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Old 18th September 2004, 06:06 PM   #1
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Smile Please help with TL431 shunt reg calculation

I have read and re-read the datasheet and I still don't know how to calcluate the values when TL431 is used as a shunt regulator.

For example, if I have a 18V source, want to regulate it down to 15V, the max load current is ~60mA, how do I calculate the R, R1, R2 and Vref?

Vo = (1+R1/R2)*Vref

and R is the current limiting resistor.

Please kindly help me, thanks.

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Old 18th September 2004, 07:34 PM   #2
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You are trying to use the TL431 just like a zener diode. I would advise against this unless you are sure that you won't be removing the load for any reason. If this happens, all the load current plus the idle current will pass through the TL431, exceeding its max ratings. A better approach if you must use a shunt regulator would be to use the TL431 to drive a PNP pass transistor using a proper heat sink if necessary. The attached schematic shows what you will need. You will need to make sure that the TL431 gets the proper idle current to work. The max spec value is 2ma - I would use 4-5. This is the purpose of the 150 ohm resistor, to make sure that the proper idle current is drawn through the 431 before the PNP turns on and diverts any excess current. Iwould give the PNP another 5 ma or so of idle current, so you have a total of about 10ma of idle current for the PNP and 431. This. plus your 60ma load current will set the value for the dropping resistor. The dropping resistor is set for the minimum value of Vin, and is (Vin-Vout)/(iload +iidletotal). The resisitive voltage divider at the TL431 reference is set to regulate at 15V.
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Old 19th September 2004, 10:46 AM   #3
Sandor is offline Sandor  Italy
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Location: Milan
A simple rule using TL431 alone:

Vref is typical of the device. TL431 has a Vref = 2.5 V

For R2 choose a value near 1 Kohm. For R1 choose the nearest value to the result of the following formula:

R1 = R2 * (Vo/2.5 - 1)

for a good regulation the current flowing through the TL431 should be the same as in the load. Say Vdrop the voltage across the series resistor R and Iload the current flowing in the load. We have:

R = Vdrop/ (2 * Iload)

In your case:
R2 = 1 Kohm
R1 = 1000 * (15/2.5 -1 ) = 1000 * (6 - 1) = 1000 * 5 = 5000 ohm.
You can choose R1 = 5.1 Kohm.

Vdrop = 18 - 15 = 3 V
Iload = 60 mA = 0.06 A
R = 3 / (2*0.06) = 25 ohm

Pay attention to the max current allowed for the anode current in the TL431.
Hope this helps.

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Old 19th September 2004, 02:50 PM   #4
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Thanks a lot!!!!!!!!!!!!
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Old 20th September 2004, 05:50 PM   #5
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Hi deskmate88,

Excel spreadsheet for shunt regulator design... TL431 Calculator

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Old 11th April 2008, 01:54 PM   #6
trmnmy is offline trmnmy  Turkey
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sandor, it is very good explanation... thanks...
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Old 11th April 2008, 02:29 PM   #7
AndrewT is offline AndrewT  Scotland
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Originally posted by josiphal
Excel spreadsheet for shunt regulator design... TL431 Calculator
what are Texas thinking about?
Macros and no signature.
Are they expecting us to activate macros for all unknown software?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 2nd January 2014, 01:25 AM   #8
Z600 is offline Z600  Viet Nam
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Originally Posted by josiphal View Post
Hi deskmate88,

Excel spreadsheet for shunt regulator design... TL431 Calculator

how to calculate I (div) / I (ref)
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