Texas Instruments | Effects of pullup and pulldown resistors on TXS and TXB devices (Rev. A) | Application notes | Texas Instruments Effects of pullup and pulldown resistors on TXS and TXB devices (Rev. A) Application notes

Texas Instruments Effects of pullup and pulldown resistors on TXS and TXB devices (Rev. A) Application notes
Application Report
SCEA054A – September 2017 – Revised March 2018
Effects of External Pullup and Pulldown Resistors
on TXS and TXB Devices
Shreyas Rao, Adrian Ozer
ABSTRACT
The TXS and TXB family of devices belong to the auto-bidirectional translation products from TI. These
devices can be sensitive to external pullup or pulldown resistors due to the internal pullup resistors
present in the TXS device and the internal serial resistors present in the TXB device. In this application
note, the effects of external pullup and pulldown resistors on VOL and VOH levels of the TXS and TXB
family of devices are examined. As external pullup or pulldown resistors impact the way the output
behaves in a design, a high value of pullup or pulldown resistors greater than 50 kΩ is beneficial to
prevent performance degradation.
1
2
Contents
External Resistors on TXS and TXB Translators ........................................................................ 2
References and Further Reading ........................................................................................ 13
List of Figures
1
TXS010x Simplified Output Circuit ........................................................................................ 2
2
TXS0108E Pullup Test Setup .............................................................................................. 3
3
TXS0108E Pullup Resistor Output ........................................................................................ 4
4
TXS0108E Pulldown Test Setup
5
TXS0108E Pulldown Resistor Output ..................................................................................... 7
6
TXB010x Simplified Output Circuit ........................................................................................ 9
7
TXB0108 Pullup Test Setup ................................................................................................ 9
8
TXB0108 Pullup Resistor Output......................................................................................... 10
9
TXB0108 Pulldown Test Setup ........................................................................................... 11
10
TXB0108 Pulldown Resistor Output ..................................................................................... 12
..........................................................................................
6
List of Tables
1
TXS0108E Pullup Results Summary ...................................................................................... 5
2
TXS0108E Pulldown Results Summary .................................................................................. 8
3
TXB0108 Pullup Results Summary ...................................................................................... 11
4
TXB0108 Pulldown Results Summary ................................................................................... 13
Trademarks
All trademarks are the property of their respective owners.
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1
External Resistors on TXS and TXB Translators
1.1
Terminology
VOL – The voltage level at the output when driving logic low. For more detailed information, see the
application note on Understanding and interpreting Standard -logic data sheets
VOH – The voltage level at the output when driving logic high. For more detailed information, see the
application note on Understanding and interpreting Standard -logic data sheets
TXS - TI auto bidirectional translation switch type device family. For more detailed information, see the
application note on A Guide to Voltage Translation With TXS-Type Translators
TXB - TI auto bidirectional translation buffered type device family. For more detailed information, see the
application note on A Guide to Voltage Translation With TXB-Type Translators
RPD - Pulldown resistor
RPU- Pullup resistor
1.2
TXS Pullup Resistor Analysis
The TXS family of translators incorporates internal pullup resistors designed to hold the output when
driving logic high. The internal pullup resistor is fixed at 10 kΩ for the TXS0101, TXS0102, and TXS0104E
translators, whereas the TXS0108E translator has dynamic pullup resistors that change value depending
on whether the output is driving a high or low. When driving a high, the pullup resistor value is 4 kΩ and
when driving a low, the pullup resistor value is 40 kΩ. See Figure 1 for a simplified diagram of the output
configuration.
TXS0101/2/4 Output
VCC
Circuit
OneShot
TXS0108E Output
Circuit
VCC
RPU
OneShot
10 k
OneShot
Output
Output
Figure 1. TXS010x Simplified Output Circuit
The one-shot circuit is designed to momentarily increase the drive strength on a transitioning edge. When
in a DC steady state, the output is held high by the internal pullup resistor. Due to this, adding external
pullup or pulldown resistors can impact the way the output behaves. To demonstrate the resulting
changes, the output of the TXS0108E is measured across four different pullup and pulldown resistor
values ranging from 4.7 kΩ to 100 kΩ. See Figure 2 for the test setup.
2
Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices
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1.8 V
1 kHz
5 ns Edge
1.8 V
1.8 V
VCCB
B1
A1
VCCA
A2
A3
A4
A5
A6
A7
A8
OE
B2
B3
B4
B5
B6
B7
B8
GND
RPU
Tektronix AFG3102
Arbitrary Waveform
Generator
To 1 0Ÿ
Active Probe
TXS0108E
Figure 2. TXS0108E Pullup Test Setup
Input A1 uses a 1-kHz signal at 1.8 V with 5-ns rising and falling edges. The output, B1, is pulled up to
3.3 V through a pullup resistor and measured using a 1-MΩ active probe. All other input channels are
grounded.
Figure 3 shows the results. Channel 1 (purple) represents the input signal and Channel 3 (green)
represents the output signal. The corresponding pullup resistor value is shown in the upper left corner of
each oscilloscope screenshot.
