Application Note 27 Issue 1 June 1996 ZR431 Application Note David Bradbury +ve The ZR431 is an enhanced version of the industry standard 431. It is a three R3 1k8 terminal shunt regulator giving excellent temperature stability and the capability +ve Input of operating at currents from 50A up (3.2 - 60V) to100mA. Its output voltage can be set to R2 IC1 24k any voltage in the range of Vref (2.5V) to ZR431 Output 20V by the addition of two external C1 (3.0V) 100nF divider resistors. The reference input R1 current is typically only 100nA, so high 120k value resistors can be used without error. Its minimum operating current -ve -ve and reference input current are ten times Figure 1 lower than industry standard 431 parts. ZR431 used as a 3V Shunt Regulator. (Please refer to Appendix A) The circuit in Figure 1 shows a 3V shunt Following are a few example regulator utilising the ZR431. The output applications of the ZR431 which show of this supply can be set to any voltage how the parts can be used. in the range 2.5V to 20V by adjusting the ratio of R1 and R2, following the Shunt Regulator relationship:- The ZR431 is ideal for providing low voltage stabilised supplies or (R1+R2) V =V out ref references. Simple low output voltage R1 supplies can be difficult to build as the components normally used, such as Where Vref is the reference voltage of the zener diodes or band gap references, are ZR431. either too poor in performance or not available at the desired voltage Note that a small correction to this (band-gap devices are usually fixed at formula is required if R1 and R2 are 2.5V or 5V with no intermediate values). given very high values, since the reference input current of the ZR431 AN 27 - 1- Applications Note 27 Applications Note 27 Issue1 June 1996 Issue 1 June 1996 flows through R2 but not R1 thereby D1 Switch-Mode Power Supply causing a small error. For the Zetex Controller +ve +ve ZR431, this reference current is 200nA maximum so as long as a current of at The circuit shown in Figure 2 is OPT1 least 20A is passed through R1, setting commonly used in the control loop of R5 its maximum value at 60k ohms, the switch-mode power supplies. The * Note 1 reference current can be ignored. For output voltage is sensed via R3/R4 by the R1 very low current applications where it is ZR431, and the IC controls the current 220 R3 C1 advantageous to set R1 and R2 at a level passed through an opto-coupler and 120k Input where the reference current must be hence feeds back output voltage status considered, the following formula takes to the switching regulator. The (From Converter C2 Output 22nF 220uF (5.0V) input current into account:- minimum supply voltage on which this R2 Transformer) IC1 10k popular circuit can be used is set by the ZR431 minimum cathode voltage of the (R1+R2) V = I x R2 + V out ref ref regulator IC, plus the forward voltage R1 drop of the opto-coupler LED. This limits R4 the minimum operating voltage of the Where I is the reference input current ref 120k circuit to around 4V. of the ZR431. -ve -ve The components C1 and R2 are not R3 is selected so as to support the Figure 2 required for correct operation of the maximum load current at minimum Control Loop of a Switched Mode Power Supply : ZR431. They have been included as they input voltage, yet still maintain adequate C1, R2 and R5 are Optional Components. are frequently needed to stabilise the operating current for the ZR431. Since overall control loop of the switch-mode the minimum cathode current of the power supply of which Figure 2 is only a ZR431 is only 50A, it rarely has an effect hence improve efficiencies. Series part. The ZR431 is unconditionally Series Regulator Power Supply on the choice of R3. stable without additional components. regulators are usually much more Although shunt regulators such as the complex than shunt circuits, requiring a V V in out (min) circuit given in Figure 1 provide very R3 = Note 1: reference, error amplifier, driver and I +I simple and low cost power supplies, out z (max) (min) Resistor R5 is only necessary if the series pass elements. However, it is they are often inefficient because the minimum operating current of the ZR431 possible to construct a very simple Where I is the minimum cathode pass resistor R3 must be selected to z(min) causes excess opto-coupler current series regulator using a ZR431 to current of the ZR431. supply the maximum load expected at before the reference functions correctly. perform all but the pass transistor the minimum input voltage. When the The minimum operating current of the function. Figure 3 shows a series The low series resistance of the ZR431 actual load is lower or the input voltage ZR431 is only 50A so R5 is rarely regulator for an automotive application. not only gives the shunt regulator good is higher than for these worst-case required when using the Zetex part. The circuit has been designed to provide load regulation but also a line rejection conditions, excess current must be (Competitors versions can pass in 30mA at 5V to a microcontroller, to of over 60dB. In this circuit, capacitor C1 shunted away by the ZR431. This is not excess of 1mA before operating operate with a normal input supply both maintains this rejection figure at usually a problem for low output current correctly, hence the occasional need for range of 7V-15V, to withstand high frequencies and ensures stability supplies or when efficiency is not a R5). load-dump supply transients of 60V and should the power supplys load be major concern, but for loads greater a sustained overvoltage input of 24V, capacitive. For the Zetex ZR431, a than a few milliamps losses can be capacitor greater than 1.5nF will ensure and tolerate reverse battery connection. significant. stability for any load. (Competitors All parts used are surface-mount so the versions of the 431 generally require However, a series regulator can supply can be constructed using little values of 3.3F or greater). significantly reduce power losses and PCB area. AN 27 - 2 AN 27 - 3