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SC453TSTRT Arkusz danych(PDF) 15 Page - Semtech Corporation |
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SC453TSTRT Arkusz danych(HTML) 15 Page - Semtech Corporation |
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15 / 26 page  15 © 2009 Semtech Corp. www.semtech.com POWER MANAGEMENT SC453 Applications Information (Cont.) DESIGN PROCEDURE Step 1 — Define Constants Maximum input voltage Minimum input voltage Maximum load current, highest output voltage Leakage current, highest output voltage The highest V CORE voltage The lowest V CORE voltage SC453 Internal reference voltage Output capacitance per cap ESR per cap Current sense resistor Parasitic resistance from the current sense resistor to the processor Voltage droop allowed for a low current to high current transient Voltage rise allowed for a high current to low current transient Desired maximum output ripple Step 2 — Output Inductor and Capacitor Selection The SC453 has passive droop. The voltage at full load is less than the DAC voltage by the voltage drop across the current sense resistor and any PCB copper losses from the sense resistor to the processor socket. The steady- state voltage at full load is as follows. VMAX_FL VMAX_NL RCS RCU + ()I MAX_FL ⋅ − := VMAX_FL 1.182 V = Output capacitance and ESR values are a function of transient requirements and output inductor value. Figure 1 illustrates the response of a hysteretic converter to a positive transient. In a hysteretic converter with passive droop, like the SC453, two conditions determine if you meet the positive transient requirements. ESR VPOS_TRANS IMAX_FL ILKGMAX − ( ) ≤ VNEG_TRANS deltaV COUT () ≥ Figure 1 — Hysteretic Converter Response to a Positive Transient The first condition is easy to see; if the ESR is too high, the instantaneous I•ESR drop seen at the output will be larger than the allowed limit. VINMAX 20 V := VINMIN 8V := IMAX_FL 20 A := ILKGMAX 5A := VMAX_NL 1.212 V := VMIN_NL 0.956 V ⋅ := VREF 1.7 V := COUT 330 μF ⋅ := RESR 6m Ω ⋅ := RCS 1m Ω ⋅ := RCU 0.5 m Ω ⋅ := VPOS_TRANS 50 mV := VNEG_TRANS 50 mV := VRIPPLE 20 m V := |
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