LXE1684 evaluation board user?s guide lx1684 buck pwm controller manufactured by: microsemi power management division garden grove telephone: 714-898-8121 evaluation board informatio n www. microsemi .com more than solutions ? enabling possibilities
LXE1684 evaluation board installation guide copyright ? 2001 page 2 rev. 0.1 9/11/01 new product information www. microsemi . com g eneral d escription the LXE1684 evaluation board is a switching buck power supply which provides a 3.3v output from a 12v power supply utilizing the lx1684 microsemi?s integrated controller. it can source 0a to 10a and requires no minimum load for proper voltage regulation. the LXE1684 uses 12v as its main power source and 5v to bias the lx1684 ic. q uick installation guide it is recommended that you use laboratory power supply with adjustable current limit. ground and power wires used to connect power supply to lx1684 should be 4 inches or less each. wires should be at least 18 gauge wire or larger, we recommend 16 or 14 gauge stranded wires for all power wires and ground wires. additional through hole power and ground pads are provided in order to reduce connector resistance. see silkscreen for more details. 1. turn off power to 5v and 12v power supplies. 2. connect 5v,12v, and gnd from power supplies to 5v,12v and gnd connectors of LXE1684. 3. connect gnd and 3.3v output to test load. 4. turn on power supply. e nvironmental ambient temperature: <70c i nput voltage requirements 12v / 10a 10% 5v / 100ma 5% e lectrical s pecifications parameter value output voltage 3.3v, 0a to 10a 5% output current 10a short circuit protected. switching frequency 175khz 75% at 500ma 92% at 1a 90% at 5a typical efficiencies 87% at 10a l l x x e e 1 1 6 6 8 8 4 4 u u s s e e r r g g u u i i d d e e
LXE1684 evaluation board installation guide copyright ? 2001 page 3 rev. 0.1 9/11/01 new product information www. microsemi . com d etailed d escription with both 5v and 12v power supplies readily available on an at or atx desktop power supply, the lx1684 provides a flexible option for choosing either 5v or 12v as power source. differential current sensing allows more accurate current sensing from 12v which is unavailable in the earlier lx1681/lx1682 controllers. the LXE1684 provides lossless hiccup mode current limiting eliminating the need for an expensive current sense resistor. the unit can survive an output short circuit condition indefinitely without overstressing onboard power mosfets and inductor. power consumption during output short-circuit and during hiccup mode is less than 10% of 10a full current load. utilizing synchronous logic level mosfet rectification, the LXE1684 features high efficiency conversion (typically 87% at full load of 10a for 12v to 3.3v application). with a switching frequency of about 200khz, it provides an optimum efficiency and low ripple output voltage over a full range of output current load. features ? high efficiency switching 3.3v power supply ? up to 92% efficiency for 12v to 3.3v application. ? internally compensated 16mhz bandwidth opamp provides fast transient response ? lossless hiccup r ds_on sensing current limit protection ? voltage mode opamp requires no external compensation ? built in mosfet drivers reduce overall cost. ? short circuit protection: can sustain output short circuit without overstressing mosfet and inductors on board, ? differential kelvin current sensing provides superior performance than lx1681/lx1682 in terms of ease of use, accuracy and noise immunity. capacitor selection the LXE1684 are shipped from factory with aluminum electrolytic capacitors in order to minimize overall cost. the designer should choose long life, 105c high temperature and high maximum ripple current electrolytic capacitors in order to meet the demand of high ripple currents inherent to dc/dc high current switching power supplies. traditional aluminum electrolytic capacitors however are bulky in comparison to newer type of special polymer dielectric or organic dielectric capacitors. the LXE1684 pcb layout therefore is designed with both surface mount pads and through hole pads for its power components such as fets, inductors and capacitors, can be easily reconfigured to use all surface mount components for high performance and low profile space constrained applications such as video cards, memory power supply and other pci bus and agp bus multimedia interface cards. the major advantage of lx1684 over lx1681 is that with