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Switching Regulators Made Simple RO UN DE DG E 0 HB SOFTWARE DOES THE DESIGN National Semiconductor’s new range of “Simple Switcher” DC switching regulators are designed to take the hassle out of power supply design. What’s more, there is a software package available which can do it all for you. By DARREN YATES Yep, switching regulators are not new. They’ve been around for quite a long time, firstly as discrete designs using clock generators, comparators and output power devices. Then came IC packages such as the Texas Instruments’ TL497 and Motorola’s MC­ 34063 which contained all the circuit elements except the power devices. Now, National Semiconductor has gone one step further by combining all of this circuitry with an output power device inside a 5-pin TO-220 package. All you need to do is add an inductor, a fast-recovery diode and a few passive components to obtain a complete switching regulator circuit. These devices are classified into four series: (1) the LM2574/2574HV 0.5A step-down series; (2) the LM­ 2575/2575HV 1A step-down series; (3) the LM2576/2576HV 3A step-down series; and (4) the LM2577 3A step-up series. We used the LM2576-ADJ device as the basis of the 40V 3A variable power supply featured in the January 1994 issue of SILICON CHIP. These “Simple Switchers” are easy Fig.1: block diagram of the LM2576 step down converter IC. It comes in fixed & adjustable output versions. 44  Silicon Chip to get going and are capable of operating at an efficiency of over 80%. The step-down switchers require only four external components to make a com­ plete circuit, however all of the devices have a similar internal structure. The LM25 XX series have their own in-built oscilla­tor fixed at 52kHz ±10%. Having a fixed frequency allows for easy selection of filtering components. The frequency is also high enough to allow a small inductor to be very efficient. The LM25XX series also include thermal shutdown and current limit protection. Being in a TO-220 package they’re easy to mount onto a heatsink but in many cases, they don’t need one. Step-down circuit Fig.1 shows the block diagram of the LM2576 step down con­ verter. Let’s take a look at how it works. Unregulated DC is applied to pin 1 which is then regulated for the internal circui­try. This includes a 1.23V band-gap reference, which is fed into the inverting input of a fixed gain error op amp. The error signal is then fed to a comparator which produces a pulse width modulated (PWM) signal at 52kHz. The PWM signal then passes through a reset gate and onto the driver circuitry which also has an input from the thermal shutdown and over-current limit pro­tection circuits. The driver controls the 3A NPN output switch connected to pin 2. The internal switch drives a filter network consisting of inductor L1, capacitor C OUT and fast recovery diode D1. The resultant DC voltage across the load is directly monitored by pin 4 in the case of fixed output voltage versions (LM2574-LM­ 2577), while for the adjustable versions, pin 4 monitors the output voltage via an external voltage divider. The devices also include an external shutdown pin which, when taken to the supply rail, closes down the switching circui­try to leave a quiescent current of about 50µA. This is ideal for battery-operated circuitry which doesn’t always need to be pow­ered up. In normal circuit operation, the current drain is typically 5mA, with no load on the output. The range of output voltages available for the LM2574/2575/2576 stepdown series of switchers is as follows: 3.3V, 5V, 12V, 15V and adjustable (1.2V-37V). Vin(max) is 40V. For the LM2574HV/2575HV/2576HV series, the corresponding figures are: 3.3V, 5V, 12V, 15V and adjustable (1.2V-57V), with Vin(max) = 60V. Step-up converters The LM2577 step-up range of converters use slightly differ­ent circuitry and are available in a variety of packages includ­ing 5-pin TO-220 (straight or bent lead), 16-pin DIP, 24-pin surface mount and 4-pin TO-3 packages. They are typically used to step up from a 12V battery to some higher value. Because of their different operation, this series includes a soft-start function which reduces the initial inrush current into the load. Maximum input supply voltage is 45V while maximum output is 65V. Maximum switching current is 3 amps but the actual output current is less than this. The reason for this is twofold. First­ ly, because it is stepping up the voltage, it has to step down the current, and so we end up with less output current. The second reason is that in stepping up the voltage there has to be a current trade-off so that the maximum power dissipation of the device is not exceeded. Fig.2: diagram showing how the LM1577-ADJ/LM2577-ADJ is used as a stepup regulator. The switching device is an internal 3A 65V NPN transistor which operates at 52kHz. The PWM of the circuit is controlled by the feedback network connected to pin 2. This series doesn’t have a standby low current capability as do the step down converters. Instead, pin 1 is connected to an RC time constant which performs two functions. Firstly, it en­sures stability of the regulator and secondly, it forms part of the soft-start function. Block diagram Let’s take a look at the block diagram in Fig.2 and see how it works. Unregulated DC is applied to pin 5 which connects to a 2.5V regulator for the internal circuitry. The 3A 65V NPN switching transistor is controlled via the driver circuitry and runs at 52kHz. It switches current via the inductor and each time it switches off, the flyback voltage generated causes the high speed diode to conduct and charge capacitor COUT. The output voltage is monitored by pin 2 via an external voltage divider consisting of R1 and R2. The voltage at pin 2 is compared against a 1.23V reference by the internal error amplifi­ er. This amplified error signal is then Fig.3: the basic flyback arrangement. Both positive & negative rails which are greater or less than the input voltage can be derived. March 1994  45 Fig.4: higher output currents can be achieved by connecting two switching regulator ICs in parallel, with one slaved to the other. This circuit allows 5V to be stepped up to 12V with an output current of 1.5A continuous. Up to six regulators can be connected in this manner. fed to the inverting input of a comparator which compares it to the sum of the correc­tive ramp voltage from the 52kHz oscillator and a voltage propor­ tional to the switch current. The current sense voltage comes from the sense resistor which is in the emitter circuit of the 3A 65V NPN switching transistor. The output from the comparator, along with the cur­rent limit, thermal limit and under­voltage shutdown circuitry control the driver