Application Notes

NXP Semiconductors
Application note
Document Number: AN5283
Rev. 1.0, 4/2016
VR5100 layout guidelines
1
Introduction
This document covers best practices for the layout of the VR5100 device
on printed circuit boards.
The VR5100 is a SMARTMOS Power Management Integrated Circuit
(PMIC) designed specifically for use with the NXP LS1 family of
communications processors.
2
Packaging
The VR5100 device supports consumer and industrial applications and is
offered in a standard 48 QFN with an area of 7.0 mm x 7.0 mm. Refer to
Table 1. for the package drawing information for both packages.
Refer to Application Note AN1902 for guidelines on the handling and
assembly of NXP QFN packages during PCB assembly. AN1902 also
provides guidelines for PCB design and rework, and package
performance information (such as Moisture Sensitivity Level (MSL) rating,
board level reliability, mechanical and thermal resistance data).
Package dimensions are provided in package drawings. To find the most
current package outline drawing, go to www.nxp.com and search for the
drawing number.
Table 1. Package drawing information
Package
Suffix
Package outline drawing
number
48-pin QFN 7X7 mm - 0.5mm pitch
EP
98ASA00719D
© 2016 NXP B.V.
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
3 Recommended layer stack . . . . . . . . . . . . . . . . . . . .2
4 Component placement hints . . . . . . . . . . . . . . . . . . .2
5 General routing guidelines . . . . . . . . . . . . . . . . . . . .3
6 I2C communication signals . . . . . . . . . . . . . . . . . . . .3
7 Switching power supply traces . . . . . . . . . . . . . . . . .4
8 Effective grounding . . . . . . . . . . . . . . . . . . . . . . . . . .7
9 Exposed pad connection . . . . . . . . . . . . . . . . . . . . . .8
10 Feedback signals . . . . . . . . . . . . . . . . . . . . . . . . . . .8
11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Recommended layer stack
3
Recommended layer stack
Table 2. and Table 3. shows the recommended layer stack-up for effective signal shielding.
Table 2. Four layer stack-up recommendation
Layer
Stack-up
Layer 1 (Top)
High di/dt nodes
Layer 2 (Inner 1)
GND
Layer 3 (Inner 2)
Small signal/Power
Layer 4 (Bottom)
Small signal/GND
Table 3. Six layer stack-up recommendation
Layer
Stack-up
Layer 1 (Top)
High di/dt nodes
Layer 2 (Inner 1)
GND
Layer 3 (Inner 2)
Small signal/Power
Layer 4 (Inner 3)
Small signal/Power
Layer 5 (Inner 4)
GND
Layer 6 (Bottom)
High di/dt nodes
Place the ground layer between the high di/dt nodes layer and the sensitive small signal trace layer. The ground layer then shields the
small signal traces from switching traces and improves the stability and accuracy of the regulation.
Note: A more detailed layer design may be required to route the LS1 processor. If the VR5100 is affected by interference from an LS1
processor, just four of the layers are needed to route it.
4
Component placement hints
Place these components as close as possible to the IC in order of priority:
1.
Input capacitor of the buck regulators (SW1, SW2, and SW3)
2.
Output diode and output capacitor of the boost converter (SWBST)
3.
VIN, VGB, VCC, VDIG capacitors
4.
LICELL capacitor (if a coin cell is used in system)
5.
VSNVS, REFOUT and linear regulators capacitors (LDOx, VSD and V33)
6.
Switching regulator inductors
VR5100 layout guidelines, Rev. 1.0
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General routing guidelines
5
General routing guidelines
•
•
•
•
•
•
Shield regulator feedback paths from noise planes and traces and connect them as close as possible to the load (i.e. SW1FB.)
The exposed pad (EP) on the VR5100 is the high-current ground return for all the buck regulators and the boost regulator. Use
vias under the EP to drop onto the ground plane(s) directly, ensuring sufficient copper for the ground return.
The SWxIN, SWxLX, and SWBSTLX nodes are high dI/dt nodes and act as antennas. They are also high current paths. Hence
their traces must be kept short and wide.
Avoid coupling traces between sensitive signal/low noise supplies (like VBG) and switching nodes.
Power components should all be placed on the same side of board and their power traces routed on the same layer in order to
reduce voltage drop.
If it is necessary to route a power trace to another layer, choose a trace in low di/dt paths (see Figure 2 and Figure 5) and use
multiple vias for interconnection. - To minimize noise propagation and connection impedance between layers.
Minimize and isolate/shield the high dv/dt SW node areas. - To minimize the EMI noise source from the high dv/dt SW nodes
Separate input current paths among supplies if there is more than one supply on the same input rail (Figure 8) and the supplies are not
synchronized. To avoid common impedance noise coupling among supplies, use a local input decoupling capacitor for each supply.
6
I2C communication signals
To prevent I2C communication signals from being contaminated by nearby high power or high frequency signals, shield I2C signals with
ground planes placed on adjacent layers. Make sure the ground plane is uniform throughout the length of the signal trace.
