Mark McClear ABSTRACT Many applications require bidirectional data tra

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Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
Mark McClear ABSTRACT Many applications require bidirectional data transfer between 3.3-V systems. methods maintaining lowest possible total system power consumption safe operation with mixed-mode signal translation compared. methods are: split-rail dual 3.3-V devices, completely tolerant, pure 3.3-V components. Standard Linear Logic
Contents Introduction Split-Rail Level Shifters Tolerant, Pure 3.3-V Components Misconception About More Savings Possible
Conclusion Acknowledgment List Figures Simplified Output Structure Versus Output Drive Comparison Basic Logic Data Transceiver Basic CMOS Input Structure Typical Current 16-Bit Device FCT164245 'LVTH16245A Total-System Power-Dissipation Impact Bus-Hold Cell
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Introduction
increasing demand lower system power consumption brought many design challenges. Among them problem safely efficiently interfacing various switching levels mixed 3.3-V systems, while maintaining lowest possible total system power consumption. competing methods accomplishing this mixed-mode signal translation have emerged:
Split-rail dual 3.3-V devices Completely tolerant, pure 3.3-V components
This application report deals with pros cons using both device types offers additional suggestions even greater system power savings.
Split-Rail Level Shifters
Split-rail level shifters class transceiver devices that have both 3.3-V rail. These transceivers specifically suitable bidirectional data transfer between CMOS interface 3.3-V systems. Products this class used effectively level shifters data-path voltage translators, following precautions usually recommended:
Some dual-VCC rail devices typically have strict power sequencing requirements prevent leakage even damage devices event that rail ramps faster than other. Having does necessarily ensure that device actually switches rail. Switching reduce power consumption memories other pure CMOS circuits that driven level-shifter device (this application report demonstrates others well).
data sheet product question reveals whether part drives rail. output high-voltage (VOH) minimum around 4.44 does drive rail (for example, AHC). Five-volt level shifters with TTL-compatible outputs typically drive only around (for example, ABT).
Tolerant, Pure 3.3-V Components
second class products created meet these design challenges offers similar voltage translation level-shifting capabilities split-rail devices previously mentioned. These transceivers suitable bidirectional data transfer between CMOS interface 3.3-V LVTTL side. Using single source, they avoid power-sequencing problems offered number functions, widths, storage options. potential drawback these single 3.3-V products that outputs pull rail. comparison 3.3-V families provided following sections this true drawback. series devices rely state-of-the-art submicron BiCMOS process provide reduction static power dissipation over devices. Figure shows simplified output shows mixed-mode signal drive designed into output stage. This combination high-drive stage with rail-to-rail CMOS switching gives devices extreme application flexibility. These parts have same drive characteristics devices (see Figure providing drive needed existing backplanes allowing simple solution reduce system power migration 3.3-V operation.
Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
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only devices operate 3-V-to-5-V level translators supporting input voltages with inputs withstand even when This permits devices used under partial system power-down applications when live insertion required.
CMOS Pullup/Pulldown Rail-to-Rail Switching
High-Drive Bipolar Backplane Driving
Output
Figure Simplified Output Structure
VOL- Output Voltage |IOH| Output Current Solid Dashed
Figure Versus Output Drive Comparison
Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
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Misconception About
component selection level shifter affects major aspects total system-power dissipation:
effect level driving part Figure power dissipation receiving device Figure commonly known power device itself
Interface Driver 3.3-V Subsystem Receiver Subsystem
Note: Unidirectional mode illustrated simplicity
Figure Basic Logic Data Transceiver added power dissipation induced into TTL-compatible device Figure level driving device Figure would correct expect that TTL-compatible product would have higher power dissipation were driven device with rather than that same device driven driver. Figure shows typical CMOS input stage current associated with switching device through input voltage range from VCC.
CMOS Input
CMOS Input Structure
Input Voltage
Figure Basic CMOS Input Structure Typical Current expected, current approaches zero ground rails peaks TTL-threshold region Figure graph (i.e., additional ICC) that induced into 16-bit device (all outputs switching) function driving device frequency.
Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
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Frequency VINH VINH VINH
Figure 16-Bit Device shown Figure fact, higher case where only than same device driven rail pure CMOS device. From this, might conclude that best possible solution would always select part that switches rail, this conclusion fails consider impact system power driving device. Figure shows devices: FCT164245 split-rail device from Integrated Device Technology, Inc. (IDT) 'LVTH16245A from
High-Output Voltage
FCT164245
'LVTH16245A
Time
Figure FCT164245 'LVTH16245A
Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
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From Figure concluded correctly that induced current part driven part would higher than device. problem with this conclusion that, from system standpoint, only components total system power dissipation that affected selection level-shifter device. Figure shows total power dissipation same split-rail device, 'LVTH16245A, worst-case (VOH plotted same vertical scale.
1200 1000 Power Dissipation FCT164245
'LVTH16245A
Worst-Case Frequency
Figure Total-System Power-Dissipation Impact From Figure seen that, even split-rail device pulls rail (which part does not), power savings more than offset huge switching currents that split rail draws from rail.
More Savings Possible
Some systems means power savings known partial power down. partial power-down mode, system basically shuts some unused circuits during inactivity, thus eliminating even standby currents. members TI's product line offer parametric specification Ioff, which ensures that output pins parts remain high-impedance state when supply voltage This prevents inactive device from dragging down active part system allows part become partition partially powered-down unused subsystem. device still functions level shifter voltage translator when power restored inactive subsystem. Another aspect system power dissipation passive resistor pullups keep local from floating causing damage devices bus. Pullups were sufficient older desktop systems where power consumption much concern, pullup resistors portable equipment have serious impact battery life and, such, must addressed.
Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
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Products like 'LVTH16245A (and others) from have circuit feature called bus-hold cell (shown Figure This cell eliminates these passive components procurement costs, board space, parasitics, power dissipation associated with them. This circuitry provides typical holding current ±100 that sufficient overcome CMOS-type leakages. Since this active circuit, does take current approximately ±500 toggle state input. This current negligible when compared magnitude current that needed charge capacitive load does affect propagation delay driving output.
Bus-Hold Input Cell
Input Inverter Stage
Figure Bus-Hold Cell
Conclusion
interface between CMOS 3.3-V system, split-rail level shifter with rail-to-rail output switching good choice. Both split-rail level shifters 5-V-tolerant single 3.3-V devices used bidirectional data transfer between LVTTL interface 3.3-V systems. When optimizing cost power total system, many issues should considered. device that contributes less DICC power might always contribute more efficient system power consumption. Although FCT164245 less DICC power contribution than LVTH16245A, overall system power consumption better with LVTH16245A.
Acknowledgment
This application report edited Sagir Hasnain David Yaeger July 2002.
Low-Cost, Low-Power Level Shifting Mixed-Voltage Systems
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