(LTM802x Series used this discussion) David LTM8020, LTM8021, LTM

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(LTM802x Series used this discussion) David LTM8020, LTM8021, LTM

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Application Note April 2008 DC/DC µModule Regulator Printed Circuit Board Design Guidelines
(LTM802x Series used this discussion)
David LTM8020, LTM8021, LTM8022 LTM8023 Modules complete easy-to-use encapsulated step down DC/DC regulators intended take pain aggravation implementing switching power supply onto system board. With Module, only need input cap, output resistors complete design. might imagine, this high level integration greatly simplifies task printed circuit board design, reducing effort four categories: component footprint generation, component placement, routing nets, thermal vias. Component Footprint Generation first things when designing printed circuit board generate footprint decal each component. components required complete LTM8020, LTM8021, LTM8022 LTM8023 designs common resistors capacitors that have industry standard footprints. basic information necessary generate footprint LTM8020, LTM8021, LTM8022 LTM8023 given package outline drawing, which found "Package Description" section data sheet, which also accessible online indexed package drawing number, which also found "Package Description" section data sheet. Next, choose size accordance with Linear Technology Application Note 100. LTM8020, LTM8021, LTM8022, LTM8023, square pads with sides measuring between 0.025" 0.029" will work most applications. recommendations application note, make pads non-solder mask defined (NMSD), with solder mask expansion zero 0.002", 0.05mm. LTM80xx series Modules have both power connections. connections typically routed with trace. power ground connections usually hooked laying planes. Module footprint uses solder mask expansion zero, pads will same size. solder mask expansion greater than zero, power pads will bigger than pads. Take moment examine pattern. Note that pads surrounding have been depopulated. reason this because each LTM8020, LTM8021, LTM8022 LTM8023 Modules rated 36VDC operation. According 2221, Generic Standard Printed Board Design, Table 6-1, uncoated external printed circuit board conductors, such solder pads, with between them must separated least 0.6mm, 0.0236". LTM80xx series Modules, square pads 0.025" side, placed 0.050" pitch. Module operates above steady state, actual printed circuit board exceed 0.0257" before violating IPC-2221 standard. this case, best make footprint opening 0.025" NMSD with zero solder mask expansion. adjacent pins operate above 30V, size with opening between 0.025" 0.029" will suffice. Component Placement general, components should placed which result traces that short possible. There very components down, located near edge device, which simplifies design. example, LTM8020, typical components Module, single output voltage resistor, along with input output cap. order keep traces short possible, place resistor RADJ adjacent pad, input next output COUT next
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Application Note
VOUT. example this shown Figure keep output noise minimum, better place COUT shown Figure where connection COUT farther away from that CIN. There large current pulses flowing through CIN, moving COUT connection away from that will reduce coupling between capacitors.
RADJ
VOUT
BIAS COUT
RADJ
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SHARE SYNC PGOOD RUN/SS
Figure LTM8020 Example Component Placement
COUT BIAS
case LTM8022 LTM8023, there caps resistors. component placement guidelines very similar those LTM8020. Place resistor RADJ adjacent pad, input next output COUT next VOUT. remaining part, needs placed close possible Module pad. This shown Figure
RADJ
VOUT
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Figure Moving COUT Capacitor Away from Reduce Output Noise
SHARE SYNC PGOOD RUN/SS BIAS
Routing Nets With number components, components placed that traces short possible, routing nets pretty straightforward. task routing nets breaks down into routing traces laying planes. Traces used rout current nets. LTM8023 example above, traces used everything except VIN, VOUT nets. simply connect RADJ resistors. BIAS RUN/SS connections depend upon specific design. LTM8022 LTM8023 patterns designed with layout mind. most applications, BIAS connected either VAUX VIN, BIAS conveniently located easy access VAUX VIN. Furthermore, those cases where connection external voltage source necessary, BIAS located adjacent edge row, allowing easy access external circuitry. RUN/SS either connected external
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VOUT
COUT
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Figure LTM8023 Example Component Placement
also possible place COUT capacitor alternate position, where connection close that CIN. most applications, location COUT relative critical. circuits where important
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Application Note
signal source, located next pads outer row. Figure shows example LTM8023 where BIAS connected VAUX RUN/SS connected VIN, connections. traces shown gray. remaining pins, SHARE, SYNC also easily accessed.
PLANE RADJ
SHARE SYNC PGOOD RUN/SS
COUT RADJ VOUT SYNC PGOOD
BIAS
SHARE
RUN/SS COUT BIAS VOUT PLANE PLANE
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VOUT
Figure Suggested Power Ground Planes LTM8023. Note Wide that Separates High Voltage From Other Nets, Compliance with IPC-2221
nets carried multiple layers, vias should added them, well.
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Figure Placement Pins Makes Them Easy Route.
Next, place power ground plane, which should wide possible good thermal performance. shown Figure these planes, shown gray, should together fill nearly remaining copper area underneath Module. example Figure there wide between other planes, which applies greater than input voltages. Thermal Electrical Interconnect Vias last task place thermal electrical interconnect vias. LTM8020, LTM8021, LTM8022 LTM8023 Modules printed circuit board spread power dissipated within product, important place vias underneath around Module distribute heat throughout layers printed circuit board. general, printed circuit board will have several ground planes, adding vias should easily accomplished. VOUT
Most printed circuit board designs consider planes both electrically thermally equipotential. That voltage gradient across plane assumed ideal zero volts, that thermal resistance from point other point negligible. This actually true, especially from thermal perspective, using vias simple inexpensive achieving design whose performance approaches this ideal. Figure shows layout example with interconnect vias VIN, VOUT planes. vias same size, with 0.010" drill hole 0.015" outer diameter. This sized fits easily between pads same net, good current carrying capability. With careful placement, vias with 0.035" outer diameter used without overlapping adjacent pads. Vias very good electrical conductors interior planes serve heat pipes allow printed circuit board heatsink. best performance reliability, Module should operated cool possible, might conclude that best design
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Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
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Application Note
PLANE THERMAL VIAs RADJ
SHARE SYNC PGOOD RUN/SS
many vias that will possibly fit. Each via, however, starts hole drilled into board, which reduces amount copper that present printed circuit board layers electrical conduction. thus possible have many vias, please consult your organization's design guidelines. Conclusion high level integration LTM8020, LTM8021, LTM8022 LTM8023 Modules simplifies task printed circuit board design your power system. whole summarized four tasks shown Table
COUT
BIAS
VOUT
VOUT PLANE PLANE
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Figure Vias Conduct Electrical Power Internal Planes Heat Pipes Distribute Heat Throughout Printed Circuit Board. Table Module Printed Circuit Board Design
TASK Footprint Generation DESCRIPTION/NOTES package outline drawing from datasheet adjacent pins have greater than 30V, 0.025" NSMD pads, otherwise, 0.025" 0.029" NSMD pads solder mask expansion between zero 0.002" Component Placement Route Nets Thermal Electrical Interconnect Vias Place components keep trace lengths short possible Physically separate COUT connections minimum switching noise required layout design features minimize routing complexity Extend copper planes practical good thermal performance vias connect internal layers Place vias conduct heat internal layers 0.010" 0.015" fits between pads same 0.035" vias used higher heat transfer with careful placement
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Linear Technology Corporation
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(408) 432-1900 FAX: (408) 434-0507
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LINEAR TECHNOLOGY CORPORATION 2008

(LTM802x Series used this discussion) David LTM8020, LTM8021, LTM

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