TMUX6212: What are the switches status during and after power on?

Hi Clemens

Thank you for the reply.

1. When I saw the "Fail-safe" function description in your datasheet, I got an impression that this device is robust, and automatically deemed the "fail-safe" feature might apply to I/O pins. 

I suggest that you add the "When the switch is powered off, you are not allowed to apply a voltage to the I/O pins" statement to the "Fail-safe" description paragraph.  The problem is that application design guys will hardly think of or look at IC devices as IC designers do. Some of us are from electrical application backgrounds.  

2. I was assuming that before the supply voltage reaches 4.5V, the I/O pins should be in high impedance just like many other ICs behave, and should not be operating at all... So what do you really mean by "Correct operation is not guaranteed"? such a statement does not help for application design.  We need clear and definite behavior/state descriptions describing what is going on before, during, and after power on.

Thanks

Tony

Hi Kameron

Thanks for your further explanation. 

To be honest, this is the first time feeling a little disappointed with TI's chip. 

Over 90% of today's applications are running at 3.3 or below. in my design, I have to use a booster to boost voltage over 4.5V from 3.3V, which means this device will never be powered up before the I/O pins voltage is applied (because my 4.5V is always lagging 3.3V).  

You may argue that all 3.3V are stepped down from 5V or above, however, do remember that many module designs are with 3.3V supply available only, and 5V or above is not accessible.  Could you design a switch IC that works from 3V? or is it possible to remove the clamping diode and allow voltage on I/O pins before powering on?

My thought may not be valid, but they are real situations of application.  Do you have any questions about dealing with such a problem?

Thank you

Tony

BTW, my supply input is 3.3V--12V(so my design needs to work when 3.3V supply only), which is why I chose this high-voltage switch. the booster is from 3.3V to 15V.

There are switches with high-impedance I/Os during power down; in the search function, this feature is not called "Fail-safe logic" but "Powered-off protection".

What are your signal voltages? It might be easier to generate the switch's power supply with an LDO.

Hi Clemens

Thank you for reply.

Unfortunately, all devices with the "powered-off protection" feature are low voltage (5V below) which will not work when my application supply input is 12V.

I am thinking of any possibility or scenario that TMUX6212 might work in my design.  for example, when I use the switch to put a signal down to the ground, such as connecting the drain to GND, connecting the source to my analog signal with a high output impedance(there will be a put-down resistor anyway), so the current flowing through the clamping diode will be limited and only for a short period of time, once my booster powered up, the clamping diode will not suffer any current...what do you think?

Thanks

Tony

What are your signal voltages?

Hello Tony and sorry for the confusion of this devices operation let me try to explain some more,

1. You are correct in your idea for how the tmux6212 will work in your application and when the booster is powered up your the device will not pass the signal if the signal path/source pin  that you are applying the input voltage(i/o voltage must within recommended operation conditions) on is unselected with the control pins.

2.So because this device has clamping diodes to supplies and when the voltage on the input is higher than the supply (like vdd=0v and source pin>vdd+0.7v) the current from the input will conduct through the esd clamping diode and go through to the supply. The problem is when the internal ESD diode is forward-biased, the voltage on the I/O path can back power the switch supply pin and damage components on the power rail. These two links talk more about powered off protection and esd diodes to supplies

https://e2e.ti.com/support/switches-multiplexers-group/switches-multiplexers/f/switches-multiplexers-forum/658358/faq-do-i-need-powered-off-protection-hi-z-when-vcc-0v

https://e2e.ti.com/support/switches-multiplexers-group/switches-multiplexers/f/switches-multiplexers-forum/1108438/faq-do-ti-analog-switches-multiplexers-have-internal-esd-protection

3.I think another solution we can try is to use a device like the TMUX4827 which has powered off protection which will make sure the switch is in HI-z when it is unpowered and not allow back powering in your system. In addition this device has a feature called beyond the supply which means it can pass signals larger than its supply. So it can pass a 12v signal with a 3v supply! 

In addition to get the 1:1 4 channel functionality, we would need to use 4 of these devices. Finally I will relay your suggestions to the rest of the mux team.

Please let me know what you think!

Regards,

Kameron

Hi Clemens

I intended to use the switch to turn on/off a high current load by controlling a P-MOSFET, there are two ways to do this, 

1. the source pin of the switch connected to Vin(3--12V), the drain pin of the switch connected to the gate of the FET. so I can turn on/off the FET(with a gate pull-down resistor).

2. the source pin of the switch connected to the gate of the FET, and the drain pin of the switch connected to GND, so I can turn on/off the FET (with a gate pull-up resister). 

I need to use both ways to switch on/off the load, so the voltage at the source pin of the switch is Vin...I want to but I can't use Vin to power the switch as it will not work when Vin=3.3V.

