HV301/HV311
Application Information
Supported External Pass Devices
Paralleling External Pass Transistors
The HV301 and HV311 are designed to support N-Channel
MOSFETs and IGBTs.
Due to variations in threshold voltages and gain characteristics
between samples of transistors reliable 50% current sharing is
not achievable. Some measure of paralleling may be accom-
plished by adding resistors in series with the source of each
device; however, it will cause increased voltage drop and power
dissipation.
Selection of External Pass Devices
Since the current limit is likely to be set just slightly higher than
maximum continuous load current in a typical system, the
continuous current rating of the device will have to be at least
equal to the current limit value.
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
HV301 with active high PWRGD is recommended where the
PWRGD pins of multiple hot swap circuits can be connected in
a wired OR configuration.
The RDS(ON) of the device is likely to be selected based on
allowable voltage drop after the hot swap action has been
completed. Thus the continuous power dissipation rating of the
device can be determined from the following equation:
P
= RDS(ON) × IL2IMIT
CONT
The peak power rating may be calculated from the following
equation:
P
= VIN × ILIMIT
PEAK
Given these values an external pass transistor may be selected
from the manufacturers data sheet.
Selection of Current Sense Resistor
The power rating of the sense resistor must be greater than
Il2oad ×R
, where Iload is the normal maximum operating load.
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 respec-
tive ends of the current sense resistor is recommended.
To
V
EE
Pin
To
SENSE
Pin
To Negative
Terminal of
Power Source
To Source
of MOSFET
Sense Resistor
9
SUPERTEX [ Supertex, Inc ]
