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MAX1904 Datasheet(PDF) 23 Page - Maxim Integrated Products |
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MAX1904 Datasheet(HTML) 23 Page - Maxim Integrated Products |
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23 / 33 page  500kHz Multi-Output, Low-Noise Power-Supply Controllers for Notebook Computers ______________________________________________________________________________________ 23 PTOTAL = The sum of the output power from all outputs ITOTAL = PTOTAL / VOUT = The equivalent output cur- rent referred to VOUT where: VSEC = the minimum required rectified sec ondary output voltage VFWD = the forward drop across the secondary rectifier VOUT(MIN) = the minimum value of the main out put voltage (from the Electrical Characteristics tables) VRECT = the on-state voltage drop across the synchronous rectifier MOSFET VSENSE = the voltage drop across the sense resistor In positive-output applications, the transformer sec- ondary return is often referred to the main output volt- age, rather than to ground, to reduce the needed turns ratio. In this case, the main output voltage must first be subtracted from the secondary voltage to obtain VSEC. Selecting Other Components MOSFET Switches The high-current N-channel MOSFETs must be logic- level types with guaranteed on-resistance specifica- tions at VGS = 4.5V. Lower gate threshold specifications are better (i.e., 2V max rather than 3V max). Drain-source breakdown voltage ratings must at least equal the maximum input voltage, preferably with a 20% derating factor. The best MOSFETs will have the lowest on-resistance per nanocoulomb of gate charge. Multiplying RDS(ON) ✕ QG provides a good figure for comparing various MOSFETs. Newer MOSFET process technologies with dense cell structures generally per- form best. The internal gate drivers tolerate >100nC total gate charge, but 70nC is a more practical upper limit to maintain best switching times. In high-current applications, MOSFET package power dissipation often becomes a dominant design factor. I2R power losses are the greatest heat contributor for both high-side and low-side MOSFETs. I2R losses are distributed between Q1 and Q2 according to duty fac- tor (see the following equations). Generally, switching losses affect only the upper MOSFET, since the Schottky rectifier clamps the switching node in most cases before the synchronous rectifier turns on. Gate charge losses are dissipated by the driver and don’t heat the MOSFET. Calculate the temperature rise according to package thermal-resistance specifications to ensure that both MOSFETs are within their maximum junction temperature at high ambient temperature. The worst-case dissipation for the high-side MOSFET occurs at both extremes of input voltage, and the worst-case dissipation for the low-side MOSFET occurs at maximum input voltage: where: On-state voltage drop VQ_ = ILOAD ✕ RDS(ON) CRSS = MOSFET reverse transfer capacitance IGATE = DH driver peak output current capability (1A typ) 20ns = DH driver inherent rise/fall time Under output short-circuit, the MAX1904 synchronous rectifier MOSFET suffers extra stress because its duty factor can increase to greater than 0.9. It may need to be oversized to tolerate a continuous DC short circuit. During short circuit, the MAX1901/MAX1902’s output undervoltage shutdown protects the synchronous recti- fier under output short-circuit conditions. To reduce EMI, add a 0.1µF ceramic capacitor from the high-side switch drain to the low-side switch source. Rectifier Clamp Diode The rectifier diode is a clamp across the low-side MOS- FET that catches the negative inductor swing during the 60ns dead time between turning one MOSFET off and each low-side MOSFET on. The latest generations of MOSFETs incorporate a high-speed Schottky diode, which serves as an adequate clamp diode. For MOSFETs without integrated Schottky diodes, place a Schottky diode in parallel with the low-side MOSFET. PD I R DUTY VI f VC I ns PD I R DUTY DUTY V V V V upperFET LOAD DS ON IN LOAD IN RSS GATE upperFET LOAD DS ON OUT Q IN Q =× × +× × × × + =× × =+ () ( ) 2 2 21 20 1 () () () / - - L VV V V f I LIR Turns Ratio N VV VV V PRIMARY OUT IN MAX OUT IN MAX TOTAL SEC FWD OUT MIN RECT SENSE = ×× × = + ++ () () () () - |
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Similar Description - MAX1904 |
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