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NCP5214A
http://onsemi.com
27
PCB Layout Guidelines
Cautious PCB layout design is very critical to ensure
high performance and stable operation of the DDR power
controller. The following items must be considered when
preparing PCB layout:
1. All highcurrent traces must be kept as short and
wide as possible to reduce power loss.
Highcurrent traces are the trace from the input
voltage terminal to the drain of the highside
MOSFET, the trace from the source of the
highside MOSFET to the inductor, the trace
from inductor to the VDDQ output terminal, the
trace from the input ground terminal to the
VDDQ output ground terminal, the trace from
VDDQ output to VTTI pin, the trace from VTT
pin to VTT output terminal, and the trace from
VTT output ground terminal to the VTTGND pin.
Power handling and heaksinking of highcurrent
traces can be improved by also routing the same
highcurrent traces in the other layers and joined
together with multiple vias.
2. Power components which include the input
capacitor, highside MOSFET, lowside
MOSFET and VDDQ output capacitor of the
buck converter section must be positioned close
together to minimize the current loop. The input
capacitor must be placed close to the drain of the
highside MOSFET and the source of the
lowside MOSFET.
3. To ensure the proper function of the device,
separated ground connections should be used for
different parts of the application circuit according
to their functions. The input capacitor ground, the
lowside MOSFET source, the VDDQ output
capacitor ground, the VCCP decoupling capacitor
ground should be connected to the PGND. The
trace path connecting the source of the lowside
MOSFET and PGND pin should be minimized.
The VTT output capacitor ground should be
connected to the VTTGND first with a short
trace, it is then connected to the ground plane of
PGND. The VCCA decoupling capacitor ground,
the ground of the VDDQ feedback resistor, the
softstart capacitor ground, the VTTREF output
capacitor ground should be connected to the
AGND. The AGND pin is then connected directly
through a sense trace to the remote ground sense
point of the PGND, which is usually the ground
of the local bypass capacitor for the load. Never
connect the AGND, PGND and VTTGND
together just under the thermal pad.
4. The thermal pad of the DFN22 package should be
connected to the ground planes in the internal
layer and bottom layer from the copper pad at top
layer underneath the package through six to eight
vias with 0.6 mm holediameter to help heat
dissipation and ensure good thermal capability. It
is recommended to use PCB with 1 oz or 2 oz
copper foil. The thermal pad can be connected to
either PGND ground plane or AGND ground
plane but not both.
5. The input capacitor ground terminal, the VDDQ
output capacitor ground terminal and the source
of the lowside MOSFET must be connected to
the PGND ground plane through multiple vias.
6. Sensitive traces like trace from FBDDQ, trace
from COMP, trace from OCDDQ, trace from
FBVTT and trace from VTTREF should be
avoided from the highvoltage switching nodes
like SWDDQ, BOOST, TGDDQ and BGDDQ.
7. Separate sense trace should be used to connect
the VDDQ point of regulation, which is usually
the local bypass capacitor for load, to the
feedback resistor divider to ensure accurate
voltage sensing. The feedback resistor divider
should be place close to the FBDDQ pin.
8. Separate sense trace should be used to connect the
VTT point of regulation, which is usually the local
bypass capacitor for load, to the FBVTT pin.
9. Separate sense trace should be used to connect
the VDDQ point of regulation to the DDQREF
pin to ensure that the reference voltage to VTT is
accurately half of the VDDQ voltage.
10. The traces length between the gate driver outputs
and gates of the MOSFETs must be minimized to
avoid parasitic impedance.
11. To ensure normal function of the device, an RC
filter should be placed close to the VCCA pin and
a decoupling capacitor should be placed close to
the VCCP pin.
12. The copper trace area of the switching node which
includes the source of the highside MOSFET,
drain of the lowside MOSFET and high voltage
side of the inductor should be minimized by using
short wide trace to reduce EMI.
13. A snubber circuit consists of a 3.3 resistor and
1.0 nF capacitor may need to be connected across
the switching node and PGND to reduce the
highfrequency ringing occurring at the rising
edge of the switching waveform to obtain more
accurate inductor current limit sensing of the
VDDQ buck converter. However, adding this
snubber circuit will slightly reduce the conversion
efficiency.
14. VTTI should be connected to VDDQ output with
wide and short trace if VDDQ is used as the
sourcing supply for VTT. An input capacitor of at
least 10
F should be added close to the VTTI
pin and bypassed to VTTGND if external voltage
supply is used as the VTT sourcing supply.