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LTC3542IS6 数据表(PDF) 8 Page  Linear Integrated Systems 

LTC3542IS6 数据表(HTML) 8 Page  Linear Integrated Systems 
8 / 16 page LTC3542 8 3542fa A general LTC3542 application circuit is shown in Figure1. External component selection is driven by the load require ment and begins with the selection of the inductor L. Once the inductor is chosen, CIN and COUT can be selected. the burst clamp. Lower inductor values result in higher ripple current which causes the transition to occur at lower load currents. This causes a dip in efﬁciency in the upper range of low current operation. In Burst Mode operation, lower inductance values cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes change the size/current and price/current relationships of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated ﬁeld/EMI requirements than on what the LTC3542 requires to operate. Table 1 shows some typi cal surface mount inductors that work well in LTC3542 applications. Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT/VIN. To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: II VV V V RMS MAX OUT IN OUT IN ≈ () – where the maximum average output current IMAX equals the peak current minus half the peaktopeak ripple cur rent, IMAX = ILIM – ΔIL/2. This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worstcase is commonly used to design because even signiﬁcant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours life time. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet the size or height requirements of the VIN LTC3542 RUN R1 3542 F01 R2 CF L CIN COUT SW VIN 2.7V TO 5.5V VOUT VFB MODE/SYNC GND Figure 1. LTC3542 General Schematic Inductor Selection The inductor value has a direct effect on ripple current ΔIL, which decreases with higher inductance and increases with higher VIN or VOUT, as shown in following equation: ΔI V L V V L OUT O OUT IN = ⎛ ⎝⎜ ⎞ ⎠⎟ ƒ • – 1 where fO is the switching frequency. A reasonable starting point for setting ripple current is ΔIL = 0.4 • IOUT(MAX), where IOUT(MAX) is 500mA. The largest ripple current ΔIL occurs at the maximum input voltage. To guarantee that the ripple current stays below a speciﬁed maximum, the inductor value should be chosen according to the follow ing equation: L V I V V OUT OL OUT IN MAX = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ƒ • – () Δ 1 The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 600mA rated inductor should be enough for most applications (500mA + 100mA). For better efﬁciency, chose a low DCresistance inductor. The inductor value will also have an effect on Burst Mode operation. The transition to low current operation begins when the inductor’s peak current falls below a level set by APPLICATIONS INFORMATION 
