In the configuration shown in Figure 8-2 TLV431 behaves as a comparator, comparing the V_ref pin voltage to the internal virtual reference voltage. When provided a proper cathode current (I_k), TLV431 has enough open loop gain to provide a quick response. With the TLV431's min Operating Current maximum (Imin) being 55uA to 100uA over temperature, operation below that can result in low gain, leading to a slow response.