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Figure 3. TXS0108E Pullup Resistor Output
The results in Figure 3 illustrate the changes that pullup resistors have on the VOL levels of TXS type
translators. The baseline VOL with no pullup resistor is 30 mV compared to the VOL of 264 mV using a 4.7kΩ pullup resistor due to the parallel combination of the external pullup resistor with the internal 40 kΩ
while driving low. This is attributed to the additional current through the pass transistor, resulting in a
larger voltage drop across the pass transistor.
REQ= (40 × 4.7) / (40 + 4.7) = 4.2 kΩ; where REQ is the equivalent resistor due to the parallel combination
of the external pullup resistor with the internal 40 kΩ while driving low.
4
Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices
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Adding a strong external pullup resistor increases the current seen at that I/O port because of the reduced
parallel resistance of the internal pullup and the external pullup. The result is that the increase in VOL is
dependent on both the current sinking capability of the external driving device and the resistance of the
internal pass transistor. See Table 1 for a summary of the pullup results.
Table 1. TXS0108E Pullup Results Summary
RESISTOR VALUE (kΩ)
VOL (V)
VOH (V)
No external resistor
0.029
3.18
4.7
0.264
3.19
9.8
0.169
3.19
47
0.059
3.19
100
0.038
3.19
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External Resistors on TXS and TXB Translators
1.2.1
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TXS Pulldown Resistor Analysis
The output of the TXS0108E is also measured across four different pulldown resistor values ranging from
4.7 kΩ to 100 kΩ. See Figure 4 for the test setup.
1.8 V
1.8 V
3.3 V
1 kHz
5 ns Edge
A1
VCCA
A2
A3
A4
A5
A6
A7
A8
OE
B1
VCCB
B2
B3
B4
B5
B6
B7
B8
GND
To 1 0Ÿ
Active Probe
RPD
TXS0108E
Figure 4. TXS0108E Pulldown Test Setup
Input A1 uses a 1-kHz signal at 1.8 V with 5-ns rising and falling edges. The output, B1, is pulled down to
ground through a pulldown resistor and measured using a 1-MΩ active probe. All other input channels
were grounded.
Figure 5 shows the results. Channel 1 (purple) represents the input signal and Channel 3 (green)
represents the output signal. The pulldown resistor value used is shown in the upper left corner of each
oscilloscope screenshot.
6
Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices
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Figure 5. TXS0108E Pulldown Resistor Output
The results in Figure 5 illustrate the changes that pulldown resistors have on the VOH levels of TXS type
translators. The baseline VOH with no pullup resistor is 3.18 V compared to the VOH of 1.68 V using a 4.7kΩ pulldown resistor.
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The internal pullup resistor and the external pulldown resistor creates a voltage divider network, which
causes a decrease in VOH. The negative impact of pulldown resistors on VOH demonstrates why the TXS
family of translators must only be used to drive high-impedance loads. See Table 2 for a summary of the
results.
Table 2. TXS0108E Pulldown Results Summary
RESISTOR VALUE (kΩ)
1.2.2
VOL (V)
VOH (V)
No external resistor
0.029
3.18
4.7
0.028
1.68
9.8
0.027
2.18
47
0.029
2.91
100
0.027
3.04
Conclusion
TXS type translators can be used with external pullup resistors without a significant impact on output
voltage levels, provided that the driving device is capable of sinking the additional required current to pull
the line low and the voltage drop across the internal pass transistor is not significant. Use Equation 1 to
estimate the required current sinking capabilities of the driver:
VCCA VCCB
I
RA
RB
(1)
RA and RB are equal to the equivalent parallel resistance of the external pullup resistor and the internal
pullup resistor. For the TXS0101, TXS0102, and TXS0104E devices, the internal pullup resistance is
equal to 10 kΩ when driving low. For the TXS0108E, the internal pullup resistance is equal to 40 kΩ when
driving low. TI recommends that this current be limited to values under 1 mA for the TXS0108E and 10
mA for the TXS0101, TXS0102, and TXS0104E .
pulldown resistors must be avoided because of decreased VOH levels. If pulldown resistors are necessary
they must be limited to values of 50 kΩ or greater. The negative impact of pulldown resistors on VOH levels
also demonstrates why the TXS family of translators must only be used to drive high-impedance loads.
8
Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices
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1.3
TXB Pullup and Pulldown Resistor Analysis
The TXB family of translators is designed to drive high-impedance loads, with the output driven by a 4-kΩ
buffer when in a DC steady state. In this section, the impact of external resistors on VOH and VOL levels of
TXB translators is examined. Figure 6 shows a simplified diagram of the TXB010x output.
TXB010x Output
Circuit
VCC
OneShot
4k
OneShot
Output
Figure 6. TXB010x Simplified Output Circuit
If strong external pullup or pulldown resistors are added, a resistor divider network is formed with the 4-kΩ
buffer resulting in adverse changes in VOH and VOL levels. The output of the TXB0108 is measured across
four different pullup and pulldown resistor configurations ranging from 4.7 kΩ to 100 kΩ. Figure 7 shows
the test setup for the pullup resistor analysis.
