DRA790 Datasheet, PDF (364/436 Pages) Texas Instruments – Infotainment Applications Processor

DRA790, DRA791
DRA793, DRA797
SPRS968A – AUGUST 2016 – REVISED FEBRUARY 2017
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vss
vdd
PCB_PO_8
Figure 7-2. Coplanar Shielding of Power Net Using Ground Guard-band
7.2.3 Step 3: Static Analysis
Delivering reliable power to circuits is always of critical importance because voltage drops (also known as
IR drops) can happen at every level within an electronic system, on-chip, within a package, and across the
board. Robust system performance can only be ensured by understanding how the system elements will
perform under typical stressful Use Cases. Therefore, it is a good practice to perform a Static or DC
Analysis.
Static or DC analysis and design methodology results in a PDN design that minimizes voltage or IR drops
across power and ground planes, traces and vias. This ensures the application processor’s internal
transistors will be operating within their specified voltage ranges for proper functionality. The amount of IR
drop that will be encounter is based upon amount power drawn for a desired Use Case and PCB trace
(widths, geometry and number of parallel traces) and via (size, type and number) characteristics.
Components that are distant from their power source are particularly susceptible to IR drop. Designs that
rely on battery power must minimize voltage drops to avoid unacceptable power loss that can negatively
impact system performance. Early assessments a PDN’s static (DC) performance helps to determine
basic power distribution parameters such as best system input power point, optimal PCB layer stackup,
and copper area needed for load currents.
The resistance Rs of a plane conductor
for a unit length and unit width is called
the surface resistivity (ohms per square).
L
t
W
1^r
Rs = =
σ×t t
l
R = Rs ×
w
SPRS906_PCB_STATIC_01
Figure 7-3. Depiction of Sheet Resistivity and Resistance
Ohm’s Law (V = I × R) relates conduction current to voltage drop. At DC, the relation coefficient is a
constant and represents the resistance of the conductor. Even current carrying conductors will dissipate
power at high currents even though their resistance may be very small. Both voltage drop and power
dissipation are proportional to the resistance of the conductor.
Figure 7-4 shows a PCB-level static IR drop budget defined between the power management device
(PMIC) pins and the application processor’s balls when the PMIC is supplying power.
• It is highly recommended to physically place the PMIC as close as possible to the processor and on
the same side. The orientation of the PMIC vs. processor should be aligned to minimize distance for
the highest current rail.
364 Applications, Implementation, and Layout
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