Figure 1. Recommended shielding for critical signals
VR5100 layout guidelines, Rev. 1.0
NXP Semiconductors
3
Switching power supply traces
7
Switching power supply traces
In the buck and boost configurations, the length of the 'critical traces' must be minimized. 'Critical traces' refers to current paths having
high dI/dt. Refer to Section 7.1 "Buck regulator" and Section 7.2 "Boost converter" for details.
7.1
Buck regulator
Figure 2 shows current paths in a buck converter in the 'on' and 'off' periods of the switching cycle. Critical traces refer to traces that either
conduct only during the 'on' periods or only during the 'off' periods, as highlighted in red.
Control FET On
Synchronous FET On
SWxIN
SWxLX
Critical Traces
EP
Figure 2. Buck converter critical traces
The top and bottom MOSFETs are integrated within the package in the VR5100 buck regulators. Placing the input capacitor close to the
SWxIN pin and the exposed pad (EP) is critical. Figure 4 and show an example layout for the buck regulators.
VR5100 layout guidelines, Rev. 1.0
4
NXP Semiconductors
Switching power supply traces
Figure 3. SW1 schematic – reference for Figure 4
C18
SW1LX
C16
Multilayer chip
inductor
C13
Figure 4. SW1 layout – top layer components and top silkscreen
VR5100 layout guidelines, Rev. 1.0
NXP Semiconductors
5
Switching power supply traces
7.2
Boost converter
Figure 5 shows the critical traces in a boost converter.
Control FET On
Diode conducting
SWBSTLX
Critical traces
EP
Figure 5. Boost converter critical traces
The switching MOSFET is integrated within the package in the VR5100’s SWBST regulator. The loop formed by the switching MOSFET,
the diode and the output capacitor must be minimized to keep parasitic inductances small. Figure 7 and Figure 8 show an example of the
SWBST layout.
Figure 6. SWBST schematic – reference for Figure 7 and Figure 8
VR5100 layout guidelines, Rev. 1.0
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Effective grounding
Figure 7. SWBST example layout. Top layer components and top silkscreen
Figure 8. SWBST example layout. Bottom layer components and top and bottom silkscreen
Notice that the critical traces (blue and yellow) are kept wide and short on the previous layout example. The return current path is reduced
by populating C16 on the bottom and with its negative terminal close to the EP ground plane. A sufficient number of vias is used when
changing the high current path from top to bottom layer.
8
Effective grounding
•
•
•
The practice of 'star grounding' must be followed to achieve optimal performance with the VR5100.
The exposed pad (EP) is the ground return for all the switching regulators and should be connected to the ground plane through
multiple vias.
SGND, SGND1 and SGND2 are signal ground pins and should be connected together though ground plane using separate vias,
not through EP. This prevents coupling from return currents of the switching regulators using the EP as a return path. Ground
return currents from the switching regulators must not flow through these pins.
VR5100 layout guidelines, Rev. 1.0
NXP Semiconductors
7
Exposed pad connection
9
Exposed pad connection
The exposed pad (EP) is the ground return for all the switching regulators and should be connected to the ground plane(s) through vias.
A minimum of 16 vias is recommended under the EP. The EP also acts as a heat sink for the VR5100, so the vias should not have thermal
relief. To reduce unnecessary thermal stress, allow sufficient copper area for the EP. An efficient way to achieve this is to duplicate the EP
ground plane on all layers. When routing high current paths, a sufficient number of vias should be placed in parallel to help reduce the
parasitic impedance. The vias must be solid thermal vias as shown in Figure 9.
Thermal Relief Via - Not Recommended
Solid Thermal Via - Recommended
Figure 9. Types of via
'Wicking' of solder through the bore in the vias increases their thermal resistance. Follow techniques such as tenting or via encroaching
to prevent solder wicking. Using a bore diameter of 0.3 mm or less also helps minimize wicking due to the surface tension of the liquid
solder.
Apply the solder paste to approximately 50% to 75% of the area of the exposed pad. Rather than applying the solder paste in one large
section, apply it in multiple smaller sections. This can be accomplished by using an array of openings in the solder stencil. Sectioning
results in a more even spreading of the solder. It also minimizes out-gassing, which can create voids and bridges under the exposed pad.
Figure 10 shows an example of how the exposed pad might be laid out.
Figure 10. Exposed pad via array
10
Feedback signals
The control loop regulates output voltage at the point where the feedback trace meets the output rail. Connect the feedback trace to the
output voltage rail near the load for best load regulation. Ensure this trace does not couple noise from other traces/layers. An efficient way
to route the feedback trace is to place it alongside the output trace.
VR5100 layout guidelines, Rev. 1.0
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References
11
References
NXP.com
Support Pages
Description
URL
VR5100
Product Summary Page
www.nxp.com/VR5100
AN1902
QFN Application Note
www.nxp.com/files/analog/doc/app_note/AN1902.pdf
VR5100 layout guidelines, Rev. 1.0
9
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Revision history
12
Revision history
Revision
1.0
Date
4/2016
Description of changes
•
Initial release
VR5100 layout guidelines, Rev. 1.0
10
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© 2016 NXP B.V.
Document Number: AN5283
Rev. 1.0
4/2016
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