DEMO CIRCUIT 852 LTC4354 QUICK START GUIDE LTC4354 200W Negative Voltage Diode-OR Controller DESCRIPTION Demonstration Circuit 852 showcases the LTC 4354 The small size of this demonstration circuit facilitates negative voltage diode-OR controller and monitor in a grafting into a working system for evaluation purposes. 200W, 36 to 72V application. Included on board are two On the backside of the board there are pads for D2-pak S-8 ORing MOSFETs and an LED to indicate fault MOSFETs. The S-8 MOSFETs included on the board conditions. The 48V inputs are separated from RTN are avalanche rated and carry up to 5A for ATCA ap- and from each other with at least 60 mils spacing, ex- plications. cept for where drain sensing resistors bridge the gap. Design files for this circuit board are available. Call 93-mil turrets make input and output connections if the LTC factory. removed, these accommodate insertion of up to 12 , LTC and LT are registered trademarks of Linear Technology Corporation. ATCA is a gauge wires for in-situ testing. registered trademark of PIC Industrial Computer Manufacturers Group. PERFORMANCE SUMMARY Specifications are at T = 25C A SYMBO PARAMETER CONDITIONS MIN TYP MAX UNITS L Input Supply Range (48VA)(48VB) 100 100 V Maximum Output Current 5 A Commutation Time 700 ns BOARD LAYOUT The top of the board contains the components necessary with only convection cooling. Thus situated and carrying for the application. The 93-mil turrets are not swaged 5A load current, either MOSFET experiences a junction and may be removed before assembly in a working sys- temperature rise of approximately 75C. tem with up to 12-gauge wire. The FDS3672 MOSFETs Copper loss is equivalent to about 2 squares of 1-ounce can handle 5A continuous load current in this board. copper, or 1.2mW, measured from turret to S-8 The bottom of the board contains no components but MOSFET lead. provisions are made for installation of optional D2-pak Dissipation for the combined MOSFET and board loss is MOSFETs. Remove the topside S-8 MOSFETs if others 700mW at 5A. In contrast, the power loss from an are installed on the bottom side. Larger MOSFETs ex- equivalent passive Schottky diode circuit would measure tend the usable range of this board to in excess of 10A. about 3.1W. This represents a power and thermal area The FDS3672 S-8 MOSFETs are capable of handling up savings of more than 4X. to 7.5A for short periods, limited by the thermal charac- A clear line of demarcation separates RTN, 48VA and teristics of the board. A continuous load current of 5A is 48VB, with a spacing of at least 60 mils to any adjacent permissible with the board laying face-up on a lab bench trace. Spacing necks down in the vicinity of R1 and R2. 1 LTC4354 To increase the voltage capability of this area, use either 13.7A to trip the FAULT pin. Although this exceeds the larger resistors (1206, for example) or apply conformal MOSFET s maximum continuous current rating, the de- coating. Since operational spacing is ultimately limited vice can handle such a current for up to 400ms. As the by the LTC4354 s DFN package, the latter is preferred. MOSFET heats up, its resistance increases to a point An S-8 package (LTC4354CS8 or LTC4354IS8) with where as little as 7.5A drain current is necessary to trip wider pad spacing is also available. the FAULT detector. Under normal operating conditions LED1 will not illuminate. The LTC4354 detects excessive Vds (>260mV) across the MOSFETs and reports this to the FAULT pin, assert- Note that the aforementioned Vds detection is performed ing it high and illuminating LED1. As a result the LED in a negative direction, asserting FAULT when Vd is not only indicates faults, but also doubles as a clamp for 260mV or more negative with respect to Vs. This corre- the FAULT pin, limiting the high-state voltage to less sponds to either an open MOSFET, or current flow far in than 2V. excess of the intended operating range. At room temperature the typical resistance of the FDS3672 MOSFET is 19mW, requiring a current of QUICK START PROCEDURE Connect 48V power sources to 48VA and 48VB, The turrets are installed without swaging in 93-mil with the combined returns connected to RTN. The 48V plated through holes. They may be removed for insertion output is taken at 48VOUT. The larger magnitude sup- of up to 12 AWG wire for in-situ testing. ply will source current as controlled by the diode action of the LTC4354. Figure 1. Proper Measurement Equipment Setup 2