circuitry which in turn drives the output transistor. Undervoltage shutdown The undervoltage shutdown circuitry sounds like a good idea since it could be used to prevent the switcher from over-discharging a battery. Unfortunately, the shut-down voltage for all the 2577 series is typically 2.9V – too low to be of any use with most battery applications and there is no way of varying it. The input supply current under no load conditions is 10mA. The maximum duty cycle is 95% and the soft start current is only 5µA. At 3A switching current, the output device saturation voltage is typically 0.7V <at> 25°C. The efficiency of the switcher is quoted as 80% when stepping 5V up to 12V with an output load of 800mA. This is quite good for a step-up switcher with so few components. Flyback circuit Unlike the step-down switchers, the LM2577 is suitable for use in a flyback design as shown in Fig.3. In this mode, the output switching transistor is used to drive the primary side of the transformer. The feedback is derived from the rectified positive output on the secondary side of the transformer. Note the phasing of the primary and secondary windings – this is critical. Because of the high switching frequency, compact trans­formers can be used, keeping the overall size of the converter down. This flyback arrangement allows the generation of both positive and negative supplies greater or less than the input voltage. Parallel switchers But what if you need an output current which is higher than the available 3A? No problem. You can easily parallel up a couple of LM2577s and Software Offer Thanks to NSD Australia, we are making available copies of the “Switchers Made Simple” software package on a floppy disc which can be 3.5-inch or 5.25-inch format. System requirements are IBM PC or compatible, DOS 2.0 or higher and 512K RAM minimum. The cost is only $12 plus $3 for postage and packing. You can obtain a copy by filling in the order form on page 25 and sending it to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097; or you can phone (02) 979 5644 or fax (02) 979 5644 and quote your credit card details (Bankcard, Master­card, or Visa). 46  Silicon Chip there is no need for ballast or current sharing resistors at the output as is usually the case with conventional regulators. Fig.4 shows how to do it and the idea could easily be extended to include up to five or six devices in parallel for even higher currents. The circuit of Fig.4 allows 5V to be stepped up to 12V with an output current of 1.5A continuous, with one regulator slaved to the other. The control regulator’s feedback error amplifier is used to control the switching of both regulators, the slave’s feedback input being tied to ground. This works because the LM2577 is current-mode controlled and by tying both compensation inputs together via the same network, the slave regulator is forced to follow the control’s waveform quite accurately. The master regulator produces a voltage on its compensation pin which is proportional to the inverse duty cycle of the output switch. What this means is that the smaller the amount of time the output switch is off, the higher the compensation voltage. Hence, this inverse duty cycle is proportional to the output voltage and by feeding this master compensation voltage back to the slave regulator, it forces the slave’s duty cycle to match the master’s and so the output voltages will be very similar. In this way, both regulators share the load. The outputs from each regulator are then fed via separate fast recovery diodes to the same filter capacitor and the output is taken from there. As with any switching regulator, there are precautions to take to make sure that the regulator produces the least amount of electromagnetic interference (EMI). Layout is crucial in keeping down the level of voltage transients. The leads of any components which carry the switching current should be kept as short as possible and to reduce the effects of ground loops, single point or “star” earthing should be used. In many circuits, the length of the leads is not all that critical but here just a few centimetres can make a big difference, due mainly to the 52kHz switching frequency used. Component choice can also make a big difference as well, particularly in the output filtering stage. The amount of ripple voltage that appears across the output is a function of the equivalent series resistance (ESR) of the filter capacitor. The lower the ESR, the lower the ripple and hash. Unfortunately there is a trade-off. Using a capacitor with a very low ESR tends to make the circuits unstable, particularly if a capacitor with an ESR of less than 50mΩ (that’s 0.05Ω) is used. With small ESRs, the load pulse response worsens. This results in increased ringing or overshoot in the output at the switching point. By using a capacitor with a higher ESR, the pulse response is decreased and so the amplitude of the high frequency transients is reduced. Another method of reducing the amount of ripple in the output is to add a second LC low pass filter at the output. By setting the cutoff frequency to a tenth of the switching frequen­cy (ie, to 5.2kHz), the amount of ripple is reduced to around a tenth of that from a single stage filter. All you have to do is type in the required input parameters & the program automatically generates the relevant component values (shown at right). This is the circuit diagram generated by the program for the above input parameters. In this case, we have a flyback converter which generates ±12V rails at up to 0.5A. Design software National Semiconductor has put all of the design equations and procedures into an easy-to-use software package. It contains all the necessary data to design any type of switcher using the complete range from the LM2574 to the LM2577 and features boost, flyback, buck and buck-boost circuits. Boost converters step up the input voltage; flyback converters can either step up or down, or invert the input voltage via a coupling transformer thus provid­ing isolation for the output; buck converters step down the voltage; and buck-boost converters create a negative voltage from a positive one, either higher or lower in magnitude. The software will tell you everything The software can also be used to design standard step-up converter circuits, as shown here. This circuit generates a 12V rail from a 5-7V input. you need to know, including the device to use and the component values. It will also tell you the maximum switching current for a given output load current and even the junction temperature of the device under the user-specified conditions. Finally, the software generates an on-screen circuit diagram (see above) which can SC be printed out via your printer. March 1994  47