TIDA-01471: Multiplexers and current source based on TLV431BQ

Victor Ilien

Part Number: TIDA-01471
Other Parts Discussed in Thread: MUX36S08, TMUX6104

Dear Ahmed,

I have two questions related to this reference design.

1. In this forum thread, you mentioned the preferred multiplexer for the current source: MUX36xxx or MUX61xxx (TMUX6104). Despite the fact that the TMUX6104 is a 36-volt multiplexer, with single supply, the maximum supply voltage (and analog inputs) is 16.5 V. This is less than the voltage of the current source (20-24 V). How can we use TMUX6104 in this case? I think the MUX36S08 is more suitable. Isn't it?

2. The second question about current source based on TLV431BQ. TLV431BQ comes in five different packages. Each of the packages has its own ability to dissipate heat. I did not find calculations on how to calculate the amount of heat dissipated by this chip in this application. Do I have to use the smallest TLV431BQ package or are there any specific requirements?

Best wishes,

Victor

over 5 years ago

0 Ahmed Noeman over 5 years ago

TI__Expert 3520 points

Hi Victor,

1. You are right, using general MUX61xxx was an oversight. Only TMUX6104 can support 36V.

You can select one of the following list based on the number of channels.

#CH

2. We have an article dedicated for designing such current sources with some proposed devices "High-side current sources for industrial applications"

Note that the TLV431BQ works as a series reference with fixed voltage and limited quiescent current. It is not the current source in this case and is not critical in thermal analysis. You should care more about the element in which the output current (e.g. 20mA) is flowing. being an opamp or a transistor. a little bit about that you will find in the aforementioned article when calculating maximum current range.

Once you determined the maximum current allowed given the maximum device power and worst-case voltage drop on the current steering element, you should make sure the circuit is not exceeding that limit. The next question in thermal analysis would be what surface temperature would the device have in case of constantly dissipating this maximum power limit. for that you need to check the package thermal properties under (Thermal information) section in the datasheet.

here are one app note discussing this topic:

How to Properly Evaluate Junction Temperature with Thermal Metrics

Thermal Characteristics of Linear and Logic Packages Using JEDEC PCB Designs

You can also use the online tool for thermal analysis

http://www.ti.com/design-resources/design-tools-simulation/models-simulators/pcb-thermal-calculator.html

which is described in this blog

Regards,

Ahmed

0 Victor Ilien over 5 years ago in reply to Ahmed Noeman

Prodigy 60 points

Thank you for your response!

Can you explain, please, how TMUX6104 can deal with 20+ Volts, if datasheet says: VDD to GND - maximum 18 V? TMUX6104 is 36V only with dual supplies.

0 Ahmed Noeman over 5 years ago in reply to Victor Ilien

TI__Expert 3520 points

Sorry, I got the device and application mixed up in mind. For single supply operation it's 16.5V as you mentioned and confirmed by Datasheet. so we can remove the TMUX6104 from possible options here.

0 Victor Ilien over 5 years ago in reply to Ahmed Noeman

Prodigy 60 points

Thank you! Now it's clear for me.

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TIDA-01471: Multiplexers and current source based on TLV431BQ