Texas Instruments | Biasing Requirements for TXS, TXB, and LSF Auto-Bidirectional Translators | Application notes | Texas Instruments Biasing Requirements for TXS, TXB, and LSF Auto-Bidirectional Translators Application notes

Texas Instruments Biasing Requirements for TXS, TXB, and LSF Auto-Bidirectional Translators Application notes
Application Report
SCEA060 – October 2017
Biasing Requirements for TXS, TXB, and LSF AutoBidirectional Translators
Shreyas Rao, Adrian Ozer
ABSTRACT
The TXS and LSF families of translators differ from the TXB translator family, because the outputs of the
TXS and LSF families are not driven by buffers. Instead, the TXS and LSF families use internal or external
pullup resistors to drive logic high, and an internal pass transistor that lets the host device drive logic low.
This application report discusses the specific requirements of bidirectional translators for a minimum
voltage separation between VCCA and VCCB. For TXS and TXB-type translators, VCCA must be less than or
equal to VCCB. For translators from the LSF family, Vref_A must be at least 0.8 V less than Vref_B. This
application report also examines the reason for these requirements and the implications when they are
violated.
2
Contents
VCCA and VCCB Bias Requirements for Bidirectional Translators ........................................................
1.1
VCCA and VCCB Separation of TXS and TXB-Type Translators .................................................
1.2
Vref_A and Vref_B Bias for LSF Type Translators...................................................................
References ...................................................................................................................
1
Simplified TXS Architecture ................................................................................................ 2
2
Simplified TXB010X Architecture .......................................................................................... 3
3
TXS and TXB VCCA and VCCB Separation Test Setup .................................................................... 3
4
TXS0101 VCCA > VCCB Leakage ............................................................................................. 4
5
TXS0108E VCCA > VCCB Leakage ........................................................................................... 4
6
TXB0108 VCCA > VCCB Leakage ............................................................................................. 4
7
LSF010x Simplified Architecture ........................................................................................... 5
8
LSF010x Voltage Clamping ................................................................................................ 6
1
2
2
5
6
List of Figures
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1
VCCA and VCCB Bias Requirements for Bidirectional Translators
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1
VCCA and VCCB Bias Requirements for Bidirectional Translators
1.1
VCCA and VCCB Separation of TXS and TXB-Type Translators
The internal architecture of the TXS family of translators contains a pass transistor, internal pullup
resistors on both the I/O ports, and One-shot edge accelerator circuitry. Figure 1 shows a simplified
diagram of this internal architecture. For more information, see the A Guide to Voltage Translation With
TXS-Type Translators application report.
VCCA
VCCB
T1
One
Oneshot
shot
One
Oneshot
shot
T2
R1
10k
R2
10k
Gate Bias
A
B
N2
Figure 1. Simplified TXS Architecture
The TXB family of translators has a weak, buffered architecture with one-shot edge accelerator circuitry to
improve the data rate. The devices can translate the CMOS push-pull logic, however, they are not suitable
for open-drain signals.
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2
Biasing Requirements for TXS, TXB, and LSF Auto-Bidirectional Translators
Copyright © 2017, Texas Instruments Incorporated
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VCCA and VCCB Bias Requirements for Bidirectional Translators
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Figure 2 shows a simplified diagram of this internal architecture. For more information, see the A Guide to
Voltage Translation With TXB-Type Translators application report.
VCCA
VCCB
One
Shot
T1
4k
One
Shot
T2
A
B
One
Shot
T3
4k
T4
One
Shot
Figure 2. Simplified TXB010X Architecture
TXS and TXB-type translators require that VCCA is less than or equal to VCCB. This is due to an internal
protection diode that can become forward biased when the voltage on VCCA exceeds the voltage on VCCB.
When this occurs, large amounts of current flows through VCCA and into the diode, increasing power
consumption and potentially damaging the device. Figure 3 shows the test setup.
4.5 V ± 5.5 V
4.5 V
Keithley
2420 3A SourceMeter
5V
HP 6624A
Power Supply
Keithley
2420 3A SourceMeter
A1
VCCA
A2
A3
A4
A5
A6
A7
A8
OE
B1
VCCB
B2
B3
B4
B5
B6
B7
B8
GND
TXS0108E/
TXB0108
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Figure 3. TXS and TXB VCCA and VCCB Separation Test Setup
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VCCA and VCCB Bias Requirements for Bidirectional Translators
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VCCA and VCCB were supplied and measured through Keithley 2420 3-A source meters, with VCCB fixed at
2.5 V and VCCA swept from 2.5 V to 3.5 V. Figure 4, Figure 5, and Figure 6 show the resulting current
through the supply pins.
