Datasheet

Layout Considerations
TPS51116
SLUS609H – MAY 2004 – REVISED JULY 2009 ................................................................................................................................................................ www.ti.com
This thermal resistance strongly depends on the board layout. TPS51116 is assembled in a thermally enhanced
PowerPAD™ package that has exposed die pad underneath the body. For improved thermal performance, this
die pad needs to be attached to ground trace via thermal land on the PCB. This ground trace acts as a heat
sink/spread. The typical thermal resistance, 39.6 ° C/W, is achieved based on a 6.5 mm × 3.4 mm thermal land
with eight vias without air flow. It can be improved by using larger thermal land and/or increasing vias number.
Further information about PowerPAD™ and its recommended board layout is described in (SLMA002). This
document is available at http:\\www.ti.com.
Certain points must be considered before designing a layout using the TPS51116.
• PCB trace defined as LL node, which connects to source of switching MOSFET, drain of rectifying MOSFET
and high-voltage side of the inductor, should be as short and wide as possible.
• Consider adding a small snubber circuit, consists of 3 Ω and 1 nF, between LL and PGND in case a
high-frequency surge is observed on the LL voltage waveform.
• All sensitive analog traces such as VDDQSNS, VTTSNS and CS should placed away from high-voltage
switching nodes such as LL, DRVL or DRVH nodes to avoid coupling.
• VLDOIN should be connected to VDDQ output with short and wide trace. If different power source is used for
VLDOIN, an input bypass capacitor should be placed to the pin as close as possible with short and wide
connection.
• The output capacitor for VTT should be placed close to the pin with short and wide connection in order to
avoid additional ESR and/or ESL of the trace.
• VTTSNS should be connected to the positive node of VTT output capacitor(s) as a separate trace from the
high current power line and is strongly recommended to avoid additional ESR and/or ESL. If it is needed to
sense the voltage of the point of the load, it is recommended to attach the output capacitor(s) at that point.
Also, it is recommended to minimize any additional ESR and/or ESL of ground trace between GND pin and
the output capacitor(s).
• Consider adding LPF at VTTSNS in case ESR of the VTT output capacitor(s) is larger than 2 m Ω .
• VDDQSNS can be connected separately from VLDOIN. Remember that this sensing potential is the reference
voltage of VTTREF. Avoid any noise generative lines.
• Negative node of VTT output capacitor(s) and VTTREF capacitor should be tied together by avoiding
common impedance to the high current path of the VTT source/sink current.
• GND (Signal GND) pin node represents the reference potential for VTTREF and VTT outputs. Connect GND
to negative nodes of VTT capacitor(s), VTTREF capacitor and VDDQ capacitor(s) with care to avoid
additional ESR and/or ESL. GND and PGND (power ground) should be connected together at a single point.
• Connect CS_GND (RGE) to source of rectifying MOSFET using Kevin connection. Avoid common trace for
high-current paths such as the MOSFET to the output capacitors or the PGND to the MOSFET trace. In case
of using external current sense resistor, apply the same care and connect it to the positive side (ground side)
of the resistor.
• PGND is the return path for rectifying MOSFET gate drive. Use 0.65 mm (25mil) or wider trace. Connect to
source of rectifying MOSFET with shortest possible path.
• Place a V5FILT filter capacitor (RGE) close to the TPS51116, within 12 mm (0.5 inches) if possible.
• The trace from the CS pin should avoid high-voltage switching nodes such as those for LL, VBST, DRVH,
DRVL or PGOOD.
• In order to effectively remove heat from the package, prepare thermal land and solder to the package ’ s
thermal pad. Wide trace of the component-side copper, connected to this thermal land, helps heat spreading.
Numerous vias with a 0.33-mm diameter connected from the thermal land to the internal/solder-side ground
plane(s) should be used to help dissipation. Do NOT connect PGND to this thermal land underneath the
package.
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