Odd 74HCT1G125 behaviour
Clash Royale CLAN TAG#URR8PPP
$begingroup$
I am using a 74HCT1G125 buffer as an '5 V output driver', but, in some cases it has a 12 V, low impedance source presented at its output pin (but in these cases its enable is false, i.e the output should be high impedance (Hi-Z)). When I do this, the 5 V supply (that the buffer uses also) is pushed up to around 8 V. Surely if its Hi-Z is an open circuit, how is the external 12 V making its way and affecting the 5 V?
I have checked and indeed the device is being 'told' to be in Hi-Z. I then put 12 V onto its output pin, and somehow, the VCC gets pushed up.
digital-logic buffer
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
$endgroup$
add a comment |
$begingroup$
I am using a 74HCT1G125 buffer as an '5 V output driver', but, in some cases it has a 12 V, low impedance source presented at its output pin (but in these cases its enable is false, i.e the output should be high impedance (Hi-Z)). When I do this, the 5 V supply (that the buffer uses also) is pushed up to around 8 V. Surely if its Hi-Z is an open circuit, how is the external 12 V making its way and affecting the 5 V?
I have checked and indeed the device is being 'told' to be in Hi-Z. I then put 12 V onto its output pin, and somehow, the VCC gets pushed up.
digital-logic buffer
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
$endgroup$
$begingroup$
Assuming that your circuit still works with the resulting high gate driver resistance, you can fix this problem by putting a series resistor on the gate output. Output protection diodes can typically withstand several milliamps without causing problems. So if you size the series resistor for say 1ma at 12V - 5V => ~7K ohms, that might fix your problem.
$endgroup$
– crj11
Feb 27 at 18:48
2
$begingroup$
@crj11 - It's generally strongly frowned upon to depend on the implementation details of an IC's output drivers. If you're going to do this, use an external schottky clamp diode too.
$endgroup$
– Connor Wolf
Feb 27 at 21:05
$begingroup$
Looking at the datasheet in the answer, it says that the diodes can take 20ma max, so 1ma should not be an issue. The only issue would be if the circuit was not drawing enough current to keep the VCC below 5V when the 12V is active.
$endgroup$
– crj11
Feb 27 at 22:01
2
$begingroup$
@ConnorWolf, the fact they actually specify a max clamping current rather than maximum pin voltage hopefully indicates you can rely on the protection structure surviving reasonably margined currents below the abs max limit. But you also have to worry about where else on your board those currents could go and what other chips they could muck up.
$endgroup$
– The Photon
Feb 27 at 22:33
$begingroup$
How about putting a diode between the 125 output and the 12V source?
$endgroup$
– Harper
Feb 27 at 22:51
add a comment |
$begingroup$
I am using a 74HCT1G125 buffer as an '5 V output driver', but, in some cases it has a 12 V, low impedance source presented at its output pin (but in these cases its enable is false, i.e the output should be high impedance (Hi-Z)). When I do this, the 5 V supply (that the buffer uses also) is pushed up to around 8 V. Surely if its Hi-Z is an open circuit, how is the external 12 V making its way and affecting the 5 V?
I have checked and indeed the device is being 'told' to be in Hi-Z. I then put 12 V onto its output pin, and somehow, the VCC gets pushed up.
digital-logic buffer
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
$endgroup$
I am using a 74HCT1G125 buffer as an '5 V output driver', but, in some cases it has a 12 V, low impedance source presented at its output pin (but in these cases its enable is false, i.e the output should be high impedance (Hi-Z)). When I do this, the 5 V supply (that the buffer uses also) is pushed up to around 8 V. Surely if its Hi-Z is an open circuit, how is the external 12 V making its way and affecting the 5 V?
I have checked and indeed the device is being 'told' to be in Hi-Z. I then put 12 V onto its output pin, and somehow, the VCC gets pushed up.
digital-logic buffer
digital-logic buffer
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
edited Feb 28 at 8:33
Peter Mortensen
1,60031422
1,60031422
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
asked Feb 27 at 18:14
MattyT2017MattyT2017
9918
9918
$begingroup$
Assuming that your circuit still works with the resulting high gate driver resistance, you can fix this problem by putting a series resistor on the gate output. Output protection diodes can typically withstand several milliamps without causing problems. So if you size the series resistor for say 1ma at 12V - 5V => ~7K ohms, that might fix your problem.
$endgroup$
– crj11
Feb 27 at 18:48
2
$begingroup$
@crj11 - It's generally strongly frowned upon to depend on the implementation details of an IC's output drivers. If you're going to do this, use an external schottky clamp diode too.
$endgroup$
– Connor Wolf
Feb 27 at 21:05
$begingroup$
Looking at the datasheet in the answer, it says that the diodes can take 20ma max, so 1ma should not be an issue. The only issue would be if the circuit was not drawing enough current to keep the VCC below 5V when the 12V is active.
