Texas Instruments | Factors Affecting VOL for TXS and LSF Auto-bidirectional Translation Devices | Application notes | Texas Instruments Factors Affecting VOL for TXS and LSF Auto-bidirectional Translation Devices Application notes

Texas Instruments Factors Affecting VOL for TXS and LSF Auto-bidirectional Translation Devices Application notes
Application Report
SCEA056 – November 2017
Factors Affecting VOL for TXS and LSF Auto-bidirectional
Translation Devices
Shreyas Rao, Adrian Ozer
ABSTRACT
The TXS and LSF families of translators differ from other translator families, because their outputs 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 to drive logic low. This results in a VOL (Logic
low output voltage) level that is a function of VIL(Logic low Input voltage), the resistance of the internal
pass transistor, and the current through the pass transistor. In Section 1 and Section 2, the relationship
between VOL, VIL, pass transistor resistance and the pass transistor current are examined in detail for the
TXS and LSF families of passive auto-bidirectional translators.
1
2
3
Contents
VOL Versus VIL of TXS Type Translators ..................................................................................
1.1
Summary for VOL Versus VIL of TXS Type Translators .........................................................
VOL Versus VIL of LSF Type Translators ..................................................................................
2.1
Summary for VOL Versus VIL of LSF-Type Translators .........................................................
References ...................................................................................................................
2
3
4
5
5
List of Figures
1
Simplified TXS Architecture ................................................................................................ 2
2
TXS0101 VOL Versus VIL .................................................................................................... 3
3
TXS0108E VOL Versus VIL
4
5
..................................................................................................
LSF010x Simplified Architecture ...........................................................................................
LSF0108 VOL Versus VIL .....................................................................................................
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3
4
4
1
VOL Versus VIL of TXS Type Translators
1
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VOL Versus VIL of TXS 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 pass transistor bias voltage, VBIAS, is referenced to VCCI, the input side supply voltage. When the
voltage on one of the I/O lines drops below the threshold of the gate bias voltage, VBIAS, the pass transistor
turns on and the input tracks the output. In general, when the pass transistor stops conducting the voltage
is roughly equal to Equation 1.
VCCI
2
(1)
When the pass transistor is on, the output voltage is equal to the input voltage plus the voltage drop
across the pass transistor. Therefore, as the current increases, the output voltage also increases, even
with a constant input voltage. Likewise, if the input voltage increases, the output voltage also increases,
even with a constant current. This implies that the VOL voltage level is a function of the VIL voltage level
and the current flowing through the pass transistor.
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2
Factors Affecting VOL for TXS and LSF Auto-bidirectional Translation Devices
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VOL Versus VIL of TXS Type Translators
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Figure 2 and Figure 3 show this relationship, where VOL is plotted against VIL at various current levels for
the TXS0108E and TXS0101 voltage translators.
1000
800
900
700
800
600
600
VOL (mV)
VOL (mV)
700
500
400
500
400
300
300
200
200
100 PA
1 mA
10 mA
100
0
100
100 PA
1 mA
0
0
50 100 150 200 250 300 350 400 450 500 550 600
VIL (mV)
D004
0
Figure 2. TXS0101 VOL Versus VIL
50 100 150 200 250 300 350 400 450 500 550 600
VIL (mV)
D005
Figure 3. TXS0108E VOL Versus VIL
As shown in Figure 2 and Figure 3, there is a linear relationship between VOL and VIL as long as the
current remains constant and the voltage at the output does not exceed the threshold voltage of the pass
transistor. The TXS0101 can maintain a reasonable VOL, with up to 10 mA of current through the pass
transistor with the input at 0 V. However, the TXS0108E device cannot maintain a reasonable VOL for
current levels above 1 mA. With 1 mA through the pass transistor, the TXS01018E device also begins to
oscillate at the output as the input voltage reaches 400 mV, because the output voltage begins to exceed
the threshold voltage of the pass transistor. At 100 µA, the TXS0101 and TXS0108E devices can both
maintain a VOL under 50 mV with VIL at 0 V.
1.1
Summary for VOL Versus VIL of TXS Type Translators
When driving the line low, the voltage at the output is equal to Equation 2.
VOL = VIL + IOL × Rpass
where
•
•
Rpass is the on-state resistance of the pass transistor.
IOL is the current flowing through the pass transistor.
(2)
Therefore, to maintain the lowest possible VOL two practices must be observed. First, the VIL must be kept
as close as possible to 0 V, and second, the current through the pass transistor must be kept as low as
possible. To prevent excessive current through the pass transistor, strong external pullup resistors of less
than 50 kΩ must be avoided. The effects of external resistors are described in the Effects of External
Pullup and Pulldown Resistors on TXS and TXB Devices application report.
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3
VOL Versus VIL of LSF Type Translators
2
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VOL Versus VIL of LSF Type Translators
Like the TXS family, the LSF family also contains a pass transistor that conducts when driving the line low.
The LSF family differs in that there are no internal pullup resistors; however, the behavior of the pass
transistor is identical with the exception of the pass transistor gate reference, which is supplied by the EN
pin, as shown in Figure 4. See the Voltage Translation With the LSF Family application report, watch the
LSF Logic Minute videos to understand more about the LSF device operation and its applications.
Vref_A
VB
EN
Vref_B
Vref_A+VTH
B1
A1
Figure 4. LSF010x Simplified Architecture
When the pass transistor is on, the output voltage is equal to the input voltage plus the voltage drop
across the pass transistor. Therefore, as the current increases, the output voltage also increases, even
with a constant input voltage. Likewise, if the input voltage increases, the output voltage also increases,
even with a constant current. This implies that the VOL voltage level is a function of the VIL voltage level
and the current flowing through the pass transistor. Figure 5 shows this relationship, where VOL is plotted
against VIL at various current levels for the LSF0108 device.
700
600
500
VOL
400
300
200
100 PA
1 mA
10 mA
100
0
0
50 100 150 200 250 300 350 400 450 500 550 600
VIL
D006
Figure 5. LSF0108 VOL Versus VIL
As shown in Figure 5, there is a linear relationship between VOL and VIL, given a constant current. The LSF
translators can maintain a VOL less than 100 mV, even at 10 mA, indicating a lower on-state resistance of
the pass transistor when compared to the TXS family of translators.
4
Factors Affecting VOL for TXS and LSF Auto-bidirectional Translation Devices
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VOL Versus VIL of LSF Type Translators
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2.1
Summary for VOL Versus VIL of LSF-Type Translators
When driving the line low, the voltage at the output is equal to Equation 3.
VOL = VIL + IOL × Rpass
where
•
•
Rpass is the on-state resistance of the pass transistor.
IOL is the current flowing through the pass transistor.
(3)
Therefore, to maintain the lowest possible VOL two practices must be observed. First, VIL must be kept as
close as possible to 0 V. Second, the current through the pass transistor must be kept as low as possible.
3
References
•
•
•
•
•
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
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