The schema of a series pass regulator, based on the TL431 have a capacitor between the cathode and the reference. It is drawn as an electrolytic, so this must be several microfarads.
What is the role of this capacitor, soft start or maybe miller compensation ?
And haw to calculate correct value of this capacitor. (V+=15V and Vout=12V; Io=0.2A). Or maybe this capacitor is pick up empirically, but then whit values I should start off.
Hi,
The capacitor is used to bypass the ripple to the reference input so that the internal amplifier reduces the output ripple by negative feed back. The value of C may be calculated to get Xc at ripple frequency to be one tenth of parallel combination of R2and R1.
You probably overrate the schematic drawing details. TI has two similar application circuits with external output transistor in the datasheet, one has no compensation capacitor, the other has 10 nF. Actually, the need for compensation depends on the output load, but TL431 already has a first order frequency response that may achieve stability in many cases.
Oki. This schema is from Philips datasheet.
But I have another question, in Hitachi datasheet I found this diagram
It seems to me that gain equation is wrong. What you think about that