DC1066A - LINEAR TECHNOLOGY

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

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

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

APPLICATIONS

*Servers, Routers, Switches

*Mass Storage

*Central Office Computing

*Fan Trays

*ATCA

Table 1.

Typical Performance Summary (TA = 25°C)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Input Operating Range

9 48 77 V

Peak Input Voltage Clipped by Transient Voltage Suppressor 77 100 V

IOUT Maximum Load Current Limited by MOSFET Dissipation

Limited by Fuses

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

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1066

POSITIVE HIGH VOLTAGE IDEAL DIODE-OR

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.

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.

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 con-

trolled by the diode action of the LTC4355.

Figure 1.

Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1066

POSITIVE HIGH VOLTAGE IDEAL DIODE-OR