I have a buck converter to get 3.3V(for the test of the control circuit) when Vin>3.3V.

Now you can see how critical a clear and definite state/behavior of the switch I/O pin is to my design during system power-up.

I have at least 4 control signals to independently turn on/off the load, so the 4 channels of the switch are connected in either a common source or a common drain connection. I assume it should not be a problem,  Please confirm.

Thanks

Tony

HI Kameron

Thank you so much for the detailed information.

1. I am happy that you confirmed the high impedance state of the I/O pin of the switch during powering on.  I will assume the high impedance of the I/O pin will keep and remain after power over 4.5V to its stable level until its SEL pin is toggled(I will make sure all SEL pins are pulled down before and during the booster power-up),  am I right?

2. Thank you for your suggestion. the powered-off protection of TMUX4827 is good (why this feature is not available in high-voltage switches?), but it requires an additional LDO when my Vin > 5V(the booster is a must-have due to my load voltage and current measurement circuit), another concern is my Vin over-voltage protection at 16V, the 12V beyond supply of the switch I/O pins may not be good enough, also to have 4 pieces of switches might be too costly from both BOM and board area perspectives. 

3. I am still thinking of the possibility of TMUX6212, I can understand the potential problems associated with back-powering of the clamping diode, in my design, the booster output is to power INA849/INA821 for load voltage/current measurement only. Please check my reply to Clemens for information on my application, and help me to find any possibility of using TMUX6212.  My idea is to limit the current flowing through the clamping diode for a short period of time during booster power-on, because back-powering is not so harmful in my circuit as long as the switch can survive the back-powering. In the worst case, I can accept all drain pins connected to GND and all source pins connected to the gate of FET via a resistor, thus, limiting the current flowing through the clamping diode within a few mA.  Please help to check if this idea is viable.

Thank you for support.

Tony

Hello Tony,

1.Yes you are correct once the device is powered to 4.5v and the select pins are pulled down to ground the io pins will be in a hi-z state.

2.Understand your concerns for using the TMUX4827 in your application and will help you use the TMUX6212

3.After looking at your explanation and your response to Clemens I understand your application. You are correct that if you limit the current through the device  to a within a few ma range the mux should not be destroyed in this application. You would just need to use a current limiting resistor in front of the signal paths to make sure the current is limited. 

In addition I know that back powering is not a concern in your application but  if you would if you would like a device with power off protection the TMUX7412F is an option.

Please let me know what you think!

Regards,

Kameron

Hi Kameron

Thanks for the help.

After a study, we still need at least one switch to be connected between Vin and the gate of the PMOSFET without or with a small resistor for load short circuit protection (to quickly turn off the power line), also allowing current to flow through the clamping diode is not a decent design, in order to be compliance with IEC61508, we have decided to give up TMUX6212. The alternative part could be ADG5412BF from ADI, although it is much more expensive, we have run out of options.  

I hope that TI can come up with a competing part at a lower cost in the near future, and I will be ready to switch back to TI.

I really appreciate your responsive technical support which is the key reason making TI my favorite supplier (at least better than ADI).

Thank you.

Tony

Hello Tony,

Thank you for your reply and much appreciated! 

I just wanted to reiterate that TI actually has a competitive solution to the ADG5412BF in the form of the TMUX7412F!

Here is a comparison table to better help you understand.

TMUX7412F vs ADG5412BF.pptx

While there are many benefits to using the TMUX7412F(highlighted in green on the PowerPoint) over the ADG device, the only  different feature the competitor device has is the bidirectional overvoltage protection  and power off protection(meaning it can protect unwanted signals from source to drain or drain to source). The TMUX7412F  is bidirectional in passing signals but is made to protect signals source to drain( which is exactly what you need for this application I believe).

Please take a look at the PowerPoint and see if these would be a suitable device.

Regards,

Kameron

Hi Kameron

Thank you for highlighting  TMUX7412F which I didn't notice.

It looks a lot better than TMUX6212, but power-off protection applying only to source pins may be a concern and could be problematic in our application

We need switches to be connected between Vin and the gate of the PMOSFET to quickly turn off the FET, thus, the drain pins (connected to the gate of the FET) need to remain high impedance to sustain gate voltage (equal to Vin) when Vin is powered-up while Vdd of the switch is ground (from the booster output). 

In order to turn on our load at a designed slew rate, we add a capacitor to the gate of the PMOSFET(with a gate pull-down resistor and a switch to GND). This capacitor together with the booster output capacitor will be charged by Vin via the clamping diode of drain pins, such charging could bring the gate voltage to the ground level and may transiently turn on the PMOSFET before booster output is established.

Regards

Tony

Hi Kameron

Thank you for your support.

Please tell the team that from an application perspective, it would be a great product if an analog switch could be used as a relay substitution.

Best Regards

Tony