differential current sensing with both positive current sense and negative current sense, the topside power mosfet can be far away from lx1684 and/or have different voltage and still provide superior performance in comparison with the lx1682. the lx1684 does not limit the power input option to 5v, if current sensing is used. with 5v readily available on desktop at and atx power supplies, the LXE1684 can provide a dc to dc power supply which can draw power from 12v, 5v or 3.3v to provide output voltages as low as 1.25v while providing lossless r ds_on sensing for all power input options. ripple voltage can be lower than 10mv if high performance poly capacitors or oscon capacitors are used. the lx1684 provides stable operation with total output capacitor esr as low as 5m ? when used in a dc to dc buck configuration. l l x x e e 1 1 6 6 8 8 4 4 u u s s e e r r g g u u i i d d e e
LXE1684 evaluation board installation guide copyright ? 2001 page 4 rev. 0.1 9/11/01 new product information www. microsemi . com mosfet selection the top and bottom fets must be logic level mosfets. for a 12v power input application the total gate capacitance of each mosfet should be less than 50nc. this is due to the fact that soic-14 package has a junction to ambient temperature coefficient of 165c per watt. maximum v ds rating of mosfets for 12v operation should be at least 30v. the LXE1684 can accommodate top and bottom mosfets with both through hole and surface mount footprints such as to-220, smd220, to-263,to-252, sot-223 such as the vishay logic mosfet sub45n05-20l. please refer to lx1684 data sheet for mosfet selections. for 12v application, care must be taken in selecting mosfet to make sure that lx1684 junction temperature is within acceptable limits. die temperature should be less than 125c. for 12v to 3.3v application , the current flow through the bottom mosfet is more than twice the current flow through the top mosfet. the power supply designer therefore should choose a larger mosfet and larger heat sink for the bottom mosfet. please refer to lx1684 data sheet for calculation of power dissipation on the top and bottom mosfets. LXE1684 evaluation board schematic fb nc ss csp gnd pgnd bdr tdr nc vc1 nc cs nc vcc css 0.1f r2 100 ohm r1 162 ohm c5 1f 5v 5% v in controller rset 3.3k l1 10f q2 sub45n05- 20l 3x 820f, 16v q1 sub45n 05-20l 12v 10% v in power* c1 0.1f 3.3v 10a 5817smt c4,c4a,c4b l1 is a powdered iron toroid 1 2 3 4 5 6 7 14 13 12 11 10 9 8 d1a0 vout c3b,c,d,e 4x 1500f 6.3v v out = v ref (1+r1/r2) *v pwr can be reconfigured to any voltage between 3v and 13.2v. this change may require different a input/output capacitance and changes to the output voltage divider resistors, etc, see data sheet. l l x x e e 1 1 6 6 8 8 4 4 u u s s e e r r g g u u i i d d e e
LXE1684 evaluation board installation guide copyright ? 2001 page 5 rev. 0.1 9/11/01 new product information www. microsemi . com LXE1684 component li LXE1684 component li LXE1684 component li LXE1684 component list st st st miscellaneous compon miscellaneous compon miscellaneous compon miscellaneous components ents ents ents part description manufacturer & part # reference designators qty buck pwm controller microsemi lx1684 u1 1 n-channel mosfet vishay sub45n05-20l q1, q2 2 schottky diode microsemi 1n5817 d1a0 1 inductor, 10uh toroid iron powder micrometal l1 1 capacitors capacitors capacitors capacitors part description manufacturer & part # reference designators qty capacitor, 0.1f, ceramic 1206, 50v any c1,css 1 capacitor, 1500f, 6.3v, al elect. panasonic eeu-fc0j152 c3b, c3c, c3d, c3e 4 capacitor, 820f, 16v, al elect. panasonic eeu-fc1c821s c4, c4b, c4c 3 capacitor, 1f, ceramic 1206 any c5 1 resistors resistors resistors resistors part description manufacturer & part # reference designators qty resistor, 162, 1%, 1206 any r1 1 resistor, 100, 1%, 1206 any r2 1 resistor, 3.3k, 5%, 1206 any r set 1 additional power and ground through hole pads for improved current conduction and ease of use. user can solder wires directly from power supply and to load without removing components. l l x x e e 1 1 6 6 8 8 4 4 u u s s e e r r g g u u i i d d e e
LXE1684 evaluation board installation guide copyright ? 2001 page 6 rev. 0.1 9/11/01 new product information www. microsemi . com silk screen silk screen silk screen silk screen solder side component side l l x x e e 1 1 6 6 8 8 4 4 u u s s e e r r g g u u i i d d e e
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