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LT3782A Datasheet(PDF) 11 Page - Linear Technology |
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LT3782A Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3782A 11 3782af The Peak and Average Input Currents The control circuit in the LT3782A measures the input cur- rent by using a sense resistor in each MOSFET source, so the output current needs to be reflected back to the input in order to dimension the power MOSFET properly. Based on the fact that, ideally, the output power is equal to the input power, the maximum average input current is: I IN(MAX) = I O(MAX) 1– D MAX The peak current is: I IN(PEAK) = 1.2 • I O(MAX) 1– D MAX The maximum duty cycle, DMAX, should be calculated at minimum VIN. Power Inductor Selection In a boost circuit, a power inductor should be designed to carry the maximum input DC current. The inductance should be small enough to generate enough ripple current to provide adequate signal to noise ratio to the LT3782A. An empirical starting of the inductor ripple current (per phase) is about 40% of maximum DC current, which is half of the input DC current in a 2-phase circuit: ΔIL ≅ 40% • I OUT(MAX) •VOUT 2V IN = 20% • I OUT(MAX) •VOUT V IN where VIN, VOUT and IOUT are the DC input voltage, output voltage and output current, respectively. And the inductance is estimated to be: L = V IN •D f s • ΔIL where fs is the switching frequency per phase. The saturation current level of inductor is estimated to be: I SAT ≥ ΔIL 2 + I IN 2 ≅ 70% • I OUT(MAX) •VOUT V IN(MIN) Sense Resistor Selection During the switch on-time, the control circuit limits the maximum voltage drop across the sense resistor to about 63mV. The peak inductor current is therefore limited to 63mV/R. The relationship between the maximum load current, duty cycle and the sense resistor RSENSE is: R ≤ VSENSE(MAX) • 1– D MAX 1.2 • I O(MAX) 2 Power MOSFET Selection 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)). APPLICATIONS INFORMATION |
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