Table 1. Design Parameters

9.2.1.2.1 Basic Operation

In the configuration shown in Figure 26 TLV431 will behave as a comparator, comparing the V_ref pin voltage to the internal virtual reference voltage. When provided a proper cathode current (I_k), TLV431 will have enough open loop gain to provide a quick response. With the TLV431's min Operating Current maximum (Imin) being 55 uA to 100 uA over temperature, operation below that could result in low gain, leading to a slow response.

9.2.1.2.2 Overdrive

Slow or inaccurate responses can also occur when the reference pin is not provided enough overdrive voltage. This is the amount of voltage that is higher than the internal virtual reference. The internal virtual reference voltage will be within the range of 1.24V ±(0.5% or 1.0% ) depending on which version is being used.

The more overdrive voltage provided, the faster the TLV431 will respond. This can be seen in figures Figure 27 and Figure 28, where it displays the output responses to various input voltages.

For applications where TLV431 is being used as a comparator, it is best to set the trip point to greater than the positive expected error (i.e. +1.0% for the A version). For fast response, setting the trip point to > 10% of the internal V_ref should suffice.

For minimal voltage drop or difference from Vin to the ref pin, it is recommended to use an input resistor < 10 kΩ to provide I_ref.

9.2.1.2.3 Output Voltage and Logic Input Level

In order for TLV431 to properly be used as a comparator, the logic output must be readable by the recieving logic device. This is accomplished by knowing the input high and low level threshold voltage levels, typically denoted by V_IH & V_IL.

As seen in Figure 27, TLV431's output low level voltage in open-loop/comparator mode is ~1 V, which is sufficient for some 3.3V supplied logic. However, would not work for 2.5 V and 1.8 V supplied logic. In order to accommodate this a resistive divider can be tied to the output to attenuate the output voltage to a voltage legible to the receiving low voltage logic device.

TLV431's output high voltage is approximately V_sup due to TLV431 being open-collector. If V_sup is much higher than the receiving logic's maximum input voltage tolerance, the output must be attenuated to accommodate the outgoing logic's reliability.

When using a resistive divider on the output, be sure to make the sum of the resistive divider (R1 & R2 in Figure 26) is much greater than R_sup in order to not interfere with TLV431's ability to pull close to V_sup when turning off.

Table 2. Design Parameters

9.2.2.2.1 Programming Output/Cathode Voltage

In order to program the cathode voltage to a regulated voltage a resistive bridge must be shunted between the cathode and anode pins with the mid point tied to the reference pin. This can be seen in Figure 29, with R1 & R2 being the resistive bridge. The cathode/output voltage in the shunt regulator configuration can be approximated by the equation shown in Figure 29. The cathode voltage can be more accuratel determined by taking in to account the cathode current:

V_O=(1+R1/R2)*V_ref–I_ref*R1

In order for this equation to be valid, TLV431 must be fully biased so that it has enough open loop gain to mitigate any gain error. This can be done by meeting the I_min spec denoted in Recommended Operating Conditions table.

9.2.2.2.2 Total Accuracy

When programming the output above unity gain (Vka=Vref), TLV431 is susceptible to other errors that may effect the overall accuracy beyond V_ref. These errors include:

• R1 and R2 accuracies
• V_I(dev) - Change in reference voltage over temperature
• ΔV_ref / ΔV_KA - Change in reference voltage to the change in cathode voltage
• |z_KA| - Dynamic impedance, causing a change in cathode voltage with cathode current

Worst case cathode voltage can be determined taking all of the variables in to account. Application note SLVA445 assists designers in setting the shunt voltage to achieve optimum accuracy for this device.

9.2.2.2.3 Stability

Though TLV431 is stable with no capacitive load, the device that receives the shunt regulator's output voltage could present a capacitive load that is within the TLV431 region of stability, shown in Figure 18. Also, designers may use capacitive loads to improve the transient response or for power supply decoupling.