LTC1772 UOPERATIO(Refer to Functional Diagram)Slope Compensation and Inductor’s Peak Current 110 The inductor’s peak current is determined by: 100 90 V – 0.7 (%) 80 I ITH PK = 70 10 R ( SENSE) OUT(MAX) 60 /I when the LTC1772 is operating below 40% duty cycle. 50 OUT IRIPPLE = 0.4IPK 40 However, once the duty cycle exceeds 40%, slope com- AT 5% DUTY CYCLE SF = I I 30 RIPPLE = 0.2IPK pensation begins and effectively reduces the peak induc- AT 5% DUTY CYCLE 20 V tor current. The amount of reduction is given by the curves IN = 4.2V 10 in Figure 2. 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 1772 F02 Figure 2. Maximum Output Current vs Duty CycleUUWUAPPLICATIONS INFORMATION The basic LTC1772 application circuit is shown in Figure 1. However, for operation that is above 40% duty cycle, slope External component selection is driven by the load re- compensation effect has to be taken into consideration to quirement and begins with the selection of L1 and RSENSE select the appropriate value to provide the required amount (= R1). Next, the power MOSFET, M1 and the output diode of current. Using Figure 2, the value of RSENSE is: D1 are selected followed by CIN (= C1) and COUT (= C2). SF R = R SENSE SENSE Selection for Output Current ( ) 10 I (OUT )( ) 100 RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- Inductor Value Calculation oped across RSENSE, the threshold of the comparator The operating frequency and inductor selection are inter- determines the inductor’s peak current. The output cur- related in that higher operating frequencies permit the use rent the LTC1772 can provide is given by: of a smaller inductor for the same amount of inductor ripple current. However, this is at the expense of efficiency 0 1 . 2 I I RIPPLE OUT = − due to an increase in MOSFET gate charge losses. RSENSE 2 The inductance value also has a direct effect on ripple where IRIPPLE is the inductor peak-to-peak ripple current current. The ripple current, IRIPPLE, decreases with higher (see Inductor Value Calculation section). inductance or frequency and increases with higher VIN or A reasonable starting point for setting ripple current is VOUT. The inductor’s peak-to-peak ripple current is given I by: RIPPLE = (0.4)(IOUT). Rearranging the above equation, it becomes: V V ⎛ V V ⎞ I IN OUT OUT D RIPPLE = − + f L ⎝⎜ V + V ⎠⎟ ( ) 1 IN D RSENSE = for Duty Cycle < 40% 10 ( ) I (OUT ) 1772fb 6
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