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does the impedance of the reactor. form PWM by modulating the leading-edge of the input voltage pulse applied to the output section. high level of design expertise. proach. Drawbacks? None. And once more the L6561 turns out to be a really versatile device. Figure 1. Typical architecture of an SMPS for desktop computer ("silver box").

semblies and in DPAK / D2PAK packages for surface mount assemblies, as summarized in Table 1. Available PC (Power Supply ’98) specification. Table 1. Suggested MOSFETs for typical output current ratings (3.3V output).

6 STP30NE06 55mohm STD20NE06 40mohm - -

8 STP36NE06 40mohm STD20NE06 40mohm - -

10 STP55NE06 22mohm - - STB55NE06 22mohm

13 STP80NE06-10 10mohm - - STB80NE06-10 10mohm

the gate-drive voltage on the duty cycle. [1] for a detailed description of the device. D1 withstands the reverse secondary voltage. covery of D1 is here of no concern. Figure 5. Circuits for the generation of the Figure 6. L6561-based switcher time diagrams.

(±1% @ T j = 25°C) voltage reference. VMUL voltage is connected to the output voltage, in this way providing foldback current limiting. This function provides a short circuit current lower then the normal load current, thus greatly reducing the stress on the power components in case of failure. The diode D3 avoids latchoff, that is prevents the output current from dropping to zero during short cir- cuits and staying at zero even when the short circuit is removed. Furthermore, at start-up, there is a soft- start action because of the limitation of the current inflow. The regulator will exhibit a better efficiency over its mag amp counterpart: the dissipation on the MOS- FET is quite limited (about 1W with the parts suggested in Tab.1), while the mag amp dissipates on the copper and the core of the reactor, as well as some hundreds mW on the reset PNP transistor. The "peak" current mode control performed by the circuit offers many more benefits: excellent load regu- lation from very low output current up to the maximum load, fast response to step-load changes, good line regulation due to the inherent input voltage feedforward and one-pole transfer function of the output stage, which simplifies frequency compensation. Additionally, since the L6561 can be disabled by grounding the ZCD pin, the entire regulator can be either enabled or shut down with a logic signal. Besides, being the L6561 on the secondary side, the in- terface needed to feed the enable/disable signal will be extremely simple, like the one shown in the schematic of fig. 4. This disable feature achieves a real zero-power shutdown. In fact, when the regulator is disabled, Q1 is kept off and the secondary winding is an open circuit . The only power consumption is a couple of mA from the supply pin of the L6561. This functionality turns out to be extremely useful in those systems were power management is required, as stated in Intel’s Power Supply ’98 specifications, for example. Conclusions The replacement of a mag amp-based post regulator with an L6561-based one for the generation of a given auxiliary power rail allows to save numerous external parts among which the bulky and expensive mag amp core. Such a regulator will benefit a dramatic cost reduction from the lower number and cost of the parts used, from the shrinkage of the PCB area needed to accommodate the circuit and from a cut of the design time. In spite of that, the result is a much more performant system which takes advantage of all the benefits inherent in switching regulation with current mode control as well as of the functionalities available from the versatile L6561. REFERENCE [1] "L6561, Enhanced Transition Mode Power Factor Corrector" (AN966) AN1007 APPLICATION NOTE

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners © 2003 STMicroelectronics - All rights reserved STMicroelectronics GROUP OF COMPANIES Australia – Belgium - Brazil - Canada - China – Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States AN1007 APPLICATION NOTE