A Buck Regulator

The SG1524A advanced regulating pulse width modulator can be configured to create a voltage mode controlled buck regulator. This type 

The schematic of the SG1524 buck regulator is shown in Fig. 4.33. The values of the capacitors and resistors are measured values. 

There are five parameters that must be passed to the average SG1524 model. 

These values were measured on the breadboard circuit and are shown in Figs. 4.35 and 4.36. 

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In Figure 2-1 we see the various possibilities related to where exactly to connect our voltage divider resistors. We have chosen a Buck regulator, set to deliver 5V at 1A (max load) for our example. 

For a buck regulator, the duty cycle is (now including the switch and diode forward drops)... 

Question 11 Can a buck regulator be used to convert a 15 V input to 14.5 V output ... 

Answer Maybe, maybe not Technically, this is a step-down conversion, since Vo < Vin. Therefore, in principle, a buck regulator should have worked. However in practice there are some limitations regarding how close we can set the output of a converter in relation to the input. 

Even if the switch of a buck regulator is turned fully ON (say in an all-out effort to produce the required output), there will still be some remaining forward-drop across the switch, Vsw, and this would effectively subtract from the applied input Vin- Note that in this fully ON state, the switcher is basically functioning just like an LDO, and so the concerns expressed in Chapter 1 regarding the minimum achievable headroom of an LDO apply to the switcher too, in this state. As an example — if the switch drop Vsw is 1 V, then we certainly can t get anything higher than 14 V output from an input of 15 V. 

But in practice, that clearly does not happen in a perfect manner, as we may have desired. For example, if we suddenly increase the input to a buck regulator, the output initially just... 

In general, remember that doing anything at the input of a Buck is usually preferable to trying to achieve the same function at its output, since the current at the input of a Buck is lower than at its output. Similarly, for a Boost, you would prefer to put a switch or pass element at its output, not at its input, for the very same reason. For example, recently I suggested that the load disconnect Fet we wanted to introduce in our new Boost regulator IC be placed at the output rather than the output. 

February 2006 I am using the 2678 as an adjustable Buck regulator and am having difficulties with the voltage output. It sets to the proper value using calculated values for resistors, but upon applying a small (<1A) load, the voltage will dip up to 1.5 V lower. 

Which components might we focus our debugging efforts on The Buck regulator is a standalone PCB with no other circuitry operated concurrently. Any suggestions are welcome. 

Figure 4.51 SIMetrix results simulated turn-on of SG1524 buck regulator using a transient model.

So we attempt to use an inductor instead of a resistor for the purpose — we don t really have many other component choices left in our bag In fact, if we manage to do that, we get our first modern LC-based switching regulator — the buck regulator (i.e. step-down converter), as also presented in Figure 1-2. 

Though the detailed functioning of the modern buck regulator of Figure 1-2 will be explained a little later, we note that besides the obvious replacement of R with an L, it looks very similar to the bucket regulator — except for a mysterious diode. The basic principles of power conversion will in fact become clear only when we realize the purpose of this diode. This component goes by several names — catch diode, freewheeling diode, commutation diode, and output diode, to name a few But its basic purpose is always the same — a purpose we will soon learn is intricately related to the behavior of the inductor itself. 

Answer One of the reasons for limiting Dmax to less than 100% is specific to synchronous buck regulators (Figure 4-3) — when it utilizes a technique called low-side current sensing. ... 

Another reason for choosing Dmax < 100% comes from the use of n-channel mosfets in any (positive-to-positive) buck regulators. Unlike an npn transistor, an n-channel mosfet s gate terminal has to be taken several volts above its source terminal to turn it ON fully. So to keep the switch ON, when the mosfet conducts, we need to drive its gate a few Volts higher than the input rail. But such a rail is not available The only way out is to create such a rail — by means of a circuit that can pump the input rail higher as required. This circuit is called the bootstrap circuit, as shown in Figure 4-3. 

The essential difference from a conventional buck regulator is that the low-side mosfet in a synchronous regulator is designed to present a typical forward drop of only around 0.1 V or less to the freewheeling current, as compared to a Schottky catch diode which has a typical drop of around 0.5 V. This therefore reduces the conduction loss (in the freewheeling path) and enhances efficiency. 

Question 42 In synchronous buck regulators, why do we sometimes use a Schottky diode in parallel to the low-side mosfet, and sometimes don t ... 

Question 43 Why do most synchronous buck regulators use a low-side mosfet with an integrated Schottky diode ... 

The DSO You see that What do you want with a DMM Let s take the buck regulator for starters (the voice of reason replies). The simplified textbook equation for duty cycle is D = Vo/Vin. So if we were say stepping down from 20 V to 5 V, the calculated duty cycle is D = 5/20 = 0.25. Suppose the load is 2 A. That s an output power of 5 V x 2 A = 10 W. Now, the center of the ramp portion (average) of the inductor current is always going to be fixed at 2 A for a buck topology, come what may. That follows from Kirchhoff s first law (and believe me, you can t afford to ever get on the wrong side of that guy ). 

FIGURE 10.167 Motor-driven line-voltage regulator using an autotransformer with a buck/boost series transformer. 

These parameters are determined experimentally on the basis of the charge and discharge of the supeicapacitors at constant current. Generally we use a testing array based on a current-regulated buck converter (see Figure 4.14) [MAR 04a]. 

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