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Switch-Mode Regulator Design
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Important Switcher Design Issues and Cautions In order to suppress damaging EMI, and
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to act as a heat sink, proper PCB layout for the switcher circuit is absolutely critical Indeed, if trace inductance within the switcher circuit is not minimized, the rapid current switching rates of these regulators can cause severe voltage transients, which means keeping CIN, SW, D1, and COUT (see Fig 873) traces as short and as fat as possible within the layout Routing the sensitive feedback line to a quiet inner or bottom PCB layer would also be very beneficial to shield this trace from possible coupling through
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D1 R2 + COUT
FIGURE 873 Switch-mode regulator design example circuit
SW, L1, D1, COUT, and their associated traces, while CIN should be as close as feasible to the IN pin For electrical and thermal reasons, all the regulator s ground pins should be soldered directly to the top groundplane, and then drop immediately to the bottom PCB groundplane(s) through multiple stitching vias Further, to dissipate the heat generated by the switcher, this top groundplane should cover as large a surface area as possible For this particular switcher model, at its maximum output current draw and voltage input level, 15 in of top copper should be adequate to keep the die sufficiently cool for long-term reliability However, the larger this copper area, the cooler the regulator die will be A point that is commonly overlooked in switcher design is that switch-mode regulators must obtain their rapid transient current demands from large-value capacitors, and these components must have low ESR and ESL The ESR and ESL requirements are due to the large value of peak currents that these capacitors must source and sink every time the regulator switches, which can be as fast as 4 MHz This obviously demands quality components, and of a value and type as recommended by the manufacturer of the switch-mode regulator chip Therefore, for EMI control, the external input capacitor of a switcher must not only confine the rapidly switching currents to a localized, tight loop area on the PCB through a compact, low-impedance layout, but this capacitor must also be of a sufficient value to supply the transient peak current needs of the rapidly switching regulator at both low and high frequencies, and with low ESR and ESL However, since this input capacitor is normally an electrolytic, it cannot be expected to operate effectively above a few megahertz because of the high value of parasitic series inductance it possesses to any high-frequency noise: a noise which is trying to escape from the regulator s own input port Thus, for the particular noise-sensitive applications that are so common to wireless communication circuits, we will need to shunt an RF capacitor in parallel with the high-value electrolytic input capacitor This higher frequency capacitor will normally be of the ceramic type, with a value as recommended by the switcher s manufacturer Additionally, we may want to insert a series input inductor, followed by another bypass capacitor, to further reduce switcher generated EMI from entering the main unregulated DC supply of the entire system This inductor may in fact only need to be a thin, meandering trace to supply the low value of added inductance needed to assist in the suppression of this high-frequency RF noise The inductive trace must be sufficiently wide to safely carry the required current draw without burning up The input and output electrolytic capacitors must not only be selected for their low ESR/ESL and particular capacitance value, but also for a sufficient ripple current rating
Support Circuit Design
(RCR) Both ESR and RCR are strongly related, since the constantly charging and discharging of the electrolytic capacitors in a switching power supply will cause power to be dissipated within the capacitor s ESR, producing internal heating and possible capacitor failure, as well as low switcher efficiency Therefore, if the switcher is run in a high-temperature ambient environment, and either the input or the output capacitor is excessively increasing in temperature during regulator operation, then the capacitor(s) should be replaced with models than enjoy better RCR (As a general rule, the capacitor s ripple current rating should never be below 80% of the expected maximum current expected from the switch-mode regulator) There must be minimal distance and low-trace inductance between the ultra fast Schottky diode of D1 and the L1 inductor, and with both placed as close as possible to the switcher s output pin This is needed so as to reduce the generated EMI caused when the internal FET of the regulator turns off, which produces a very rapid transient voltage overshoot and strong ringing effect that is created by the diode s nonzero switching time and the parasitic inductance of the components and traces between these two parts Even with a good layout, some of this EMI is bound to occur through the above mechanism, but can be further suppressed by employing a series RC snubber network placed directly across the switcher s external output pins for the IC s internal FET drain and source This snubber will absorb much of the EMI generating voltage overshoot and ringing caused by the rapid FET switching speeds, and consists simply of a resistor and capacitor in series The value of the snubber s R and C must be optimized for maximum transient suppression, while minimizing switcher losses, but can be a value of between 10 to 1 k for the R, and 001 to 1 F for the C
Designing with the Micrel MIC4680 SuperSwitcher (Fig 873)
As an excellent demonstration of the procedure to design a modern, low-cost switch-mode regulator, the Micrel MIC4680 SuperSwitcher is a perfect example This particular device requires only four external parts when utilized as a fixed voltage regulator; by adding two resistors we can construct a fully voltage-adjustable switcher The MIC4680 comes in a lowcost plastic-surface-mount SOIC-8 package, and is capable of supplying up to 13 A output, has a 4 to 34 V safe input voltage range, a 125 to 6 V output voltage range, is thermal and overcurrent protected, and can be shut down with a logic high signal Pin 1 (SHDN) of the MIC4680 is the enable/disable port, with any voltage greater than 16 V placed at this pin forcing the switcher to turn off, and any voltage below 1 V turning it on Pin 2 (IN) is the port for the unregulated input voltage of anywhere between +4 to +34 V (this value should be 15 V if we demand maximum switcher efficiency) Pin 3 (SW) is the internal switching transistor s output port, with a frequency of 200 kHz Pin 4 (FB) is for the feedback line back from the two external programming resistors, with this pin internally connected to a comparator circuit All other pins are for electrical ground, which also remove heat from the chip s die junction
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