For all switching power supplies, the
layout is an important step in the design. If the layout is not carefully done, the
regulators can have stability and EMI issues. Therefore, use wide and short traces
for the main current path and for the power ground tracks. The input capacitors,
output capacitors, and inductors must be placed as close as possible to the device.
The output capacitors must have a low impedance to ground. Use multiple VIAS (at
least three) directly at the ground landing pad of the capacitor. Here are some
layout guidelines:
- PVIN_Bx: Place the input
capacitor as close to the IC as allowed by the layout DRC rules. Any extra
parasitic inductance between the input cap and the PVIN_Bx pin can create a
voltage spike. It is recommended to have wide a short traces or polygon to help
minimize trace inductance. Do not route any sensitive signals close to the input
cap and the device pin as this node has high frequency switching currents. Add
3-4 vias per amp of current on the GND pads for each DCDC. If the space is
limited and does not allow to place the input capacitors on the same layer as
the PMIC, then place the input capacitors on the opposite layer with VIAS, close
to the IC, and add a small input capacitor (0.1uF) on the same layer as the
PMIC. This small capacitor must be placed close to the PVIN_Bx pin.
- LX_Bx: Place the inductor
close to the PMIC without compromising the PVIN input caps and use short &
wide traces or polygons to connect the pin to the inductor. Do not route any
sensitive signals close to this node. The inductor must be placed in the same
layer as the IC to prevent having to use VIAS in the SW node. Since the SW-node
voltage swings from the input voltage to ground with very fast rise and fall
times, it is the main generator of EMI. If needed, to reduce EMI, a RC snubber
can be added to the SW node.
- FB_Bx: Route each of the
FB_Bx pins as a trace to the output capacitor. Do not extend the output voltage
polygon to the FB_Bx pin as this pin requires to be routed as a trace. The trace
resistance from the output capacitor to the FB_Bx pin must be less than 1 Ω. The
TPS6521905 does not support remote sensing so the FB_Bx pins
must be connected to the local capacitor of the PMIC. Avoid routing the FB_Bx
close to any noisy signals such as the switch node or under the inductor to
avoid coupling. If space is constraint, FB_Bx pin can be routed through an inner
layer. See example layout.
- Bucks Cout: The local output capacitors must be placed as close to the
inductor as possible to minimize electromagnetic emissions.
- PVIN_LDOx: Place the input
capacitor as close as possible to the PVIN_LDOx pin.
- VLDOx: Place the output
capacitor close to the VLDOx pin. For the LDO regulators, the feedback
connection is internal. Therefore, it is important to keep the PCB resistance
between LDO output and target load in the range of the acceptable voltage, IR,
drop for LDOs.
- VSYS: Connect VSYS directly to a quiet system voltage node. Place the
decoupling capacitor as close as possible to the VSYS pin.
- VDD1P8: Place the 2.2 uF
cap as close as possible to the VDD1P8 pin. This capacitor needs to be placed in
the same layer as the IC. Two to Three VIAS can be used to connect the GND side
of the capacitor to the GND plane of the PCB.
- Power Pad: The thermal pad
must be connected to the PCB ground plane with a minimum of nine VIAS.
- AGND: Do not connect AGND to the power pad (or thermal pad). The AGDN pin
must be connected to the PCB ground planes through a VIA . Keep the trace from
the AGDN pin to the VIA short.