hump shifts to lower or higher temperatures; in the worst outliers it degenerates into a monotonously rising or falling curve. Owing to finite output impedance, changes in VCA voltage affect ICA and, indirectly, VREF, just like they do in transistors or triodes. For a given fixed ICA, a 1 V rise in VCA must be offset with ≈1.4 mV (2.7 mV worst-case maximum) decrease in VREF. The ratio μ = 1 V / 1.4 mV ≈ 300–1000, or ≈ 50–60 dB is the theoretical maximum open-loop voltage gain at DC and low frequencies; Owing to finite transconductance, a rise in ICA causes a rise in VREF at a rate of 0.5–1 mV/mA. Speed and stability The open-loop frequency response of a TL431 can be reliably approximated as a first-order low-pass filter. The dominant pole is provided by a relatively large compensation capacitor in the output stage. An equivalent model contains an ideal 1 A/V voltage-to-current converter, shunted with a 70 nF capacitor. For a typical cathode load of 230 Ohm, this translates to open-loop cutoff frequency of 10 kHz and unity gain frequency of 2 MHz. Owing to various second-order effects, actual unity gain frequency is only 1 MHz; in practice, the difference between 1 and 2 MHz is unimportant. The slew rates of ICA, VCA and the settling time of VREF are not specified. According to Texas Instruments, power-on transient lasts for around 2 μs. Initially, VCA quickly rises to ≈2 V, and then locks at this level for around 1 μs. Charging internal capacitances to steady-state voltages takes up 0.5–1 μs more. Capacitive cathode loads (CL) may cause instability and oscillation. According to stability boundary charts published in the original datasheet, TL431 is absolutely stable when CL is either less than 1 nF, or greater than 10 μF. Inside the 1 nF–10 μF range the likelihood of oscillation depends on the combination of capacitance, ICA and VCA. The worst-case scenario occurs at low ICA and VCA. On the contrary, combinations of high ICA and high VCA, when the TL431 operates close to its maximum dissipation rating, are absolutely stable. However, even a regulator designed for high ICA and high VCA may oscillate at power-on, when VCA has not yet risen to a steady-state level. In a 2014 application note, Texas Instruments admitted that their stability boundary charts are unreasonably optimistic. They describe a "typical" IC sample at zero