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The Flexibility of the Monolithic L6590 Flexibility, of course, is a term much loved by marketing people because it is a positive way of representing non-optimization i.e. a flexible solution is often one that is not optimized for anything in particular. There is often a good reason for this - e.g. there may be a large market for an “almost optimized” solution while the market for any particular fully optimized solution may be too small to justify developing the product. There’s nothing wrong with that, provided we bear in mind that flexibility has a cost and we still have to pay for any potential functionality that is not used.  For designers of offline switching power supplies, one of the most useful developments was the introduction of PWM control chips, with the combination of the integrated control chip and a discrete power switching transistor offering a good compromise between flexibility, reliability, efficiency and cost. However, with today’s increasing cost and time-to-market pressures, the flexibility of the “two-chip” approach is often more of an overhead than a real benefit - if the product does not require any sophisticated functionality, there is no point in adopting a more flexible but more expensive solution. Enter the L6590 family, ST’s latest range of fully integrated power supplies. Designed to simplify offline power supplies up to 15W, the L6590 integrates all of the control circuits needed to build a converter plus a 700V power MOSFET switch. It’s certainly less flexible than the two-chip approach but the freedom to choose the external power switch is one that many designers will happily give up in favor of increased reliability, reduced design effort and lower overall cost. How flexible do you need to be? Furthermore, flexibility is a relative term. The L6590 may be less “flexible” than traditional solutions but it can be used in flyback, boost or forward converter topologies and operates not only over the universal input voltage range of 85VAC to 265VAC and also with DC inputs, making it suitable for consumer applications in all world markets. Typical applications of the L6590 include wall plug power supplies, AC-DC adapters, auxiliary supplies for CRTs, LCD monitors, desktop PCs and servers, fax machines and laser printers. The L6590’s impressive level of integration was achieved by using a ST proprietary BCD offline technology. This is a high voltage implementation of ST’s Bipolar-CMOS-DMOS (BCD) technology family that is applied in a broad range of smart power applications ranging from simple motor drivers to complex smart power subsystems with embedded microcontrollers. It allows the integration of 700V power devices and control circuits on the same chip, making it ideal for applications such as offline power supplies and energy-saving electronic lamp ballasts. BCD offline is ideal for these applications because it allows many different kinds of components to be integrated, including high voltage n-channel LDMOS transistors, medium voltage (20V) PNP and NPN bipolar transistors, CMOS devices up to 10V, Zener voltage references, resistors and capacitors. The waste of silicon area due to lateral diffusion in thick epitaxial layers, normally required for high voltage operation, is eliminated by a technique known as RESURF (Reduced Surface Field) that allows the IC to withstand high voltages in very thin epitaxial layers. One of the many benefits of this technique is that current flows laterally in the high voltage DMOS transistor, so the substrate of the chip is grounded, leading to much less noise generation compared to a discrete MOSFET.  Figure 1 Figure 1 shows the block diagram of the L6590D, which is housed in a 16-lead Small-Outline package (SO-16W) and provides both brownout and error amplifier connections. Two other versions are also offered in 8-lead DIP packages; the L6590N provides the error amplifier function for both primary and opto-isolated feedback but no brownout function, while the L6590AN, which is optimized for PC auxiliary SMPS applications, provides the brownout function but not the error amplifier.  Figure 2 All three devices include functions such as cycle-by-cycle current limiting, output overvoltage protection and thermal shutdown that lead to a robust design, while their very low operating current, low parasitics and built-in standby function make it easy to comply with Blue Angel and similar norms for standby power consumption. The internal oscillator normally operates at a fixed frequency of 65kHz with a precision of 10%. This value was chosen because the second harmonic falls below the 150kHz limit above which some international EMC standards impose severe limits. The standby function, which is optimized for the flyback topology that is the most popular for 15-20W mains converter applications, automatically detects a light load condition and reduces the oscillator frequency to 22kHz. In this way, power losses related to switching frequency, which represent the majority of losses in a lightly loaded flyback configuration, are minimized without sacrificing the benefits of a higher switching frequency at heavier loads. Design support for the L6590 includes an evaluation package - order code EVAL6590N - that contains samples, an evaluation board and PC software, which simulates the application function  A comprehensive application note containing many application examples and ideas can be downloaded from the ST website, www.st.com. |
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