A Product Line of Diodes Incorporated AN62 Designing with Shunt Regulators - Other applications Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Zetex Ltd Introduction Shunt regulators or voltage references can be applied to other applications beyond the obvious PSU ones. Some of these are shown below. A simple voltage comparator V R3 V+ Flag IS H Vin R1 VREF V+ Flag VIN TLV431 R2 VTH GND VKA(min) t 0 R1 ⎞ ⎛ VTH = VREF ⎜1 + ⎟ R 2⎠ ⎝ R3 = V + − VKA(min) 0.1mA ≤ I SH ≤ 15mA ISH Figure 1 Using the TLV431 as a level detector In its open loop state, the 3-terminal reference is analogous to a line-powered comparator with its non-inverting input internally connected to a reference voltage. This means the remaining inverting input can be used for comparator functions. Figure 1 above shows the TLV431 being used as a level comparator. Its output (Flag) is normally high and goes low when the input reaches or exceeds the threshold (VTH) determined by R1 and R2. Issue 1 - September 2008 © Diodes Incorporated, 2008 1 www.zetex.com www.diodes.com AN62 A window comparator V V+ R1H R3 R1L IS H VRE F VREF REF2 R2H Flag V HI V a lid l id sup ply ra n ge ge REF1 V IINN V LO R2L F la g V K A (m in ) GND t 0 V + − K A(min) R1H ⎞ ⎛ VHI = VREF ⎜1 + ⎟ R3 = R 2H ⎠ I SH ⎝ R1L ⎞ ⎛ VLO = VREF ⎜1 + ⎟ ⎝ R 2L ⎠ 0.1mA ≤ I SH ≤ 15mA Figure 2 Window comparator for PSU supervision or Power-On Reset An extended variation of Figure 1 is the use of two references to implement a window comparator. It is effectively two level comparators in series. It is a circuit that gives an output only when the input is within a window defined by a lower (VLO) and a higher (VHI) limit. The window comparator is used either in general PSU supervision, status indicator or as a power-on reset (POR) in many types of applications. Circuit explanation The graph shows how it works. At input voltages below VLO, both devices are off and so the output (Flag) simply follows the input. At input voltages above VLO, REF1 switches on taking Flag low. The circuit remains in this state until the input voltage reaches or exceeds VHI. At this point, REF2 switches on, inhibiting the input to REF1 which therefore switches off causing Flag to go high again. Thus the flag represents an indication of the input voltage lying in the range of an acceptable window. Simple current sources Vin Q1 ZXTN2038F Rs R1 VREF I R1 IB ⎛ hFE ⎞ ⎜⎜ ⎟⎟ + I R1 ⎝ hFE + 1⎠ By making I R1 << VREF and hFE > 100 RS Iout REF1 0V IOUT = VREF RS I OUT ≈ VREF RS R1 = VIN (min) − (VOUT (max) + VREF + VBE ) I KA(min) + I B GND Figure 3 Constant current source Constant current circuits are used in many applications, e.g. relaxation oscillators, biasing circuits, active loads, battery chargers, test and measurement, etc. Issue 1 - September 2008 © Diodes Incorporated, 2008 2 www.zetex.com www.diodes.com AN62 Figure 3 above shows a very simple constant current source. The output current expression includes two sources of error, one of which is the effect of the finite current gain, hFE, of transistor Q1. This error can be minimised by using a transistor with the highest possible gain. If necessary a Darlington pair may be used which will practically remove this error. A far more dominant error source is the IR1 term in the expression. This is largely influenced by the requirement of the reference device that is used. A reference with very small IKA(min) such as the TLV431 will help in keeping this error current down to a minimum. Ultimately, this error term cannot be got rid of and it puts a lower limit on how effectively the circuit can be used as a constant current source for very small currents. Significant errors can be expected for currents below 10mA. A constant current source that does not have this problem is shown in Figure 4 below. It is more appropriately a constant "current sink" and has eliminated the IR1 error altogether. IR1 still flows and has same requirements but it is not seen by the load which is connected between VIN and Q1's collector. The circuit is good enough down to at least 10µA or less depending on the transistor used. Vin Iout IOUT = R1 I R1 REF1 IB Q1 ZXTN2038F VREF RS ⎛ hFE ⎞ ⎟⎟ ⎜⎜ ⎝ hFE + 1⎠ By making I OUT ≈ VREF VREF RS VIN (max) − (VOUT (min) + VREF + VBE ) Rs I KA(max) + I B ≤ R1 ≤ VIN (min) − (VOUT (max) + VREF + VBE ) I KA(min) + I B GND Figure 4 Constant current sink Conclusion The preceding examples illustrate the flexibility of 3-terminal voltage references beyond the obvious and intended applications. These examples can either be used on their own or as building blocks for more complex applications. Recommended further reading AN58 - Designing with Shunt Regulators - Shunt Regulation AN59 - Designing with Shunt Regulators - Series Regulation AN60 - Designing with Shunt Regulaors - Fixed Regulators and Opto-Isolation AN61 - Designing with Shunt Regulators - Extending the operating voltage range AN63 - Designing with Shunt Regulators - ZXRE060 Low Voltage Regulator Issue 1 - September 2008 © Diodes Incorporated, 2008 3 www.zetex.com www.diodes.com AN62 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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Life support devices or systems are devices or systems which: 1. are intended to implant into the body or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in significant injury to the user. B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. Reproduction The product specifications contained in this publication are issued to provide outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. 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