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LTC1872 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1872 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LTC1872 APPLICATIONS INFORMATION The basic LTC1872 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L1 and RSENSE (= R1). Next, the power MOSFET and the output diode D1 is selected followed by CIN(= C1) and COUT(= C2). RSENSE Selection for Output Current RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- oped across RSENSE, the threshold of the comparator determines the inductor’s peak current. The output cur- rent the LTC1872 can provide is given by: I R IV VV OUT SENSE RIPPLE IN OUT D =− + 012 2 . where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation section) and VD is the forward drop of the output diode at the full rated output current. A reasonable starting point for setting ripple current is: IO I VV V RIPPLE OUT OUT D IN = ()( ) + .4 Rearranging the above equation, it becomes: R I VV SENSE OUT OUT D = ()( ) + 1 10 V for Duty Cycle < 40% IN However, for operation that is above 40% duty cycle, slope compensation’s effect has to be taken into consideration to select the appropriate value to provide the required amount of current. Using the scaling factor (SF, in %) in Figure 2, the value of RSENSE is: R SF I V VV SENSE OUT IN OUT D = ()( )( ) + 10 100 Inductor Value Calculation The operating frequency and inductor selection are inter- related in that higher operating frequencies permit the use of a smaller inductor for the same amount of inductor ripple current. However, this is at the expense of efficiency due to an increase in MOSFET gate charge losses. The inductance value also has a direct effect on ripple current. The ripple current, IRIPPLE, decreases with higher inductance or frequency and increases with higher VOUT. The inductor’s peak-to-peak ripple current is given by: I V fL VV V VV RIPPLE IN OUT D IN OUT D = () +− + where f is the operating frequency. Accepting larger values of IRIPPLE allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is: II VV V RIPPLE OUT MAX OUT D IN = + () 04 . In Burst Mode operation, the ripple current is normally set such that the inductor current is continuous during the burst periods. Therefore, the peak-to-peak ripple current must not exceed: I R RIPPLE SENSE ≤ 003 . This implies a minimum inductance of: L V f R VV V VV MIN IN SENSE OUT D IN OUT D = +− + 003 . A smaller value than LMIN could be used in the circuit; however, the inductor current will not be continuous during burst periods. |
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