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TL431: TL431

Siddharth Sangam
Intellectual 640 points
Part Number: TL431
Other Parts Discussed in Thread: TLV431, TPS3700

Hi,

I am using TL431 as a fixed voltage reference in my design.

but as per datasheet below configuration is specified for cathode voltage (V.KA) > V.ref .

Need to know what should be considered if V.KA is less than V.ref reference voltage. 

and its corresponding formula.

 

Also need to know what will be the V.kA if keeping resistors the same and changing 

5Vcc to 3V3 or vice- versa.

 

with regards

Siddharth

 

  • 0
    TI__Mastermind 19370 points

    Hi Siddharth,

    When the TL431 is regulating in closed loop, the Vka will always be greater or equal the VREF. VREF should always be 2.495V.

    The only condition where Vka might be lower than VREF is if the Ika(min) is not met. If the biasing resistor is too large that the TL431 does not have enough current to turn on, then the VKA might be lower than VREF but this is poor design. You can look at figure 4 to see Vka voltage depending on Ika.

    The resistor values are not important when setting Vka only the ratio. Larger resistors will burn less current but too large will make the output less accurate.

    You design your circuit to work at the minimum operating condition. If 5V for minimum operation VCC then any voltage under 5V might not have enough current for the load or the TL431.

    If you have an example circuit or requirements, we can discuss it with values.

    -Marcoo

  • 0 in reply to Marcoo Z
    Intellectual 640 points

    Hi Marcoo,

    Thanks for the response.

    I need to understand that as per your explanation,is that means this device cannot be used as reference voltage source below 2.5V.

    If my circuit is window comparator which requires two reference thresholds.I am setting this reference using two TL431.

    Ex. Upper threshold 3V and lower threshold 1.5V so can you tell me how can I achieve it.

    with regards

    Siddharth

  • 0 in reply to Siddharth Sangam
    TI__Mastermind 19370 points

    Hi Siddharth,

    If you want to use discrete circuits, you can use the TLV431 and figure 36 from the TL431 datasheet. In this example Vref will be 1.25V instead of 2.5V.

    The issue with using TLV431 or a TL431 is that the output is from VDD to 1V not VDD to GND. So logic low cannot be achieved in certain designs.

    If you need a more integrated solution that can output VDD to GND, the TPS3700 can be used for both OV and UV with easy to set resistors.

    I would highly recommend to use the TPS3700 due its quick response time and high accuracy. Section 8.2.2. has a highly detailed and easy to follow instructions on how to set the values for the resistors.

    -Marcoo