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ADP3156JR-18 Datasheet(PDF) 8 Page - Analog Devices |
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ADP3156JR-18 Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page REV. 0 ADP3156 –8– The converter operates at the nominal operating frequency only at the VOUT specified above, and at light load. At higher load conditions, the operating frequency decreases due to the para- sitic voltage drops across the power devices. The actual mini- mum frequency at VOUT = 1.8 V is calculated from Equation 1, and is a function of the finite resistances of various components in the power converter. COUT Selection—Determining the ESR The required ESR and capacitance drive the selection of the type and quantity of the output capacitors. The ESR must be small enough that both the resistive voltage deviation due to a step change in the load current and the output ripple voltage stay below the values defined in the specification of the supplied circuitry. The capacitance must be large enough that the output is held up while the inductor current ramps up or down to the value corresponding to the new load current. Taking into account the ±1% setpoint accuracy of the ADP3156, and assuming a 1% (or 15 mV) peak-to-peak ripple, the allowed static voltage deviation of the output voltage when the load changes between the minimum and maximum values is: (2 × 5% × 1.5 V) – (2 × 1% × 1.5 V) – (1% × 1.5 V) = 105 mV This sets the maximum ESR at 105 mV/7 A = 15 m Ω. Four parallel capacitors of 470 µF with a maximum ESR of 60 mΩ will achieve the 15 m Ω maximum net ESR. Whether or not the capacitance is sufficient must be determined after the inductor is selected. Inductor Selection The minimum inductor value can be calculated from ESR, off- time, dc output voltage and allowed peak-to-peak ripple voltage using the following equation: L Vt R V sm mV H MIN O OFF E MAX RIPPLE p p 1 15 32 7 15 224 == ×× =µ − () .. . µΩ The minimum inductance gives a peak-to-peak ripple current of 2.14 A, or 35% of the maximum dc output current IOMAX. The inductor peak current in normal operation is: ILPEAK = IOMAX + IRPP/2 = 8.07 A The inductor valley current is: ILVALLEY = ILPEAK – IRPP /2 = 5.93 A The inductor for this application should have an inductance of not less than 2.24 µH at full load current and should not satu- rate at the worst-case overload or short circuit current at the maximum specified ambient temperature. For this example, it is assumed the inductance might drop as much as 33% due to load current, so its initial value might be as high as 3.36 µH. Tips for Selecting the Inductor Core Ferrite designs have very low core loss, so the design should focus on copper loss and on preventing saturation. Molypermalloy, or MPP, is a low loss core material for toroids, and it yields the smallest size inductor, but MPP cores are more expensive than ferrite cores or the Kool M µ® cores from Magnetics, Inc. The lowest cost core is made of powdered iron, for example the #52 material from Micrometals, Inc., but yields a larger size inductor. COUT Selection—Determining the Capacitance The minimum capacitance of the output capacitor is determined from the requirement that the output be held up while the in- ductor current ramps up (or down) to the new value. The mini- mum capacitance should produce an initial dv/dt which is equal (but opposite in sign) to the dv/dt obtained by multiplying the di/dt in the inductor and the ESR of the capacitor. C II Rdi dt C AA mV H F MIN OMAX OMIN E MIN = = − () = – (/ ) ./ . 70 15 18 336 871 Ωµ µ In the above equation the value of di/dt is calculated as the smaller voltage across the inductor (i.e., the smaller of VIN – VOUT and VOUT) divided by the maximum inductance (3.36 µH) of the inductor. The four parallel-connected 470 µF capacitors have a total capacitance of 1880 µF, so the minimum capacitance requirement is met with ample margin. RSENSE The value of RSENSE is based on the required output current. The current comparator of the ADP3156 has a threshold range that extends from 0 mV to 125 mV (minimum). Note that the full 125 mV range cannot be used for the maximum specified nominal current, as headroom is needed for current ripple and transients. The current comparator threshold sets the peak of the inductor current yielding a maximum output current IOMAX, which equals the peak value less half of the peak-to-peak ripple current. Solv- ing for RSENSE and allowing a 20% margin for overhead, and using the minimum current sense threshold of 125 mV yields: RSENSE = (125 mV)/[1.2(IOMAX + IRPP/2)] = 12.9 mΩ Once RSENSE has been chosen, the peak short-circuit current ISC(PK) can be predicted from the following equation: ISC(PK) = (145 mV)/RSENSE = (145 mV)/(12.9 mΩ)= 11.2 A The actual short-circuit current is less than the above-calculated ISC(PK) value because the off-time rapidly increases when the output voltage drops below 1 V. The relationship between the off-time and the output voltage is: t CV V k A OFF T O ≈ × + 1 360 2 Ω µ With a short circuit across the output, the off-time will be about 104 µs. During that time the inductor current gradually decays. The amount of decay depends on the L/R time constant in the output circuit. With an inductance of 2.24 µH and total resis- tance of 40 m Ω (the inductor’s series resistance plus the sense resistor), the time constant will be 56 µs. This yields a valley f t VI R I R R R V VI R I R R R R MIN OFF IN IN IN OMAX DS ON HSF SENSE L O IN IN IN OMAX DS ON HSF SENSE L DS ON LSF =× ++ ++ 1 –– ( ) – –– ( – ) () () () (1) |
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