Magic Blue Smoke

 

Header Vs Footer Switch Selection

As I said one of the key questions asked by many customers is “How do I decide on whether to Switch VDD or Switch VSS”

Here are some facts about the same.

The header switch is implemented by PMOS transistors to control Vdd supply. PMOS transistor is less leaky than NMOS transistor of a same size. Header switches turn off VDD and keep VSS on. As the result, it allows a simple design of a pull-down transistor to isolate power-off cells and clamp output signals in “0� state. The “0� state isolation complies with reset state requirement in most designs. The disadvantage of the header switch is that PMOS has lower drive current than NMOS of a same size, though difference is reduced by strained silicon technology. As a result, a header switch implementation usually consumes more area than a footer switch implementation.

The footer switch is implemented by NMOS transistor to control VSS supply. The advantage of footer switch is the high drive and hence smaller area. However, NMOS is leakier than PMOS and Designs become more sensitive to ground noise on the virtual ground (VIRTUAL_VSS) coupled through the footer switch. The isolation on “0� state becomes complex due to loss of the virtual ground in sleep mode and necessity of bypassing footer switch to reach permanent VSS.