A Product Line of Diodes Incorporated AN60 Designing with Shunt Regulators Fixed voltage regulators and opto isolation Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Incorporated Introduction An often used application of the 3-terminal shunt regulators or references is taking advantage of their precision function to turn otherwise medium accuracy linear regulators into precision ones. Figure 1 and Figure 2 show how this is done. A major benefit is that the series regulator maintains all its in-built functions whilst behaving like a precision device by virtue of the reference taking over the primary control function. Improved accuracy and extended voltage range The circuits are very simple and the output voltage, VOUT, is computed exactly as in other examples (see references at the end of this document). AP1084 Vin Vin VR1 Vout Vout GND VREG R1 VOUT ≥ (VREG + VREF ) VREF TLV431 (All features of the regulator such as output current, short circuit protection, thermal shutdown, etc, are maintained.) R2 C1 R1 ⎞ ⎛ VOUT = VREF ⎜1 + ⎟ R2 ⎠ ⎝ GND Figure 1 Increasing output voltage of a fixed linear regulator One such application of references is in using it to improve the accuracy and/or extend the range and flexibility of fixed voltage regulators. In the circuit in Figure 1 above, both the output voltage and its accuracy are entirely determined by the TLV431, R1 and R2. However the rest of the features of the regulator (up to 5A output current, output current limiting and thermal shutdown) are all still available thus combining the best of both devices. AP1117 or AP1084 Vin Vin IB VR1 Vout Vout GND R3 1.2V R1 C1 VOUT ≥ (VREG + VREF ) R3 = VRE F TLV431 R1 ⎞ ⎛ VOUT = VREF ⎜1 + ⎟ R2 ⎠ ⎝ VIN − (VOUT − VREG ) IB 0.1mA ≤ I B ≤ 18mA R2 (All features of the regulator such as short circuit protection, thermal shutdown, etc, are maintained.) GND Figure 2 Adjustable linear voltage regulator Issue 1 - September 2008 © Diodes Incorporated, 2008 1 www.zetex.com www.diodes.com AN60 Figure 2 is similar to Figure 1 with adjustability added. Note the addition of R3, which is only required for the AP1117 due to the fact that its ground or adjustment pin can only supply a few micro-Amps of current at best. R3 is therefore needed to provide sufficient bias current for the TLV431. Opto-isolated control Regulated supply Regulated supply Optocoupler IF To controller VOUT VF To controller R3 R1 ⎞ ⎛ VOUT = VREF ⎜1 + ⎟ ⎝ R2 ⎠ R1 VOUT (min) − (V F + V KA(min) ) VREF REF1 I F (min) R2 ≥ R3 ≥ VOUT (max) − (V F + V KA(min) ) 15mA GND Figure 3 Using a reference as the regulating element in an isolated PSU A frequent application of three-terminal references is in isolated power supplies where the regulated output is galvanically isolated from the controller. In this case, an isolated feedback to the controller is implemented as shown in Figure 3. REF1 drives current, IF, through the optocoupler's LED which in turn drives the isolated transistor which is connected to the controller on the primary side of the power supply. The opto-isolated transistor is configured to suit the requirements of the controller on the primary side. An equilibrium state is only achieved when REF1 has achieved VREF across its feedback pin. This loop completes the feedback path that ensures that a stable isolated supply given by the expression in Figure 3 above is maintained and the reference helps to "trim" out the non-linear transfer function of the opto-isolator. In this way, the reference functions as a high gain amplifier, and, as part of the feedback loop, ensures that variations in the opto-coupler over time and temperature are not significant. As an example, the Vishay SFH610 series opto-coupler requires an input current of between 1mA and 15mA to achieve a useful current transfer ratio for control purposes. This range of current is well within the capability of the TLV431. For the SFH610, VF is about 1.2V, and IFmin is taken to be 1mA. For the TLV431, VKAmin is taken to be 1.24V, though it can be slightly lower in practice. If for example a 5V output is required, then suitable values of R1 and R2 are easily found to be: R1 = 30.5k⍀ R2 = 10k⍀ The value of R3 determines the normal regulating input current IF of the opto-isolator. Assuming 10mA: VOUT (min) − (VF + VKA(min) ) IF = 5 − (1.2 + 1.24) 0.01 = 256 ohms This value is not critical and R3 could be set to 240 or 270⍀. Issue 1 - September 2008 © Diodes Incorporated, 2008 2 www.zetex.com www.diodes.com AN60 Conclusion The above examples show how the superior parameters of the Diodes' 3-terminal references can be used to improve COTS voltage regulators. It also shows how they can be used in an isolated power supply application. Recommended further reading AN58 - Designing with Shunt Regulators - Shunt Regulation AN59 - Designing with Shunt Regulators - Series Regulation AN61 - Designing with Shunt Regulators - Extending the operating voltage range AN62 - Designing with Shunt Regulators - Other Applications AN63 - Designing with Shunt Regulators - ZXRE060 Low Voltage Regulator Issue 1 - September 2008 © Diodes Incorporated, 2008 3 www.zetex.com www.diodes.com AN60 Definitions Product change Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or service. Customers are solely responsible for obtaining the latest relevant information before placing orders. Applications disclaimer The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for the user’s application and meets with the user’s requirements. 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