40
35
3000
ICCA
ICCB
ICCA
ICCB
2500
30
2000
ICC (PA)
ICC (PA)
25
20
15
10
1500
1000
500
5
0
0
-5
2.5
2.6
2.7
2.8
2.9
3
3.1
VCCA (V)
3.2
3.3
3.4
-500
2.5
3.5
2.6
2.7
D003
Figure 4. TXS0101 VCCA > VCCB Leakage
2.8
2.9
3
3.1
VCCA (V)
3.2
3.3
3.4
3.5
D001
Figure 5. TXS0108E VCCA > VCCB Leakage
1250
1000
ICCA
ICCB
ICC (PA)
750
500
250
0
-250
2.5
2.6
2.7
2.8
2.9
3
3.1
VCCA (V)
3.2
3.3
3.4
3.5
D002
Figure 6. TXB0108 VCCA > VCCB Leakage
As shown in Figure 4, Figure 5, and Figure 6, when the voltage on VCCA exceeds the voltage on VCCB by
roughly 0.6 V, the diode begins to conduct and the ICCA current begins to drastically increase as VCCA
increases.
1.1.1
Summary: Bias Requirements for TXS and TXB Translators
TXS and TXB translators require that VCCA be less than or equal to VCCB. When this requirement is
violated, there is potential for the internal protection diode to become forward biased, resulting in
increased current consumption and potential damage to the device. When using separate power supplies
for VCCA and VCCB, special care must be taken if VCCA and VCCB will operate at the same voltage node. The
system designer must ensure that power supply tolerances will not result in a potential difference between
VCCA and VCCB large enough to bias the internal protection diode.
4
Biasing Requirements for TXS, TXB, and LSF Auto-Bidirectional Translators
Copyright © 2017, Texas Instruments Incorporated
SCEA060 – October 2017
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VCCA and VCCB Bias Requirements for Bidirectional Translators
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1.2
Vref_A and Vref_B Bias for LSF Type Translators
For translators from the LSF family, TI recommends that Vref_A is at least 0.8 V lower than Vref_B. Figure 7
shows a simplified diagram of the LSF architecture. See the Voltage Translation With the LSF Family
application report, and watch the LSF Logic Minute videos to understand the LSF device operation and it's
applications.
Vref_A
VB
EN
Vref_B
Vref_A+VTH
B1
A1
Figure 7. LSF010x Simplified Architecture
As shown in Figure 7, the LSF010x device contains a reference FET between Vref_A and Vref_B, as well as a
pass FET on each channel. The reference FET is designed to set the gate bias voltage of each pass
transistor equal to Equation 1.
Vref_A + VTH
(1)
When the proper bias of Equation 2 is maintained, the reference FET conducts, allowing VB to pull the
voltage at EN to that of Equation 1, and setting the gate voltage on the pass transistor of the channel to
that of Equation 1. The result is that the output port clamps at Equation 3.
Vref_A ≤ Vref_B – 0.8 V
(2)
Vref_A + VTH – VTH = Vref_A
(3)
When the proper bias between Vref_A and Vref_B is not maintained, the gate bias of the pass transistor can
no longer be accurately predicted and the voltage at which the pass transistor turns off is no longer
known. By maintaining the proper bias between Vref_A and Vref_B, the system designer can accurately set
the gate voltage of the pass transistor, allowing for predictable down-translation from the B-port to the Aport without the use of external pullup resistors.
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References
www.ti.com
As an example, the LSF0108 device was tested with Vref_A = Vref_B = 3.3 V. A 25-MHz, 3.3-V square wave
was applied to B1 (blue) and the output was measured at A1 (green). Figure 8 shows the resulting
waveform.
Figure 8. LSF010x Voltage Clamping
When the 3.3 V signal is applied to B1, the output A1 clamps at 2.3 V even though Vref_A is set to 3.3 V.
When the bias between Vref_A and Vref_B is not maintained, the system designer can no longer accurately
set the gate voltage of the pass transistor. Therefore, the system designer can no longer accurately
predict the voltage at which the pass transistor stops conducting. This can result in reduced flexibility and
reduced signal integrity.
1.2.1
Bias Requirements Summary for LSF Translators
When the proper bias between Vref_A and Vref_B is not maintained, the gate bias of the pass transistor can
no longer be accurately predicted, and the voltage at which the pass transistor turns off can no longer be
known. By maintaining the proper bias between Vref_A and Vref_B, the system designer can accurately set
gate voltage of the pass transistor, allowing for predictable down-translation from the B-port to the A-port
without the use of external pullup resistors.
2
References
•
•
•
•
•
6
Texas Instruments, Basics of Voltage-Level Translation, application report
Texas Instruments, Voltage Translation With the LSF Family, application report
Texas Instruments, A Guide to Voltage Translation With TXB-Type Translators, application report
Texas Instruments, A Guide to Voltage Translation With TXS-Type Translators, application report
Texas Instruments, Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices,
application report
Biasing Requirements for TXS, TXB, and LSF Auto-Bidirectional Translators
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