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AP-477
APPLICATION
NOTE
Low Voltage
Embedded Design
JOHN WILLIAMS
APPLICATIONS ENGINEER
February 1993
Order Number 272324-001
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Low Voltage Embedded Design
CONTENTS PAGE CONTENTS PAGE
3 2 2 Disadvantages of Mixed
1 0 INTRODUCTION 1 Voltage Systems 6
1 1 What is 3V 1
3 2 2 1 Multiple Supplies in
Mixed Voltage Systems 7
2 0 REASONS FOR 3V OPERATION 1
3 2 2 2 Additional Current
2 1 Device Geometries 1 Consumption in Mixed Voltage
2 2 System Benefits 1 Systems 7
3 2 3 Summary Mixed Voltage
3 0 LOW VOLTAGE SYSTEM Systems 8
DESIGN 2
3 3 Single Voltage System Design 8
3 1 Power Supply Design 2
3 3 1 Devices Designed for Low
3 1 1 Batteries 2
Voltage Operation 8
3 1 2 Voltage Regulation 3
3 3 2 Devices Derated for Low
3 1 3 Calculating System Power Voltage Operation 8
Consumption 3 3 3 3 Noise Generation by Low
3 1 3 1 Device Current 3 Voltage Devices 9
3 1 3 2 Discrete Component 3 3 4 Noise Margins in Low Voltage
Current 4 Systems 9
3 1 3 3 Leakage Current 4
4 0 SYSTEM POWER MANAGEMENT 9
3 1 4 Summary Calculating Power
Consumption 5
4 1 Device Power Management 9
3 2 Mixed Voltage System Design 5 4 2 Software Power Management 10
3 2 1 Interfacing 3V and 5V 4 2 1 Implementing Delays 10
Components 5 4 2 2 Code Optimization 10
3 2 1 1 3V to 5V Interface 5
3 2 1 2 5V to 3V Interface 5
3 2 1 3 Voltage Translation with
Open Drain Outputs 6
3 2 1 4 BidirectionaI
Translation 6
AP-477
10 INTRODUCTION 2 1 Device Geometries
Currently integrated circuit manufacturers are specify- One of the driving factors to reduce component operat-
ing devices for low voltage operation Most devices are ing voltages is technology As device channel lengths
specified for operation around 3V This can indicate and oxide thicknesses decrease (Figure 2-1) lower op-
operation centered around 3 0V 3 3V or other voltages erating voltages are required to maintain component
Many devices are even specified for operation at 2V and reliability Three main reliability concerns are oxide
below breakdown punch-through and hot-electron effects
In many embedded designs the designer wants the low- As feature length and width in a component decrease
est system power dissipation possible Embedded low depth also decreases With thinner gate oxides lower
voltage designs require maximum battery life which is gate voltages are required to avoid dielectric breakdown
directly proportional to the current consumption of the in the oxide If oxide breakdown occurs the gate of the
system System current consumption is a linear func- device begins to draw current
tion of operating voltage
As the channel lengths shorten lower source and drain
This application note discusses why low voltage devices voltages are required to keep the depletion regions
and systems are beneficial and how to design low volt- around the wells from meeting at this point the cur-
age systems Power supplies mixed voltage systems rent through the device is no longer controlled by the
single voltage systems and power management schemes voltage on the gate
are all discussed Although 3V systems are specifically
discussed the concepts apply to all low voltage designs
1 1 What is 3V
The term ``3V'' usually refers to 3 3V g 0 3V This is
the JEDEC standard for regulated supplies The major-
ity of devices are specified for 3 3V operation the stan-
272324