SMALL FORM FACTOR DESIGN CHALLENGES Dealing with Power Supply Noise Presenter: Mike Caubarreaux Design Work: AESD Beijing Technical Design Center Octo...
SMALL FORM FACTOR DESIGN CHALLENGES Dealing with Power Supply Noise Presenter: Mike Caubarreaux Design Work: AESD Beijing Technical Design Center October 2011
POWER SUPPLY NOISE Electromagnetic interference (EMI) is the disruption of operation of an electronic device (victim) when it is in the vicinity of the electromagnetic field (EM field) of another electronic device (culprit)
Current flow through a conductor generates an electromagnetic field A conductor can be a trace on a printed circuit board (PCB) or a device such as a FET or inductor Radiated coupling – magnetic field
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2 | Small Form Factor Design Challenges | October 2011
EXAMPLE 1 – AMD SOCKET AM3 PROCESSOR WITH ATI RADEON™ E4690 GPU AND 780E/SB710 CHIPSET REFERENCE DESIGN KIT DDR3
PWR Supplies
DDR3
H T 3
ATI Radeon™ HT3
LVTM
E4690 GPU
PCIe® 2.0
HDMI™
Dual Independent Display Controllers
UVD 2.0
DirectX®LVTM 10.1
Socket AM3 Processor
PCIe® x8
GFX PCIe
GP PCIe®
512MB 128-bit GDDR3
HDMI™
PCIe® x1
AMD 780E IPG/ Northbridge
PCIe® x1
AMD SB710 HD Audio
SPI
1Gb Eth
BIOS ROM
Southbridge RS-232 eSATA
USB 2.0 I/O Backplane
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DC PWR
Design requirement: Support AMD Socket AM3 processor up to 65W TDP
ISSUE, FINDINGS AND HYPOTHESIS Issue: Ethernet controller was non-functional Findings: LAN controller device was not the problem – Issue remained after swapping LAN device to another board and installing a known functional device – LAN controller worked with 25W processor installed – LAN controller worked if the closest low-side CPU core FET was removed from the board
Hypothesis: Power supply noise disrupting LAN circuitry subsection
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ORIGINAL PCB LAYOUT
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REVISED PCB LAYOUT Solution: Separate the victim from the EMI source
Moved processor power MOSFETs and inductor to right side of the board Swapped the location of the Ethernet controller and RJ45 port
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EXAMPLE 2 – AMD ASB1 PROCESSOR WITH 780E/SB710 CHIPSET xSTB REFERENCE DESIGN KIT
ASB1 BGA
DDR2
PWR Supplies
H T 1
Side port
RS780E (AMD 785E Southbridge)
AMD SB710 Audio
miniPCI® x1
GP PCIe
PCIe x8
GFX PCIe
HT1
PCIe® x1
1Gb Eth
SPI BIOS ROM
Southbridge
IR
USB 2.0
VGA
Analog Audio Digital Audio
DC PWR
I/O Backplane
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170mm x 152.4mm
ISSUE, FINDINGS AND HYPOTHESIS Issue: Unstable HDMI™ port – Display on HDMI™ LDC monitor flickers on and off – Audio on HDMI™ LCD monitor cuts in and out
Findings: Northbridge power inductor and MOSFET were the EMI source – Resolved the issue by reworking the board to move the inductor and FET further away from HDMI™ traces
Hypothesis: Power supply noise disrupted the HDMI™ signals
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ORIGINAL PCB LAYOUT
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REVISED PCB LAYOUT
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RECOMMENDATIONS Pay close attention to power supply component locations relative to sensitive circuits Use shielded inductors
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