Texas Instruments AM335 Series Design Manual page 8

Hardware Low Power Design Guidelines
2.3 Core Leakage
Core leakage is a result of the physical properties of transistors. On some level it is unavoidable since it is
impossible to remove thermal noise from a system. However it can become an issue when junction
temperature on the device increases. Therefore, if operating conditions for your application include high
temperatures, or if significant load on the processor is expected, special attention to the cooling of the
device should be taken.
These efforts could include using thicker copper layers on the ground plane to provide better thermal
sinking, the inclusion of a heatsink, or even incorporating an active cooling solution. Higher cooling
capacity will enable the processor to run at a lower temperature which would improve core leakage. For
more detailed information about how to manage thermal considerations, see the
DSP and ARM Application
2.4 DDR System Design
The DDR interface can be a major consumer of power on both the System-on-Chip (SoC) and at the
system level. For new system designs, it is recommended to use a single DDR3L memory configured in a
point-to-point topology without VTT termination. Using only a single DRAM module reduces the
component count that in turn reduces power consumption. In addition, eliminating VTT is a significant
power savings on its own. Since a single DDR3L chip now can have densities of 1 GB, there is no
capacity tradeoff for using only one DRAM chip. Actual memory capacity should still be tailored to the
needs of the specific application.
If other topologies are needed for a specific application, see the device-specific technical reference
manual (TRM) for other supported topologies, as well as more information on supporting DDR2, and
LPDDR1.
For existing designs with VTT, if possible, ensure that VTT is turned off when DDR enters self-refresh as
this generates a significant power savings.
2.5 Design for measurability
During the development and prototyping stage it is useful to integrate some features to make measuring
power consumption easier. The AM335x has the following power rails:
• VDD_MPU
• VDD_CORE
• VDDS
• VDDSHV1-6
• VDDS_DDR
All of these rails can all be measured independently. There are two basic ways to monitor power
consumption on the AM335x: first is through test points and shunt resistors, and second is by integrating
in INA226 analog-to-digital converters to monitor power.
2.5.1 Shunt Resistors
The simplest way to implement power monitoring capabilities is to include low resistance, high precision
shunt resistors in the power supply rails. By measuring the voltage drop across the resistor and the supply
rail voltage, it is simple to calculate the power consumed by the measured rail. Using a shunt to compute
the current through the power rail instead of measuring current directly, the impact of unknown voltage
drops is negated due to probe resistances. Typical values for these resistors can be found in the
Evaluation Module (EVM) Hardware User's
of the multimeter used to ensure that the expected voltage range is measurable with the desired precision.
It may be convenient to use jumpers or test points placed close to the shunt resistors to monitor the
voltage across the resistor. Otherwise, measurement can be made directly at the terminals of the shunt
resistor.
8
AM335x Low Power Design Guide
Processors.
Guide, it can also be advantageous to consider the sensitivity
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Thermal Design Guide for
SPRAC74A – February 2017 – Revised March 2017
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