PI2127 datasheet(11/19 Pages) VICOR

11 / 19 page



Picor Corporation • picorpower.com

PI2127

Rev 1.3

Page 11 of 19

PGMin

−

: Controller minimum clamp voltage, 11V

max

Controller maximum bias current, use

2.0mA

0.1mA is added for margin

Example: 40V <VS-PG <50V

−

PGMax

max

min

()

PdR

PGMin

116

)

(

max

−

Alternative Bias Circuit with Device Enable:

Constant current circuit

In a wide operating input voltage range the size of RPG

may be become large to support power dissipation. A

simple constant current circuit can be used instead of

RPG to reduce power dissipation and can be used as a

device enable.

As shown in Figure 20, the constant current circuit

consists of an NPN transistor (Q1), Zener diode DZ,

current limit resistor (RLIMIT) and Zener bias resistor

(RZ). RLIMIT and RZ can be very low power resistors

and Q1 is a signal transistor where its Collector-

Emitter Voltage (VCEO) is equal or greater than the

input operating voltage and supports 2.5mA at the

operating input voltage.

Figure 20: Constant current bias circuit

Pulling the Q1 base (EN) to the system return (RTN)

will turn off the transistor and the controller return (PG

pin) will float and eventually the MOSFET will be

turned off. An open collector device can be used to

enable and disable the PI2127.

The constant current circuit should guarantee current

greater than the PI2127 maximum Quiescent current

(IVC), 2.0mA.

RLIMIT can be calculated from the following equation:

MAX

MIN

LIMIT

)

(

−

Where:

MIN

: Minimum Zener diode voltage

)

(on

: Q1 Base-Emitter On maximum voltage, for

default use

)

(on

=0.7V

Zener Diode Selection:

Select a Zener diode with a low reverse current

requirement to minimize RZ. Zener diodes with higher

break down voltage will have lower reverse current

and reduce Q1 collector current variation.

Zener

diodes with a breakdown voltage of 6V and higher will

require low bias current for accurate voltage

breakdown.

RZ maximum value can be calculated with the

following equation:

Note that the surface mount resistors have limited

operating voltage capability. Be sure to pick a resistor

package that can meet the maximum operating

voltage (Vin).

MAX

MIN

−

Where:

MIN

: Min input voltage

MAX

: Zener diode maximum breakdown voltage

I :

Zener diode required reverse current

MAX

: Q1 required maximum base current which

calculated from the following equation:

MIN

MAX

: Q1 maximum expected collector current.

MIN

: Q1 minimum gain.

Internal N-Channel MOSFET BVDSS:

The

PI2127’s

internal

N-Channel

MOSFET

breakdown voltage (BVDSS) is rated for 60V at 25°C

and will degrade to 55.5V at -40°C, refer to Figure 10.

Drain to source voltage should not exceed BVDSS in

nominal operation. During a fast switching transient

the MOSFET can tolerate voltages higher than its

BVDSS rating under avalanche conditions, refer to the

Absolute Maximum Ratings table.

In Active ORing applications when one of the input

power sources is shorted, a large reverse current is

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