PI2127 Datasheet, PDF (11/19 Pages) Vicor Corporation – 60 Volt, 12 Amp Full-Function Active ORing Solution


V_{S −PGMin} : Controller minimum clamp voltage, 11V
I_{VC max} : Controller maximum bias current, use
2.0mA
0.1mA : 0.1mA is added for margin
Example: 40V <V_S-PG <50V
R_PG
=
V_{S min} − V_{S −PGMax}
IC_max + 0.1mA
=
40V −12.5V
2.1mA
= 13.1kΩ
( ) _PdRPG
=
(V_{S −max}
− V_{S −PGMin} ) ²
R_PG
=
50V −11V
13.1kΩ
2
= 116mW
Alternative Bias Circuit with Device Enable:
Constant current circuit
In a wide operating input voltage range the size of R_PG
may be become large to support power dissipation. A
simple constant current circuit can be used instead of
R_PG 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 D_Z,
current limit resistor (R_LIMIT) and Zener bias resistor
(R_Z). R_LIMIT and R_Z can be very low power resistors
and Q1 is a signal transistor where its Collector-
Emitter Voltage (V_CEO) 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
(I_VC), 2.0mA.
R_LIMIT can be calculated from the following equation:
R_LIMIT
= ^{VZ _ MIN − VBE (on)}
I _{VC _ MAX}
Where:
V_{Z _ MIN} : Minimum Zener diode voltage
V_BE (on) : Q1 Base-Emitter On maximum voltage, for
default use V_BE (on) =0.7V
Zener Diode Selection:
Select a Zener diode with a low reverse current
requirement to minimize R_Z. 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.
R_Z 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).
R_Z
= ^{Vin _ MIN − VZ _ MAX}
I _Z + I _{B _ MAX}
Where:
V_{in _ MIN} : Min input voltage
V_{Z _ MAX} : Zener diode maximum breakdown voltage
I_Z :
Zener diode required reverse current
I _{B _ MAX} : Q1 required maximum base current which
calculated from the following equation:
I = _{hI B _ MAX}
C _ MAX
FE _ MIN
I _{C _ MAX} : Q1 maximum expected collector current.
h_{FE _ MIN} : Q1 minimum gain.
Internal N-Channel MOSFET BV_DSS:
The PI2127’s internal N-Channel MOSFET
breakdown voltage (BV_DSS) 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 BV_DSS in
nominal operation. During a fast switching transient
the MOSFET can tolerate voltages higher than its
BV_DSS 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
Picor Corporation • picorpower.com
PI2127
Rev 1.3
Page 11 of 19