This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TL431-Q1: Question about STABILITY BOUNDARY CONDITIONS

Part Number: TL431-Q1
Other Parts Discussed in Thread: TL431LI-Q1, TL431

Hello Folks,

I have a question about the stability boundary conditions. I my design i have VKA from around 5.1 V and Ika of 4.2 mA.

In the datasheet Figure 16, recommends that for this condition the load capacitance CL should be higher than around 2µF (Curve B). Because the device may oscillate.

As depicted in my design, the Vref output is after a voltage divider with also a 1µF connected and my CL is 1µF. 

My question is to know how critical is this oscillations and the stability. If the output voltage can goes to 0 V or the device can be damaged or the output voltage will drift or oscillations of x% can happen.

Thank in advance for the support.

My  question is if we need to follow strictly this rule? because im my design I have a CL of 1µF and after a voltage divider with a capacitor of 1µF

  • Hi Tobias,

    This is extremely critical. The 1uF capacitive load on VKA can cause the device to have oscillations of at least 10% can happen or more. I would increase this or decrease this value to an acceptable range.

    Figure 13 on the datasheet shows the boundary is around 2uF. Keep in mind that his boundary is for 0 degree phase margin and it is a typical value. Due to variations in temp and lot to lot variations, I would suggest to add at least half decade to get a reasonable conservative value. So I would use a value of 4.7uF or greater.

    Another option is to use the TL431LI-Q1 because its boundary at 5V, according to figure 12, can allow you to use a 1uF/2uF capacitor range.

    I would design the system to the worst case current.

  • Hello,

    One more question about this topic. As I seen the curve is from 0 to 10µF capacitor. within the curve? In case that i used NO capacitor or smaller than 10nF, it would be stable. Or I see something wrong?

     

  • Hi Tobias,

    The TL431 from No Cap to 1nF the TL431 is expected to be stable. Above 10uF the device should be stable.

    Between 1nF and 10nF it depends on voltage output due to variations on the stability line.

  • Hi Marcoo,

    Thank you for the reply.