PI2007 datasheet(11/19 Pages) VICOR

11 / 19 page



Picor Corporation • picorpower.com

PI2007

Rev 1.3

Page 11 of 19

MAX

MIN







Where:

MIN

: Min input voltage

MAX

: Zener diode maximum breakdown voltage

I :

Zener diode required reverse current

MAX

: Q2 required maximum base current which

calculated from the following equation:

MIN

MAX



MAX

: Q2 maximum expected collector current.

MIN

:Q2 minimum gain.

Fault Indication:

FT is an open collector output and its return is

referenced to SGND. When SGND is referenced to

system ground, FT should be pulled up to the logic

voltage via a re

sistor (10KΩ). When the SGND pin is

floating on a bias resistor (RPG) or in a constant

current circuit, a level shift circuit can be added to

make the FT pin output referenced to the system

ground.

See Figure 19. Leave FT unconnected if

not used.

Note that in case of an input fault condition, where

the input voltage (Vin) and the VC pin are at

ground and the SN pin is at a high voltage, a

parasitic path between SN and VC will draw bias

current (leakage current) from the output as a

function of the voltage between SN and grounded

VC (VSN-GND) based on the following equation:

PAR

GND







Where:

SN leakage current during input short

GND

V 

: Voltage difference between SN pin (or load

voltage) and ground.

PAR

Resistor in the parasitic path, 10KΩ typical

and 8kΩ minimum

Figure 15: SN leakage current vs. SN voltage during

input fault condition (input short)

N-Channel MOSFET Selection:

Several factors affect MOSFET selection including

cost, on-state resistance (RDS(on)), DC current rating,

short pulse current rating, avalanche rating, power

dissipation,

thermal

conductivity,

drain-to-source

breakdown voltage (BVdss),

gate-to-source voltage

rating (Vgs), and gate threshold voltage (Vgs(TH)).

The first step is to select a suitable MOSFET based on

the BVdss requirement for the application. The BVdss

voltage rating should be higher than the applied Vin

voltage plus expected transient voltages.

Stray

parasitic inductance in the circuit can also contribute

to significant transient voltage conditions, particularly

during MOSFET turn-off after a reverse current fault

has been detected.

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 get very high in some

conditions before the MOSFET is turned off.

Make

sure that the MOSFET pulse current capability can

withstand the peak current.

Such high current

conditions will store energy even in a small parasitic

element. Note that PI2007 has a very fast response

time to a fault condition achieving 80ns typical and

150ns maximum.

This fast response time will

minimize the reverse peak current to keep stored

energy and MOSFET avalanche energy very low to

avoid damage (breakdown) to the MOSFET.

Peak current during input short is calculated as

follows, assuming that the output has very low

impedance and it is not a limiting factor:

PARASITIC

RVS

PEAK



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