DRA790 Datasheet, PDF (365/436 Pages) Texas Instruments – Infotainment Applications Processor

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PCB
DRA790, DRA791
DRA793, DRA797
SPRS968A – AUGUST 2016 – REVISED FEBRUARY 2017
Static IR drop and Effective Resistance
Source Component
Load Component
BGA pad on PCB
Figure 7-4. Static IR Drop Budget for PCB Only
SPRS906_PCB_STATIC_02
The system-level IR drop budget is made up of three portions: on-chip, package, and PCB board. Static IR
or DC analysis/design methodology consists of designing the PDN such that the voltage drop (under DC
operating conditions) across power and ground pads of the transistors of the application processor device
is within a specified value of the nominal voltage for proper functionality of the device.
A PCB system-level voltage drop budget for proper device functionality is typically 1.5% of nominal
voltage. For a 1.35-V supply, this would be ≤20 mV.
To accurately analyze PCB static IR drop, the actual geometry of the PDN must be modeled properly and
simulated to accurately characterize long distribution paths, copper weight impacts, electro-migration
violations of current-carrying vias, and “Swiss-cheese” effects via placement has on power rails. It is
recommended to perform the following analyses:
• Lumped resistance/IR drop analysis
• Distributed resistance/IR drop analysis
NOTE
The PMIC companion device supporting this processor has been designed with voltage
sensing feedback loop capabilities that enable a remote sense of the SMPS output voltage at
the point of use.
The NOTE above means the SMPS feedback signals and returns must be routed across PCB and
connected to the Device input power ball for which a particular SMPS is supplying power. This feedback
loop provides compensation for some of the voltage drop encountered across the PDN within limits. As
such, the effective resistance of the PDN within this loop should be determined in order to optimize
voltage compensation loop performance. The resistance of two PDN segments are of interest: one from
the power inductor/bulk power filtering capacitor node to the Processor’s input power and second is the
entire PDN route from SMPS output pin/ball to the Processor input power.
In the following sections each methodology is described in detail and an example has been provided of
analysis flow that can be used by the PCB designer to validate compliance to the requirements on their
PCB PDN design.
Copyright © 2016–2017, Texas Instruments Incorporated
Applications, Implementation, and Layout 365
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