TL1431 Reference Voltage

Muthu,

At 0.11uF, there will be oscillation on the cathode.

The stability chart lines represent zero degrees of phase margin for a typical device at room temperature.

With no capacitance the TL1431 is stable because the internal compensation capacitor attenuates the gain (20db/decade) before parasitic internal poles add much lag. As the output capacitance is increased the output capacitor pole adds more lag. With three poles in play there will be oscillation. With more capacitance the bandwidth will decrease such that that the parasitic poles no longer add lag; with two poles there will be only a couple of degrees of phase margin but that is enough to be stable (with transient ringing and gain peaking). The TL1431 has no zeros in the frequency compensation. If more phase margin is desired the most practical place to add a zero is the output capacitor effective ESR.

There are two ways to ensure stability for all devices over the temperature range.
1) Stay one decade away from stability line (0.1x capacitance of left line or 10x capacitance of right line).
2) Add series resistor between cathode and output capacitor (add zero). R = 1 / (pi * C * F); (pi=3.14, C is load cap, F is oscillation or ringing frequency)
Examining response from a step change in cathode current is a good way to judge phase margin. (load/line transient)

I recommend less than 2nF or greater then 22uF. Other values can be used if the correct series resistance is added.

Muthu,

If the oscillation is sinusoidal shaped then yes it would be close to the cross over frequency.
Saw-tooth oscillation wave shape may be farther away from cross over.