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LTC3862 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3862 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3872-1 9 38721f For more information www.linear.com/LTC3872-1 the inductor value can be determined using the following equation: L = VIN(MIN) ∆IL • f •DMAX where: ∆IL = c• IO(MAX) 1–DMAX Remember that boost converters are not short-circuit protected. Under a shorted output condition, the induc- tor current is limited only by the input supply capability. The minimum required saturation current of the inductor can be expressed as a function of the duty cycle and the load current, as follows: IL(SAT) ≥ 1+ χ 2 • IO(MAX) 1–DMAX The saturation current rating for the inductor should be checked at the minimum input voltage (which results in thehighestinductorcurrent)andmaximumoutputcurrent. Operating in Discontinuous Mode Discontinuous mode operation occurs when the load cur- rent is low enough to allow the inductor current to run out during the off-time of the switch. Once the inductor current is near zero, the switch and diode capacitances resonate with the inductance to form damped ringing at 1MHz to 10MHz. If the off-time is long enough, the drain voltage will settle to the input voltage. Depending on the input voltage and the residual energy in the inductor, this ringing can cause the drain of the power MOSFET to go below ground where it is clamped by the body diode. This ringing is not harmful to the IC and it has been shown not to contribute significantly to EMI. Any attempt to damp it with a snubber will degrade the efficiency. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. Actual core loss is independent of core size for a fixed inductor value, but is very dependent on the inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore, copper losses will in- crease. Generally, there is a tradeoff between core losses and copper losses that needs to be balanced. Ferrite designs have very low core losses and are pre- ferred at high switching frequencies, so design goals can concentrate on copper losses and preventing saturation. Ferrite core material saturates “hard,” meaning that the inductancecollapsesrapidlywhenthepeakdesigncurrent is exceeded. This results in an abrupt increase in inductor ripple current and consequently, output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/ currentandprice/currentrelationshipofaninductor.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 characteristics. The choice of which style inductor to use mainly depends on the price vs size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, Toko and Sumida. Power MOSFET Selection ThepowerMOSFETservestwopurposesintheLTC3872-1: it represents the main switching element in the power path and its RDS(ON) represents the current sensing ele- ment for the control loop. Important parameters for the power MOSFET include the drain-to-source breakdown voltage (BVDSS), the threshold voltage (VGS(TH)), the on- resistance (RDS(ON)) versus gate-to-source voltage, the gate-to-source and gate-to-drain charges (QGS and QGD, respectively), the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistances (RTH(JC) and RTH(JA)). Logic-level (4.5V VGS-RATED) threshold MOSFETs should be used when input voltage is high, otherwise if low input voltage operation is expected (e.g., supplying power from alithium-ionbatteryora3.3Vlogicsupply),thensublogic- level(2.5VVGS-RATED)thresholdMOSFETsshouldbeused. Pay close attention to the BVDSS specifications for the MOSFETs relative to the maximum actual switch voltage in the application. Many logic-level devices are limited applicaTions inForMaTion |
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Similar Description - LTC3862 |
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