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Old 19th July 2007, 04:58 AM   #1
Archwn is offline Archwn  United Kingdom
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Question Need help understanding TL431

Having a while reading TL431 datasheet several times and could not find the proper answer. I guess it is beyond my knowledge now and would like to leave here the questions. Need your help.

1. In the datasheet there is a typical application schematics. The very first one should be simple shunt regulator schematic. The thing is there is always a resistor series between source and the shunt set. What is the purpose of this R? I would like to know how can we calculate this resistor value. The best clue I got is from some datasheets say to get 10mA min into TL431. Is this mean the minimum sink current?

I would guess that if I have 10v from source and I need 5v regulated. then I have to put 5 volts over this R. Does this mean I have to know the total current flow before I can get this R? What if the current is not constant?


2. What will be the output current to the load? Does this relate to the sink current? any limitations? any mistakes of my understanding?

3. If I have 5v and I need 5v out, can I remove this resistor?


4. On some implementation, there will be an LM317 or LT1085 configure as a CCS then the serial R then the TL431. How to calculate the Current required and the value of the serial R

So confuse now and hope someone could help.
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Old 19th July 2007, 07:08 AM   #2
AndrewT is online now AndrewT  Scotland
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Default Re: Need help understanding TL431

Quote:
Originally posted by Archwn
1. In the datasheet there is a typical application schematics. The very first one should be simple shunt regulator schematic. The thing is there is always a resistor series between source and the shunt set. What is the purpose of this R? I would like to know how can we calculate this resistor value. The best clue I got is from some datasheets say to get 10mA min into TL431. Is this mean the minimum sink current?

I would guess that if I have 10v from source and I need 5v regulated. then I have to put 5 volts over this R. Does this mean I have to know the total current flow before I can get this R? What if the current is not constant?
Think of the total current into the load + the shunt as constant. The minimum for the 431 is usually 1mA. Using your example of Vin=10 and Vout=5. Take a class A discrete opamp consuming 20mA that varies it's current in each supply rail over the range of 10mA up or down. The 431 must pull at least 1mA when the opamp is drawing a maximum of 30mA the total becomes 31mA. Check the minimum opamp rail current @ 10mA, the 431 must now pull 21mA. We know that 31mA flows through the feed resistor, so it's R=161r, set to 160r. Check the power dissipation in the TL431 at maximum input voltage and at minimum load current and the dissipation in the resistor.
Quote:
Originally posted by Archwn


2. What will be the output current to the load? Does this relate to the sink current? any limitations?
You must be able to calculate the load current. The MAXIMUM VARIATION in the load current <=99mA
Quote:
Originally posted by Archwn

3. If I have 5v and I need 5v out, can I remove this resistor?
It will not work. All regulators need to drop voltage to achieve regulation.

Quote:
Originally posted by Archwn


4. On some implementation, there will be an LM317 or LT1085 configure as a CCS then the serial R then the TL431. How to calculate the Current required and the value of the serial R
You don't need a CCS and a series R. But, the CCS probably has a resistor to measure it's output current. This resistor is part of the CCS. A 317 requires 1.25V across out to ref. It also needs between 1.3V and 2V (graph in datasheet) between in and out. The total voltage to operate 317 CCS is between 2.5V and 3.2V depending on output current.
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Old 19th July 2007, 07:11 AM   #3
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First, go to datasheetarchive.com and download the motorola datasheet. Type in TL431 and scroll down near the bottom of the page to get the motorola sheet.

From the way you spoke it sounds like you are confusing a current reg with a voltage reg. The 431 keeps a constant current if used as a CCS (constant current source). It is actually a programmable zener.

Get the motorola datsheet and then post any problems that you have. We will then have common data.

Mark
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Old 19th July 2007, 07:24 AM   #4
johnrtd is offline johnrtd  Belgium
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Default shunt regulators

The big advantage of a shunt regulator is that the series resistor isolates the load from the main supply. With a larger difference between supply and shunt that resistor will have a larger value so isolation factor increases.

In some of our circuits (DAC and Phono preamp) we use an LM317/337 bringing the supply voltage down from 45 Volt to 30 Volt. Then a shunt regulator follows bringing the Voltage down to 18 - 24 Volt.

The TL431 is a nice thing but it has the disadvantage that at frequencies over 100 kHz the regulation is less effective. So it's a good thing always to have a capacitor there with good HF specifications. In our DAC we use an Os-Con 10 F and a polyester 10 NF in parallel for the digital circuits.

When designing audio circuits, discrete or with op amps, a better option is to use a discrete shunt. Two transistors and a zener diode do the trick far better then the TL431.

In tube circuits a tube can be used for the shunt regulation.
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Old 19th July 2007, 07:51 AM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
I breadboarded a 317 + 431 + BD140 (as PMA's schematic) a few days ago.
I scoped the DC and could not lock onto any frequency due to the low level of noise coming out of the assembly.
I estimate the wideband noise is about 0.3mVpp ~=100uVrms.
Not very low noise, but much better than many IC series regulators.

My next is adding a 20V Zener to the tail of the 431 to see if I can get +52Vdc out of the assembly (for a Krell Klone front end).
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Old 19th July 2007, 08:01 AM   #6
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Archwn, I see now that you are building the shunt regulator. The R you asked about is to keep from burning out the 431. It is to restrict the current to within specifications.

Some of your source current will be used by the 431, some of it by the R1+R2 divider, and the remainder by your load.

If the load will not always be connected one must choose the resistor values so that the 431 will not get too much current with load disconnected. This is a disadvantage of the zener regulator, which is what this basically is.


The series pass regulator (fig 21 on motorola sheet) may be more practical. Unlike using a zener on the base of the transistor, the 431 puts feedback on the B-E junction such that the output voltage is not affected by the load.

The shunt regulator has the same load regulation, but the series pass configuration is not so delicate.
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Old 19th July 2007, 08:07 AM   #7
AndrewT is online now AndrewT  Scotland
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The really big advantage for the CCS + shunt regulator is, it is short circuit proof.

The CCS prevents serious damage downstream no matter how ham fisted one is.

It also happens to perform rather well.
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Old 19th July 2007, 08:33 AM   #8
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Quote:
CCS + shunt regulator
AndrewT: where is the CCS?
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Old 19th July 2007, 11:28 AM   #9
AndrewT is online now AndrewT  Scotland
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Default Re: Need help understanding TL431

Quote:
Originally posted by Archwn

4. On some implementation, there will be an LM317 or LT1085 configure as a CCS then the serial R then the TL431.
here is the CCS. Did you read the thread?
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Old 19th July 2007, 06:44 PM   #10
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AndrewT: Thanks for the reply.

I'm having a little trouble with the concept of a constant current-constant voltage power supply.

What is actually going on is that the CCS is a current limiter for a shunt regulator?
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