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LTC1772 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1772 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LTC1772 APPLICATIONS INFORMATION The basic LTC1772 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 LTC1772 can provide is given by: I R I OUT SENSE RIPPLE =− 01 2 . where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation section). A reasonable starting point for setting ripple current is IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it becomes: R I SENSE OUT = ()( ) 1 12 for Duty Cycle < 40% However, for operation that is above 40% duty cycle, slope compensation effect has to be taken into consideration to select the appropriate value to provide the required amount of current. Using Figure 2, the value of RSENSE is: R SF I SENSE OUT = ()( )( ) 12 100 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 VIN or VOUT. The inductor’s peak-to-peak ripple current is given by: I VV fL VV VV RIPPLE IN OUT OUT D IN 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 IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE occurs at the maximum input voltage. In Burst Mode operation on the LTC1772, 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 ≤ 0 0288 . This implies a minimum inductance of: L VV f R VV VV MIN IN OUT SENSE OUT D IN D = − + + 0 0288 . (Use VIN(MAX) = VIN) A smaller value than LMIN could be used in the circuit; however, the inductor current will not be continuous during burst periods. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy |
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