Tektronix AFG3102
Arbitrary Waveform
Generator
1.8 V
3.3 V
1.8 V
1 kHz
5 ns Edge
RPU
A1
VCCA
A2
A3
A4
A5
A6
A7
A8
OE
B1
VCCB
B2
B3
B4
B5
B6
B7
B8
GND
To 1 0Ÿ
Active Probe
TXB0108
Figure 7. TXB0108 Pullup Test Setup
Input A1 uses a1-kHz signal at 1.8 V with 5-ns rising and falling edges. The output, B1, is pulled up to
3.3 V through a pullup resistor and measured using a 1-MΩ active probe. All other input channels are
grounded.
Figure 8 shows the results. Channel 1 (green) represents the input signal and Channel 3 (purple)
represents the output signal. The corresponding pullup resistor value is shown in the upper left corner of
each oscilloscope screenshot.
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Figure 8. TXB0108 Pullup Resistor Output
The results in Figure 8 illustrate the impact that pullup resistors have on the VOL levels of TXB type
translators. The baseline VOL with no pullup resistor is –6.8 mV compared to the VOL of 1.52 V using a 4.7kΩ pullup resistor. See Table 3 for a summary of the results.
10
Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices
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Table 3. TXB0108 Pullup Results Summary
RESISTOR VALUE (kΩ)
1.3.1
VOL (V)
VOH (V)
No External Resistor
–0.007
3.19
4.7
1.52
3.23
9.8
0.977
3.21
47
0.26
3.19
100
0.11
3.19
TXB Pulldown Resistor Analysis
The output of the TXB0108 is also measured across four different pulldown resistor values ranging from
4.7 kΩ to 100 kΩ. Figure 9 shows the test setup.
Tektronix AFG3102
Arbitrary Waveform
Generator
1.8 V
1.8 V
3.3 V
1 kHz
5 ns Edge
A1
VCCA
A2
A3
A4
A5
A6
A7
A8
OE
B1
VCCB
B2
B3
B4
B5
B6
B7
B8
GND
To 1 0Ÿ
Active Probe
RPD
TXB0108 E
Figure 9. TXB0108 Pulldown Test Setup
Input A1 uses a 1-kHz signal at 1.8 V with 5-ns rising and falling edges. The output, B1, is pulled down to
ground through a pulldown resistor and measured using a 1 MΩ active probe. All other input channels are
grounded.
Figure 10 shows the results. Channel 1 (green) represents the input signal and Channel 3 (purple)
represents the output signal. The pulldown resistor value used is shown in the upper left corner of each
oscilloscope screenshot.
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Figure 10. TXB0108 Pulldown Resistor Output
The results in Figure 10 illustrate the changes that pulldown resistors have on the VOH levels of TXB type
translators. The baseline VOH with no pullup resistor is 3.98 V compared to the VOH of 1.71 V using a 4.7kΩ pulldown resistor. The negative impact of pulldown resistors on VOH levels demonstrates why the TXB
family of translators must only be used to drive high-impedance loads. See Table 4 for a summary of the
results.
12
Effects of External Pullup and Pulldown Resistors on TXS and TXB Devices
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References and Further Reading
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Table 4. TXB0108 Pulldown Results Summary
RESISTOR VALUE (kΩ)
1.3.2
VOL (V)
VOH (V)
No External Resistor
–0.007
3.19
4.7
–0.01
1.71
9.8
–0.01
2.22
47
–0.01
2.91
100
–0.012
3.04
Conclusion
Pullup and pulldown resistors of less than 50 kΩ must not be used with TXB translators because the
internal 4-kΩ buffer and external resistors create a resistor divider network. Equation 2 provides an
estimated calculation for the resulting VOL based on the pullup value.
4 k:
VOL
u VCCO
4 k: Rpu
(2)
In Equation 2, substitute in the external pullup resistor value for Rpu and the VCC voltage at the output port
for VCCO to obtain the estimated VOL value.
Similarly, the resulting VOH value based on an external pulldown resistor can be estimated using
Equation 3.
Rpd
VOH
u VCCO
Rpd 4 k:
where
•
•
Rpd is the pulldown resistor value
VCCO is the voltage supply of the output port
(3)
From these results, only weak pullup or pulldown resistors (> 50 kΩ) must be used with TXB devices. If
stronger external pullup resistors are required, refer to the TXS family or LSF family of devices.
2
References and Further Reading
2.1
Relevant Technical Documents
•
•
•
•
•
•
Basics of Voltage-Level Translation
Selecting the Right Level-Translation Solution
Voltage-Level Translation With the LSF Family
A Guide to Voltage Translation With TXS-Type Translators
A Guide to Voltage Translation With TXB-Type Translators
Voltage Translation Between Different Logic Voltage Standards
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