The Circuit Designer's Companion - diagramas.diagram...

242 The Circuit Designer’s CompanionI RV+ @ I dcFigure 7.12 Incorrect reservoirconnectionR gI RGround AGround B10mΩ of R g will give a peak difference of 50mV between grounds A and B. If someparts of the circuit are grounded to A and some to B, then tens of millivolts of huminjection are included in the design at no additional cost, and increasing the reservoirvalue to try and reduce it will actually make matters worse as the peak ripple current isincreased. You can check the problem easily, by observing the output ripple on a’scope; if it has a pulse shape then wiring is the problem, if it looks more like a sawtooththen you need more smoothing.Correct reservoir connectionThe solution to this problem, and the correct design approach, is to ground all parts ofthe supplied circuit on the supply side of the reservoir capacitor, so that the ripplecurrent ground path is not common to any other part of the circuit (Figure 7.13). Thesame applies to the V+ supply itself. The common impedance path is now reduced tothe capacitor’s own ESR, which is the best you can do.I RI RV+ @ I dcFigure 7.13 Correct reservoirconnectionGround AGround B7.2.13 Transient responseThe transient response of a power supply is a measure of how fast it reacts to a suddenchange in load current. This is primarily a function of the bandwidth of the regulator’sfeedback loop. The regulator has to maintain a constant output in the face of loadchanges, and the speed at which it can do this is set by its frequency response as withany conventional operational amplifier. The trade-off that the power supply designerhas to worry about is against the stability of the regulator under all load conditions; aregulator with a very fast response is likely to be unstable under some conditions ofload, and so its bandwidth is “slugged” by a compensation capacitor within theregulator circuit. Too much of this and the transient response suffers. The same effectcan be had by siting a large capacitor at the regulator output, but this is a brute-forceand inefficient approach because its effect is heavily load-dependent. Note that the78XX series of three-terminal regulators should have a small, typically 0.1µF capacitorat the output for good transient response and HF noise decoupling. This is separate fromthe required 0.33−1µF capacitor at the input to ensure stability.