HV302 / HV312
Design Information - continued
Supported External Pass Devices
The HV302 and HV312 are designed to support N-Channel
MOSFETs and IGBTs.
Selection of External Pass Devices
The RDS(ON) of the device is likely to be selected based on
allowable voltage drop at maximum load (ILOAD(MAX)) after the
Hotswap action has been completed.
Thus the required
continuous power dissipation rating (PCONT) of the device can be
determined from the following equation:
PCONT = RDS(ON) ×I2
LOAD(MAX)
The peak power rating (PPEAK) should be based on the highest
current level, which is always the circuit breaker trip set point (ICB),
and on the assumption that a output is shorted. The peak power
rating may be calculated from the following equation:
PPEAK = VIN ×ICB
Given these values an external pass transistor may be selected
from the manufacturers data sheet.
Paralleling External Pass Transistors
Due to variations in threshold voltages and transconductance
characteristics between samples of MOSFETs, reliable 50%
current sharing is not achievable. Some measure of paralleling
may be accomplished by adding resistors in series with the source
of each device; however, it will cause increased voltage drop and
power dissipation.
Paralleling of external Pass devices is not recommended!
If a sufficiently high current rated external pass transistor cannot
be found then increased current capability may be achieved by
connecting independent Hotswap circuits in parallel, since they act
as current sources during the load capacitor charging time when
the circuits are in current limit. For this application the HV302 with
active high PWRGD is recommended where the PWRGD pins of
multiple Hotswap circuits can be connected in a wired OR
configuration.
Kelvin Connection to Sense Resistor
Physical layout of the printed circuit board is critical for correct
current sensing. Ideally trace routing between the current sense
resistor and the VEE and SENSE pins should be direct and as short
as possible with zero current in the sense traces. The use of
Kelvin connection from SENSE pin and VEE pin to the respective
ends of the current sense resistor is recommended.
To
To
VEE SENSE
Pin Pin
To Negative
Terminal of
To Source
of MOSFET
Power Source
Sense Resistor
10
Rev. D
04/17/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com
SUPERTEX [ Supertex, Inc ]
