QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1066
POSITIVE HIGH VOLTAGE IDEAL DIODE-OR
LTC4355
DESCRIPTION
Demonstration Circuit 1066 showcases the
LTC4355 positive high voltage diode-OR controller
and monitor in a 200W, 48V application. Included
on board are two S-8 ORing MOSFETs and five
LEDs to indicate a variety of fault conditions.
The 48V inputs are separated from ground and
from each other with at least 60 mils spacing wher-
ever possible. Input and output connections are
FEATURES
*Tiny Size Facilitates Grafting into Working System
*Two 5A Diode Channels Controlled by One LTC4355
*Dual Layout for S-8 or D2Pak MOSFETs
*0.093-inch Turret Holes Accommodate 12 AWG
Wire
made by 93 mil turrets which if removed, accom-
modate insertion of up to 12 gauge wire for in-situ
testing.
Design files for this circuit board are available.
Call the LTC factory.
LTC is a trademark of Linear Technology Corporation
APPLICATIONS
*Servers, Routers, Switches
*Mass Storage
*Central Office Computing
*Fan Trays
*ATCA
Table 1.
VIN
IOUT
Typical Performance Summary (TA = 25°C)
CONDITIONS
MIN
TYP
MAX
UNITS
SYMBOL PARAMETER
Input Operating Range
Peak Input Voltage
Maximum Load Current
Clipped by Transient Voltage Suppressor
Limited by MOSFET Dissipation
Limited by Fuses
9
77
5
7
48
77
100
V
V
A
A
Board Layout
The 93-mil turrets are not swaged and may be re-
moved for in-situ testing. The plated-through holes
can accommodate up to 12 gauge wire. The
FDS3672 MOSFETs handle 5A continuous load cur-
rent with no air flow and a total temperature rise of
about 30-40 Celsius across the board. The bottom
of the board contains pads for optional D2Pak
MOSFETs for applications up to 20A. Remove the
top-side S-8 MOSFETs if D2Paks are installed.
Current Capability
The FDS3672 S-8 MOSFETs are capable of handling
up to 7.5A for short periods, limited by the thermal
characteristics of the package. A continuous load
current of 5A is permissible with the board laying
face-up on a lab bench and convection cooling.
Thus situated and carrying 5A load current, either
MOSFET experiences a junction temperature rise of
approximately 50-55 Celsius.
Resistive losses total about 15 milliohms exclusive
of MOSFET resistance; the fuse accounts for about
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1066
POSITIVE HIGH VOLTAGE IDEAL DIODE-OR
Operating Voltage Range
DC1066 is designed for 48V applications. Neverthe-
less, the "diode" action of the board operates down
to the minimum supply voltage for the
LTC4355CDE of 9V. Other functionality will be af-
fected or lost including dim LEDs, indication of
power faults on each channel (the threshold is
34.1V), and reduced gate drive.
To modify the board for a new operating range
simply resize the LED resistors (R5-R9) to a value
of Vinmax/5mA, and change R2 and R4 to detect
under voltage at the desired point. The simple volt-
age divider calculations so efficiently covered in the
LTC4355 data sheet need not be repeated here. For
applications below 20V where minimum gate drive
is guaranteed at 4.5V, use logic level MOSFETs.
Modifying for Other Current Ranges
To modify the board for other current ranges, re-
place the fuses and MOSFETs. A good rule of
thumb for selecting MOSFETs is to select an Rds
(on) which produces 100 to 200mV drop at maxi-
mum load current. This gives a substantial im-
provement in losses over a Schottky diode solution.
Sufficient copper is available on DC1066 to handle
about 20A. Suggested devices for currents in the 5-
20A range include IRFS4710, IRF3710S,
IRF1310NS and FDB3632. The bottom of the board
contains pads for D2Pak MOSFETs. Remove the
top-side S-8 MOSFETs if D2Paks are installed.
In high current applications verify your choice of
MOSFET by checking its current ratings and calcu-
lating the dissipation. For example, 200mV drop at
20A (4W) makes DC1066 fairly warm with a maxi-
mum board temperature rise of 80-90 Celsius.
Above 4W, abandon DC1066 and attach the MOS-
FETs to a heatsink.
13 milliohms of this figure and is not an insignifi-
cant source of dissipation.
At 5A load, MOSFET dissipation is approximately
600mW. In contrast, the power loss of an equiva-
lent passive Schottky diode would measure about
3W. This represents a power and thermal area sav-
ings of 5X. At 3A load current the improvement is
even more dramatic.
Fault Pins
Green LEDs show the status of the five fault pins. If
the LED is on there is no problem; extinguished
LEDs indicate problems with a fuse, low input volt-
age, or excessive Vds across one or both of the
MOSFETs.
The LTC4355 detects excessive Vds across one or
both MOSFETs. Optional resistor R10 located on
the bottom of the board allows for three choices of
Vds fault threshold:
R10=0; Vds fault threshold=250mV
R10=100k; Vds fault threshold=500mV
R10=open; Vds fault threshold=1.5V
DC1066 is built with R10=open for a Vds fault
threshold of 1.5V.
Note that the fault pins are limited to 8V abs/max,
yet the pull-up resistors (R5-R9) are powered by
the 48V output. The LEDs serve as clamps and if
you remove them, the fault pins will be destroyed. If
you want to interface the fault pins to logic circuits,
remove BOTH the LED and the attendant pull-up
resistor.
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