Application Report
SCEA035A - June 2004
Selecting the Right Level-Translation Solution
Prasad Dhond
ABSTRACT
Supply voltages continue to migrate to lower nodes to support today’s low-power,
high-performance applications. While some devices are capable of running at lower supply
nodes, others might not have this capability. To have switching compatibility between these
devices, the output of each driver must be compliant with the input of the receiver that it is
driving. There are several level-translation schemes to interface these devices with one
another. Depending on application needs, one approach might be more suitable than the
other. This application report gives an overview of the methods and products used to translate
logic levels and lists the advantages and disadvantages of each Texas Instruments (TI)
level-translation solution.
Keywords: Dual-supply, split-rail, level translation, level shifter, mixed-voltage, T45, T245,
4245, 3245, open-drain, overvoltage tolerant, TTL, CMOS, TVC, CB3T, CBTD
Standard Linear & Logic
Contents
1
2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Dual-Supply Level Translators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Product Portfolio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Open-Drain Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Application Example − Level Translation Using the SN74LVC2G07 . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Do Not Use Pullup Resistors at Outputs of CMOS Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
FET Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 CBT and CBTD Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2 Using Translation Voltage Clamp (TVC) Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Overvoltage-Tolerant Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Devices With TTL-Compatible Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Summary of Translation Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3
4
5
6
7
8
9
Trademarks are the property of their respective owners.
1
SCEA035A
List of Figures
1. Typical Situation in Which a Level Translator Is Needed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Digital Switching Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SN74AVCB324245 Translating From 1.8 V to 3.3 V (Bank 1) and 3.3 V to 1.8 V (Bank 2)
at the Same Time (C
L
= 15 pF, R
L
= 2 kW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Dual-Supply Level-Translation Device Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Level Translation Using an Open-Drain Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Use of an Open-Drain Device in a Level-Translation Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Output Edge Slows With Increasing Value of R
PU
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Use of Pullup Resistors at Output of CMOS Drivers Is Not Recommended . . . . . . . . . . . . . . . . . . . . 11
9. CB3T Device Used to Interface a 3-V Bus With a 5-V (TTL) Bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10. SN74CBT1G384 In 5-V to 3.3-V Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. 5-V to 3.3-V Translation Using CBT Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12. Simplified Schematic of a Typical TVC-Family Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
13. SN74TVC3306 Used in a Level-Translation Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
14. Waveforms For Bidirectional Translation Using a TVC Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
15. Down-Translation Using a Logic Device With Overvoltage-Tolerant Inputs . . . . . . . . . . . . . . . . . . . 18
16. Shift in Output Duty Cycle When Using An Overvoltage-Tolerant Device
For Level Translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
17. V
IN
vs I
CC
Characteristics of SN74HCT541 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
List of Tables
1.
2.
3.
4.
Possible Voltage-Translation Combinations Using Dual-Supply Translators . . . . . . . . . . . . . . . . . . . . 6
Possible Voltage-Translation Combinations Using an FET Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Possible Voltage-Translation Combinations Using a TVC Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Possible Voltage-Translation Combinations Using Overvoltage-Tolerant Devices . . . . . . . . . . . . . . . 20
2
Selecting the Right Level-Translation Solution
SCEA035A
1
Introduction
The need for voltage level translation is prevalent on most electronic systems today. For
example, an ASIC might be operating with supply-voltage V
CCA
, while an I/O device operates
with supply-voltage V
CCB
. To enable these devices to communicate with each other, a
level-translation solution is needed as shown in Figure 1.
VCCA*
VCCB*
ASIC
TI Level
Translation
Solution
I/O
Devices
*VCCA not equal to VCCB
Figure 1. Typical Situation in Which a Level Translator Is Needed
Input-voltage thresholds and output-voltage levels of electronic devices vary, depending on the
device technology and supply voltage used. Figure 2 shows the threshold levels for different
supply voltages and device technologies. To interface two devices successfully, certain
requirements must be met:
1. The V
OH
of the driver must be greater than the V
IH
of the receiver.
2. The V
OL
of the driver must be less than the V
IL
of the receiver.
3. The output voltage from the driver must not exceed the I/O voltage tolerance of the
receiver.
Selecting the Right Level-Translation Solution
3
SCEA035A
5V
4.44 V
VCC
VOH
5V
VCC
0.7
y
VCC
VIH
3.3 V
VCC
0.5
y
VCC
VT
2.4 V
2V
VOH 2.4 V
VIH
VT
VIL
2V
1.5 V
0.8 V
VOH 2.5 V
VIH
VT
VIL
2.0 V
1.7 V
VCC
VOH
1.8 V
VIH V −0.45 V
CC
0.65yVCC
VIL 0.35yV
CC
0.45 V
VOL
GND
0V
VCC
1.5 V
VOH
VIH 0.65yV
CC
VIL 0.35yV
CC
VOL
0V
GND
VCC
1.2 V
VIH
0.65yVCC
VIL
0.35yVCC
0V
GND
0.3
y
VCC
VIL 1.5 V
0.8 V
VCC
VIH
VIL
GND
0.7 V
0.5 V
0V
VOL 0.4 V
GND
0V
VOL 0.4 V
GND
0V
VOL 0.4 V
0V
GND
5−V CMOS
5−V TTL
3.3−V LVTTL
2.5−V CMOS
1.8−V CMOS
1.5−V CMOS
1.2−V CMOS
Figure 2. Digital Switching Levels
2
2.1
Dual-Supply Level Translators
Features
Dual-supply devices are designed for asynchronous communication between two buses or
devices operating at different supply voltages. These devices use two supply voltages: V
CCA
to
interface with the A side and V
CCB
to interface with the B side. For bidirectional level translators,
data is transmitted from A to B or B to A, depending on the logic level at the DIR input. On
devices with an output enable (OE) control input, the A and B buses effectively are isolated
when OE is inactive.
Dual-supply devices from TI are available in a variety of bit widths and cover nearly every
supply-voltage node in use today. These devices are flexible, easy to use, and can translate
bidirectionally (up-translate and down-translate), which makes them an ideal choice for most
level-translation applications. Their active current-drive capability makes them suitable for
applications with long trace lengths and heavy output loads.
The SN74AVCB324245 is a 32-bit dual-supply level translator that is organized as four banks of
eight bits each. Figure 3 shows one bank of the SN74AVCB324245 translating from 1.8 V to
3.3 V while, at the same time, another bank translates from 3.3 V to 1.8 V.
4
Selecting the Right Level-Translation Solution
SCEA035A
3
3.3 V Output
Voltage − V
2
1.8 V Input
1
0
0
10
20
30
40
Time − ns
50
60
70
80
3
3.3 V Input
Voltage − V
2
1
1.8 V Output
0
0
10
20
30
40
50
Time − ns
60
70
80
Figure 3. SN74AVCB324245 Translating From 1.8 V to 3.3 V (Bank 1) and
3.3 V to 1.8 V (Bank 2) at the Same Time (C
L
= 15 pF, R
L
= 2 kW)
Advantages of dual-supply devices:
•
•
•
Flexibility in translating to/from a variety of voltage nodes
Active current drive capability
Available in a variety of bit widths
Selecting the Right Level-Translation Solution
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