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SS6577CGOTR Arkusz danych(PDF) 9 Page - Silicon Standard Corp. |
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SS6577CGOTR Arkusz danych(HTML) 9 Page - Silicon Standard Corp. |
9 / 14 page www.SiliconStandard.com 9 of 14 SS6577 Rev.1.02 3/26/2004 MAX I 100mV S R = in the voltage of VFB pin. Thus the ITH/RUN voltage remains increasing until the average inductor cur- rent matches new load current. While the high-side MOSFET turns off, the low-side MOSFET is turned on to recharge bootstrap capacitor C3. Inductor Selection With the high operating frequency of 500KHz, smaller inductor values are possible. In general, operating at high frequency will cause low efficiency because of large MOSFET switching loss. Thus the effect of inductor value on ripple current and low current operation must be considered as well. Main control loop is shutdown when ITH/RUN goes below 0.8V. When ITH/RUN pulled up to 0.8V or up by error amplifier, main control loop is enabled. Low Current Operation During heavy load current operation, the SS6577 op- erates in PWM mode with a frequency of 500KHz. Decreasing of the current will cause a drop in ITH/RUN below 1.33V so that the SS6577 enters PFM mode operation for better efficiency. If the voltage across RS does not exceed the offset of current comparator within a cycle, then the high-side and internal MOSFETs will disable until ITH/RUN goes over 1.33V. The inductor value has a direct influence on ripple current ( Δ IL), which decreases with high induc- tance and increases with high VIN or VOUT: + + × − = D V IN V D V OUT V L f OUT V IN V L ∆I VD is the drop voltage of the output Schottky diode. Accepting a large value ofΔIL allows the use of low inductance, but yields high output ripple voltage and large core loss. The inductor value also has an effect on low current operation. Low inductor value causes the PFM operation to begin at high load cur- rent. The efficiency of the circuit decreases at the beginning of low current operation. Generally speaking, low inductance in PFM mode will cause the efficiency to decrease. Component Selection The SS6 577 can be used in many switching regulator applications, such as step-down, step-up, SEPIC and positive-to-negative converters. Among these step-down converter is the most common applica- tion. External component selection, beginning with selecting RS, depends on load requirement of the application. Once RS is decided, the choice of in- ductor, which is followed by selecting power MOSFET and diode, can be easily chosen. Finally, CIN and COUT can be determined. Power MOSFET Selection For an application of SS65577, an external N- channel power MOSFET, used as the high-side switch, must be properly selected. To prevent MOSFET damage during high input voltage operation, attention should be given to the BVDSS specification of the MOSFET. RS Selection The choice of RS has substantial connection with required output current. The threshold voltage of current comparator decides peak inductor current, which yields a maximum average output current (IMAX), and the peak current is less than half of the peak-to-peak ripple current, ΔIL. Other important selection criteria for the power MOSFET include the “ON” resistance RDS(ON), in- put voltage and maximum output current. Allowing a margin for variation of the SS6577, external components can be calculated as: |
Podobny numer części - SS6577CGOTR |
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Podobny opis - SS6577CGOTR |
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