$endgroup$
– crj11
Feb 27 at 22:01
2
$begingroup$
@ConnorWolf, the fact they actually specify a max clamping current rather than maximum pin voltage hopefully indicates you can rely on the protection structure surviving reasonably margined currents below the abs max limit. But you also have to worry about where else on your board those currents could go and what other chips they could muck up.
$endgroup$
– The Photon
Feb 27 at 22:33
$begingroup$
How about putting a diode between the 125 output and the 12V source?
$endgroup$
– Harper
Feb 27 at 22:51
add a comment |
$begingroup$
Assuming that your circuit still works with the resulting high gate driver resistance, you can fix this problem by putting a series resistor on the gate output. Output protection diodes can typically withstand several milliamps without causing problems. So if you size the series resistor for say 1ma at 12V - 5V => ~7K ohms, that might fix your problem.
$endgroup$
– crj11
Feb 27 at 18:48
2
$begingroup$
@crj11 - It's generally strongly frowned upon to depend on the implementation details of an IC's output drivers. If you're going to do this, use an external schottky clamp diode too.
$endgroup$
– Connor Wolf
Feb 27 at 21:05
$begingroup$
Looking at the datasheet in the answer, it says that the diodes can take 20ma max, so 1ma should not be an issue. The only issue would be if the circuit was not drawing enough current to keep the VCC below 5V when the 12V is active.
$endgroup$
– crj11
Feb 27 at 22:01
2
$begingroup$
@ConnorWolf, the fact they actually specify a max clamping current rather than maximum pin voltage hopefully indicates you can rely on the protection structure surviving reasonably margined currents below the abs max limit. But you also have to worry about where else on your board those currents could go and what other chips they could muck up.
$endgroup$
– The Photon
Feb 27 at 22:33
$begingroup$
How about putting a diode between the 125 output and the 12V source?
$endgroup$
– Harper
Feb 27 at 22:51
$begingroup$
Assuming that your circuit still works with the resulting high gate driver resistance, you can fix this problem by putting a series resistor on the gate output. Output protection diodes can typically withstand several milliamps without causing problems. So if you size the series resistor for say 1ma at 12V - 5V => ~7K ohms, that might fix your problem.
$endgroup$
– crj11
Feb 27 at 18:48
$begingroup$
Assuming that your circuit still works with the resulting high gate driver resistance, you can fix this problem by putting a series resistor on the gate output. Output protection diodes can typically withstand several milliamps without causing problems. So if you size the series resistor for say 1ma at 12V - 5V => ~7K ohms, that might fix your problem.
$endgroup$
– crj11
Feb 27 at 18:48
2
2
$begingroup$
@crj11 - It's generally strongly frowned upon to depend on the implementation details of an IC's output drivers. If you're going to do this, use an external schottky clamp diode too.
$endgroup$
– Connor Wolf
Feb 27 at 21:05
$begingroup$
@crj11 - It's generally strongly frowned upon to depend on the implementation details of an IC's output drivers. If you're going to do this, use an external schottky clamp diode too.
$endgroup$
– Connor Wolf
Feb 27 at 21:05
$begingroup$
Looking at the datasheet in the answer, it says that the diodes can take 20ma max, so 1ma should not be an issue. The only issue would be if the circuit was not drawing enough current to keep the VCC below 5V when the 12V is active.
$endgroup$
– crj11
Feb 27 at 22:01
$begingroup$
Looking at the datasheet in the answer, it says that the diodes can take 20ma max, so 1ma should not be an issue. The only issue would be if the circuit was not drawing enough current to keep the VCC below 5V when the 12V is active.
$endgroup$
– crj11
Feb 27 at 22:01
2
2
$begingroup$
@ConnorWolf, the fact they actually specify a max clamping current rather than maximum pin voltage hopefully indicates you can rely on the protection structure surviving reasonably margined currents below the abs max limit. But you also have to worry about where else on your board those currents could go and what other chips they could muck up.
$endgroup$
– The Photon
Feb 27 at 22:33
$begingroup$
@ConnorWolf, the fact they actually specify a max clamping current rather than maximum pin voltage hopefully indicates you can rely on the protection structure surviving reasonably margined currents below the abs max limit. But you also have to worry about where else on your board those currents could go and what other chips they could muck up.
$endgroup$
– The Photon
Feb 27 at 22:33
$begingroup$
How about putting a diode between the 125 output and the 12V source?
$endgroup$
– Harper
Feb 27 at 22:51
$begingroup$
How about putting a diode between the 125 output and the 12V source?
$endgroup$
– Harper
Feb 27 at 22:51
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
You aren't allowed to put more than VCC+0.5V on the output pin, even if it's in high-Z state. If you do, you could damage the chip.
