Datasheet TLV431 (Diodes) - 8

TLV431 Application Notes
In a conventional shunt regulator application (Figure 1), an external series resistor (R3) is connected between the supply voltage, VIN, and the TLV431. R3 determines the current that flows through the load (IL) and the TLV431 (IK). The TLV431 adjusts how much current it sinks or “shunts” to maintain a voltage equal to VREF across its feedback pin. Because load current and supply voltage may vary, R3 should be small enough to supply at least the minimum acceptable IKMIN to the TLV431, even when the supply voltage is at its minimum and the load current is at its maximum value. When the supply voltage is at its maximum and IL is at its minimum, R3 should be large enough so that the current flowing through the TLV431 is less than 15mA. R3 is determined by the supply voltage, (VIN), the load and operating current, (IL and IK), and the TLV431’s reverse breakdown voltage, VKA. R3 IL Vin Vout  I VN KA V  I R K R1 3  L I K I V REF where TLV431  R  V  V  1 KA REF 1  C1  R2  0.1µF R2 and VKA = VOUT GND
Figure 1
The values of R1 and R2 should be large enough so that the current flowing through them is much smaller than the current through R3, yet not too large that the voltage drop across them causes IREF to affect the reference accuracy. The most frequent application of the TLV431 is in isolated, low-output voltage power supplies where the regulated output is galvanically isolated from the controller. As shown in Figure 2, the TLV431 drives current, IF, through the optocoupler’s LED, which in turn drives the isolated transistor that is connected to the controller on the primary side of the power supply. This completes the feedback path through the isolation barrier and ensures that a stable isolated supply is maintained. Assuming a forward drop of 1.4V across the optocoupler diode allows output voltages as low as 2.7V to be regulated. Regulated Supply Regulated Supply Optocoupler IF To controller To controller  1 R  V  V 1  OUT REF  2 R  R3 R1 V  2.7 V  2.7 OUT (max) OUT  3 R  TLV431 I mA 15 ( F min) R2 GND
Figure 2. Using the TLV431 as the Regulating Element in an Isolated PSU
TLV431 8 of 17 March 2024 Document number: DS32088 Rev. 9 - 2
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