turned off

due to a reverse fault, the series parasitic elements

in the circuit may contribute to the MOSFET being

exposed to a voltage higher than its voltage rating.

In Active ORing applications when one of the input

power sources is shorted, a large reverse current is

sourced from the circuit output through the

MOSFET. Depending on the output impedance of

the system, the reverse current may reach over 60A

in some conditions before the MOSFET is turned off.

Such high current conditions will store energy even

in a small parasitic element. For example: a 1nH

parasitic inductance with 60A reverse current will

generate 1.8µJ (½Li

2). When the MOSFET is turned

off, the stored energy will be released and produce a

high negative voltage ringing at the MOSFET

source. At the same time the energy stored at the

drain side of the internal MOSFET will be released

and produce a voltage higher than the load voltage.

This event will create a high voltage difference

between the drain and source of the MOSFET. To

reduce the magnitude of the ringing voltage, add a

ceramic capacitor very close to the source that can

react to the voltage ringing frequency and another

capacitor close to the drain. Recommended values

for the ceramic capacitors are 1µF, refer to C4 and

C6 in Figure 24.

Slave

For a high current application where one PI2125 can

not handle the total load current, multiple PI2125’s

can be paralleled in a master / slave configuration to

support the total current per input. In the Master /

Slave mode, one PI2125 is configured as the master

and the rest are configured as slaves. The slave

( SL ) pin of the master unit will act as an output

driving the units configured in slave mode. The SL

pins of the slave units will act as inputs under the

control of the master.

Tie the BK pin to VC to configure the unit in slave

mode.

Power dissipation:

In active ORing circuits the MOSFET is always on in

steady state operation mode and the power

dissipation is derived from the total source current

and the on-state resistance of the internal MOSFET.

Picor Corporation • picorpower.com

PI2125

Rev 1.0

Page 14 of 20

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