What's happening in your circuit is there's a diode connected from the output pin to VCC, oriented so that it will be reverse biased in normal operation (cathode connected to VCC). This diode is there to shunt current during ESD events.
When you connect 12 V to the output pin, you forward bias this diode and deliver current out of the VCC pin. Depending what else is connected to the same VCC net, you could damage those other parts, or high current through the ESD diode could damage the 1G125.
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
$endgroup$
add a comment |
$begingroup$
Many chips are poorly specified with regard to output clamping behavior. There is generally some amount of current that would be sourced or sinked to indefinitely without disrupting operation or causing the pin to reach a voltage that would damage it, and there is usually some upper and lower voltage bounds which would need to be exceeded to cause the part to pass enough current to disrupt it or cause damage, but many parts only specify the latter.
Note that the absolute maximum rating may suggest a level of current below which damage would be unlikely, but not necessarily a level that would disrupt operation. I've observed at least one 74HCxx part where pairs of adjacent pins' positive-rail clamp circuits behaved somewhat like PNP transistors, so driving one pin above the upper rail while the adjacent pin was below it would cause current to flow between the two pins.
I'm not sure why manufacturers decline to say anything useful about clamp diodes' actual behavior. I would have no qualm about manufacturers' Recommended Operating Conditions limiting current to values well below what the part could actually take, but in many cases designing external circuitry to limit clamp currents to e.g. 100uA or less would be much easier than trying to design external circuitry to guarantee that clamp currents could never exceed 1nA.
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
$endgroup$
add a comment |
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2 Answers
2
active
oldest
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2 Answers
2
active
oldest
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votes
$begingroup$
You aren't allowed to put more than VCC+0.5V on the output pin, even if it's in high-Z state. If you do, you could damage the chip.
What's happening in your circuit is there's a diode connected from the output pin to VCC, oriented so that it will be reverse biased in normal operation (cathode connected to VCC). This diode is there to shunt current during ESD events.
When you connect 12 V to the output pin, you forward bias this diode and deliver current out of the VCC pin. Depending what else is connected to the same VCC net, you could damage those other parts, or high current through the ESD diode could damage the 1G125.
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
$endgroup$
add a comment |
$begingroup$
You aren't allowed to put more than VCC+0.5V on the output pin, even if it's in high-Z state. If you do, you could damage the chip.
What's happening in your circuit is there's a diode connected from the output pin to VCC, oriented so that it will be reverse biased in normal operation (cathode connected to VCC). This diode is there to shunt current during ESD events.
When you connect 12 V to the output pin, you forward bias this diode and deliver current out of the VCC pin. Depending what else is connected to the same VCC net, you could damage those other parts, or high current through the ESD diode could damage the 1G125.
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
$endgroup$
add a comment |
$begingroup$
You aren't allowed to put more than VCC+0.5V on the output pin, even if it's in high-Z state. If you do, you could damage the chip.
What's happening in your circuit is there's a diode connected from the output pin to VCC, oriented so that it will be reverse biased in normal operation (cathode connected to VCC). This diode is there to shunt current during ESD events.
When you connect 12 V to the output pin, you forward bias this diode and deliver current out of the VCC pin. Depending what else is connected to the same VCC net, you could damage those other parts, or high current through the ESD diode could damage the 1G125.
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
$endgroup$
You aren't allowed to put more than VCC+0.5V on the output pin, even if it's in high-Z state. If you do, you could damage the chip.
What's happening in your circuit is there's a diode connected from the output pin to VCC, oriented so that it will be reverse biased in normal operation (cathode connected to VCC). This diode is there to shunt current during ESD events.
When you connect 12 V to the output pin, you forward bias this diode and deliver current out of the VCC pin. Depending what else is connected to the same VCC net, you could damage those other parts, or high current through the ESD diode could damage the 1G125.
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
answered Feb 27 at 18:21
The PhotonThe Photon
86.5k398198
86.5k398198
add a comment |
add a comment |
$begingroup$
Many chips are poorly specified with regard to output clamping behavior. There is generally some amount of current that would be sourced or sinked to indefinitely without disrupting operation or causing the pin to reach a voltage that would damage it, and there is usually some upper and lower voltage bounds which would need to be exceeded to cause the part to pass enough current to disrupt it or cause damage, but many parts only specify the latter.
Note that the absolute maximum rating may suggest a level of current below which damage would be unlikely, but not necessarily a level that would disrupt operation. I've observed at least one 74HCxx part where pairs of adjacent pins' positive-rail clamp circuits behaved somewhat like PNP transistors, so driving one pin above the upper rail while the adjacent pin was below it would cause current to flow between the two pins.
I'm not sure why manufacturers decline to say anything useful about clamp diodes' actual behavior. I would have no qualm about manufacturers' Recommended Operating Conditions limiting current to values well below what the part could actually take, but in many cases designing external circuitry to limit clamp currents to e.g. 100uA or less would be much easier than trying to design external circuitry to guarantee that clamp currents could never exceed 1nA.
