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LTM4605 Datasheet(PDF) 11 Page - Linear Technology |
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LTM4605 Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LTM4605 11 4605fa APPLICATIONS INFORMATION For a buck converter, the switching duty-cycle can be estimated as: D V V OUT IN = Without considering the inductor current ripple, the RMS current of the input capacitor can be estimated as: I I DD CIN RMS OUT MAX () () •• ( ) =− η 1 In the above equation, η is the estimated efficiency of the power module. CIN can be a switcher-rated electrolytic aluminum capacitor, OS-CON capacitor or high volume ceramic capacitors. Note the capacitor ripple current rat- ings are often based on temperature and hours of life. This makes it advisable to properly derate the input capacitor, or choose a capacitor rated at a higher temperature than required. Always contact the capacitor manufacturer for derating requirements. Output Capacitors In boost mode, the discontinuous current shifts from the input to the output, so the output capacitor COUT must be capable of reducing the output voltage ripple. For boost and buck modes, the steady ripple due to charg- ing and discharging the bulk capacitance is given by: V IV V C RIPPLE BOOST OUT MAX OUT IN MIN OUT , () ( ) • = − () ••• Vf OUT V VV V LC V RIPPLE BUCK OUT IN MAX OUT OUT , () • •• • = − () 8 IIN MAX f () • 2 The steady ripple due to the voltage drop across the ESR (effective series resistance) is given by: V I ESR ESR BUCK L MAX ,( ) • =Δ V I ESR ESR BOOST L MAX ,( ) • = The LTM4605 is designed for low output voltage ripple. The bulk output capacitors defined as COUT are chosen with low enough ESR to meet the output voltage ripple and transient requirements. COUT can be a low ESR tanta- lum capacitor, a low ESR polymer capacitor or a ceramic capacitor. Multiple capacitors can be placed in parallel to meet the ESR and RMS current handling requirements. The typical capacitance is 300μF. Additional output filtering may be required by the system designer, if further reduction of output ripple or dynamic transient spike is required. Table 3 shows a matrix of different output voltages and output capacitors to minimize the voltage droop and overshoot at a current transient. Inductor Selection The inductor is chiefly decided by the required ripple cur- rent and the operating frequency. The inductor current ripple ΔIL is typically set to 20% to 40% of the maximum inductor current. In the inductor design, the worst cases in continuous mode are considered as follows: L VV V Vf I BOOST IN OUT MAX IN OUT MAX OUT ≥ − () • •• () () (M MAX Ripple ) •% L VV V Vf I BUCK OUT IN MAX OUT IN MAX OUT M ≥ − () • •• () () ( A AX Ripple ) •% where: f is operating frequency, Hz Ripple% is allowable inductor current ripple, % VOUT(MAX) is maximum output voltage, V VIN(MAX) is maximum input voltage, V VOUT is output voltage, V IOUT(MAX) is maximum output load current, A The inductor should have low DC resistance to reduce the I2R losses, and must be able to handle the peak inductor current without saturation. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. Please refer to Table 3 for the recommended inductors for dif- ferent cases. |
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