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While there are multiple techniques for reducing power consumption, shutting off power domains is the main method used to reduce leakage power consumption. In power shut-off designs, there are multiple aspects designers need to take care of, including IR drop, turn-on time, rush current, and the number of power switches added. An earlier post in this Low Power blog, How Easy Is It to Switch Off Power? explored some of those issues.This post focuses on a specific challenge, the rush current.
To control rush current, designers need to run some trials and implement the enable chain in such way that the rush current is less than the average current drawn by the design. It also should have a minimal impact on the turn-on time.
When more simultaneous chains are implemented, there is smaller resistance per time step, leading to shorter ramp up times-- but there will be a larger rush current. If a longer chain depth is implemented, then it will take a longer time to turn on all switches with a longer ramp-up time, but there will be a smaller rush current. So ideally the user will balance the enable chain depth and the use of simultaneous chains for optimal rush current control versus ramp-up time.
There are multiple ways a designer can implement the enable chain. Below are some examples of how enable chains can be built.
Figure 1 - Ways to implement enable chains
The different ways to implement enable chains have their pros and cons, as shown in the illustrations below.
Figure 2 -- Pros and cons of different enable chain approaches
In order to support enable chaining requirements, there is a flexible power switch enable unchaining/chaining feature available with the Encounter Digital Implementation system. It allows the user to change the enable chaining with already placed power switches. The feature also allows the user to change the enable chain for all power switches in a power domain, or by area, or per selection.
Along with the enable connection styles shown in the pictures above, the feature allows the user to define the enable chaining in preferred horizontal or vertical direction, and also allows user to specify the maximum distance a switch can drive/connect in a left or right direction.
This feature set is very useful for users to change the power switch enable connection, and there are multiple examples of users using this command set to change the enable sequence/chain. The Encounter Digital Implementation System also has a GUI available under the Power-Multi Supply Voltage tab for users to try chaining options interactively.