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
$endgroup$
add a comment |
$begingroup$
Many chips are poorly specified with regard to output clamping behavior. There is generally some amount of current that would be sourced or sinked to indefinitely without disrupting operation or causing the pin to reach a voltage that would damage it, and there is usually some upper and lower voltage bounds which would need to be exceeded to cause the part to pass enough current to disrupt it or cause damage, but many parts only specify the latter.
Note that the absolute maximum rating may suggest a level of current below which damage would be unlikely, but not necessarily a level that would disrupt operation. I've observed at least one 74HCxx part where pairs of adjacent pins' positive-rail clamp circuits behaved somewhat like PNP transistors, so driving one pin above the upper rail while the adjacent pin was below it would cause current to flow between the two pins.
I'm not sure why manufacturers decline to say anything useful about clamp diodes' actual behavior. I would have no qualm about manufacturers' Recommended Operating Conditions limiting current to values well below what the part could actually take, but in many cases designing external circuitry to limit clamp currents to e.g. 100uA or less would be much easier than trying to design external circuitry to guarantee that clamp currents could never exceed 1nA.
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
$endgroup$
add a comment |
$begingroup$
Many chips are poorly specified with regard to output clamping behavior. There is generally some amount of current that would be sourced or sinked to indefinitely without disrupting operation or causing the pin to reach a voltage that would damage it, and there is usually some upper and lower voltage bounds which would need to be exceeded to cause the part to pass enough current to disrupt it or cause damage, but many parts only specify the latter.
Note that the absolute maximum rating may suggest a level of current below which damage would be unlikely, but not necessarily a level that would disrupt operation. I've observed at least one 74HCxx part where pairs of adjacent pins' positive-rail clamp circuits behaved somewhat like PNP transistors, so driving one pin above the upper rail while the adjacent pin was below it would cause current to flow between the two pins.
I'm not sure why manufacturers decline to say anything useful about clamp diodes' actual behavior. I would have no qualm about manufacturers' Recommended Operating Conditions limiting current to values well below what the part could actually take, but in many cases designing external circuitry to limit clamp currents to e.g. 100uA or less would be much easier than trying to design external circuitry to guarantee that clamp currents could never exceed 1nA.
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
$endgroup$
Many chips are poorly specified with regard to output clamping behavior. There is generally some amount of current that would be sourced or sinked to indefinitely without disrupting operation or causing the pin to reach a voltage that would damage it, and there is usually some upper and lower voltage bounds which would need to be exceeded to cause the part to pass enough current to disrupt it or cause damage, but many parts only specify the latter.
Note that the absolute maximum rating may suggest a level of current below which damage would be unlikely, but not necessarily a level that would disrupt operation. I've observed at least one 74HCxx part where pairs of adjacent pins' positive-rail clamp circuits behaved somewhat like PNP transistors, so driving one pin above the upper rail while the adjacent pin was below it would cause current to flow between the two pins.
I'm not sure why manufacturers decline to say anything useful about clamp diodes' actual behavior. I would have no qualm about manufacturers' Recommended Operating Conditions limiting current to values well below what the part could actually take, but in many cases designing external circuitry to limit clamp currents to e.g. 100uA or less would be much easier than trying to design external circuitry to guarantee that clamp currents could never exceed 1nA.
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
answered Feb 27 at 22:28
supercatsupercat
38.3k163112
38.3k163112
add a comment |
add a comment |
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$begingroup$
Assuming that your circuit still works with the resulting high gate driver resistance, you can fix this problem by putting a series resistor on the gate output. Output protection diodes can typically withstand several milliamps without causing problems. So if you size the series resistor for say 1ma at 12V - 5V => ~7K ohms, that might fix your problem.
$endgroup$
– crj11
Feb 27 at 18:48
2
$begingroup$
@crj11 - It's generally strongly frowned upon to depend on the implementation details of an IC's output drivers. If you're going to do this, use an external schottky clamp diode too.
$endgroup$
– Connor Wolf
Feb 27 at 21:05
$begingroup$
Looking at the datasheet in the answer, it says that the diodes can take 20ma max, so 1ma should not be an issue. The only issue would be if the circuit was not drawing enough current to keep the VCC below 5V when the 12V is active.
$endgroup$
– crj11
Feb 27 at 22:01
2
$begingroup$
@ConnorWolf, the fact they actually specify a max clamping current rather than maximum pin voltage hopefully indicates you can rely on the protection structure surviving reasonably margined currents below the abs max limit. But you also have to worry about where else on your board those currents could go and what other chips they could muck up.
$endgroup$
– The Photon
Feb 27 at 22:33
$begingroup$
How about putting a diode between the 125 output and the 12V source?
$endgroup$
– Harper
Feb 27 at 22:51