Layout Considerations

TPS51123

Certain points must be considered before starting a layout work using the TPS51123.

• TPS51123 has only one GND pin and special care of GND trace design makes operation stable, especially

when both channels operate. Group GND terminals of output voltage divider of both channels and the VREF

capacitor as close as possible, and connect them to an inner GND plane with PowerPad and the overcurrent

setting resistor, as shown in the thin GND line of Figure 37. This trace is named Signal Ground (SGND).

Group ground terminals of VIN capacitor(s), VOUT capacitor(s) and source of low-side MOSFETs as close as

possible, and connect them to another inner GND plane with GND pin of the device and the GND terminal of

VREG3 and VREG5 capacitors, as shown in the bold GND line of Figure 37. This trace is named Power

Ground (PGND). SGND should be connected to PGND at the middle point between ground terminal of VOUT

capacitors.

• Inductor, VOUT capacitor(s), VIN capacitor(s) and MOSFETs are the power components and should be

placed on one side of the PCB (solder side). Power components of each channel should be at the same

distance from the TPS51123. Other small signal parts should be placed on another side (component side).

Inner GND planes should shield and isolate the small signal traces from noisy power lines.

• PCB trace defined as LLx node, which connects to source of high-side MOSFET, drain of low-side MOSFET

and high-voltage side of the inductor, should be as short and wide as possible.

• VREG5 requires capacitance of at least 33-µF and VREG3 requires capacitance of at most 10-µF. VREF

requires a 220-nF ceramic bypass capacitor which should be placed close to the device and traces should be

no longer than 10 mm.

• Connect the overcurrent setting resistors from TRIPx to SGND and close to the device, right next to the

device if possible.

• The discharge path (VOx) should have a dedicated trace to the output capacitor; separate from the output

voltage sensing trace. When LDO5 is switched over Vo1 trace should be 1.5 mm with no loops. When LDO3

is switched over and loaded Vo2 trace should also be 1.5 mm with no loops. There is no restriction for just

monitoring Vox. Make the feedback current setting resistor (the resistor between VFBx to SGND) close to the

device. Place on the component side and avoid vias between this resistor and the device.

• Connections from the drivers to the respective gate of the high-side or the low-side MOSFET should be as

short as possible to reduce stray inductance. Use 0.65-mm (25 mils) or wider trace and via(s) of at least 0.5

mm (20 mils) diameter along this trace.

• All sensitive analog traces and components such as VOx, VFBx, VREF, GND, EN0, TRIPx, PGOOD,

TONSEL and SKIPSEL should be placed away from high-voltage switching nodes such as LLx, DRVLx, and

DRVHx nodes to avoid coupling.

• Traces for VFB1 and VFB2 should be short and laid apart each other to avoid channel to channel

interference.

• In order to effectively remove heat from the package, prepare thermal land and solder to the package’s

thermal pad. Three by three or more vias with a 0.33-mm (13 mils) diameter connected from the thermal land

to the internal ground plane should be used to help dissipation. This thermal land underneath the package

should be connected to SGND, and should NOT be connected to PGND.

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