DC1066A - Demo Manual

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 wherever possible. Input and output connections are
made by 93 mil turrets which if removed, accommodate 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
FEATURES
APPLICATIONS
*Tiny Size Facilitates Grafting into Working System
*Servers, Routers, Switches
*Two 5A Diode Channels Controlled by One LTC4355
*Mass Storage
*Dual Layout for S-8 or D2Pak MOSFETs
*Central Office Computing
*0.093-inch Turret Holes Accommodate 12 AWG
Wire
*Fan Trays
Table 1.
Typical Performance Summary (TA = 25°C)
SYMBOL PARAMETER
VIN
IOUT
*ATCA
Input Operating Range
Peak Input Voltage
Maximum Load Current
CONDITIONS
MIN
TYP
MAX
UNITS
48
Clipped by Transient Voltage Suppressor
Limited by MOSFET Dissipation
Limited by Fuses
9
77
5
7
77
100
V
V
A
A
Board Layout
Current Capability
The 93-mil turrets are not swaged and may be removed for in-situ testing. The plated-through holes
can accommodate up to 12 gauge wire. The
FDS3672 MOSFETs handle 5A continuous load current 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.
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
13 milliohms of this figure and is not an insignificant source of dissipation.
At 5A load, MOSFET dissipation is approximately
600mW. In contrast, the power loss of an equivalent passive Schottky diode would measure about
3W. This represents a power and thermal area savings 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 voltage, 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.
Operating Voltage Range
DC1066 is designed for 48V applications. Nevertheless, the "diode" action of the board operates down
to the minimum supply voltage for the
LTC4355CDE of 9V. Other functionality will be affected 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 voltage 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, replace 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 maximum load current. This gives a substantial improvement in losses over a Schottky diode solution.
Sufficient copper is available on DC1066 to handle
about 20A. Suggested devices for currents in the 520A 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 calculating the dissipation. For example, 200mV drop at
20A (4W) makes DC1066 fairly warm with a maximum board temperature rise of 80-90 Celsius.
Above 4W, abandon DC1066 and attach the MOSFETs to a heatsink.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1066
POSITIVE HIGH VOLTAGE IDEAL DIODE-OR
QUICK START PROCEDURE
Connect 48V power sources to +48VA and +48VB,
with the combined returns connected to GROUND.
Connect the load between +48VOUT and GROUND.
A 10 ohm, 250W resistor makes a good load. The
larger magnitude supply will source current as controlled by the diode action of the LTC4355.
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1066
POSITIVE HIGH VOLTAGE IDEAL DIODE-OR
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