Datasheet LTC1622 (Analog Devices) - 7

LTC1622
U U W U APPLICATIONS INFORMATION
The basic LTC1622 application circuit is shown in Figure VOUT. The inductor’s peak-to-peak ripple current is given 1. External component selection is driven by the load by: requirement and begins with the selection of L and RSENSE.   Next, the Power MOSFET and the output diode D1 are V V V + V I IN OUT OUT D RIPPLE = − selected followed by C ( )  + IN and COUT. f L V V IN D 
R
where f is the operating frequency. Accepting larger values
SENSE Selection for Output Current
of IRIPPLE allows the use of low inductances, but results in RSENSE is chosen based on the required output current. higher output voltage ripple and greater core losses. A With the current comparator monitoring the voltage devel- reasonable starting point for setting ripple current is oped across RSENSE, the threshold of the comparator IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE determines the inductor’s peak current. The output cur- occurs at the maximum input voltage. rent the LTC1622 can provide is given by: With Burst Mode operation selected on the LTC1622, the 0 0 . 8 I ripple current is normally set such that the inductor I RIPPLE OUT = − current is continuous during the burst periods. Therefore, RSENSE 2 the peak-to-peak ripple current should not exceed: where IRIPPLE is the inductor peak-to-peak ripple current (see Inductor Value Calculation section). IRIPPLE ≤ 0 036 . R A reasonable starting point for setting ripple current is SENSE IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it This implies a minimum inductance of: becomes: V V  V + V  1 L IN OUT OUT D = − R MIN SENSE = for Duty Cycle < 40%    V + V  0 036 . 15 I ( )( ) IN D OUT fRSENSE However, for operation that is above 40% duty cycle, slope (Use V compensation has to be taken into consideration to select IN(MAX) = VIN) the appropriate value to provide the required amount of A smaller value than LMIN could be used in the circuit; current. Using Figure 2, the value of RSENSE is: however, the inductor current will not be continuous during burst periods. SF RSENSE = 15
Inductor Core Selection
I ( )( )(100) OUT Once the value for L is known, the type of inductor must be
Inductor Value Calculation
selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, The operating frequency and inductor selection are inter- forcing the use of more expensive ferrite, molypermalloy related in that higher operating frequencies permit the use or Kool Mu® cores. Actual core loss is independent of core of a smaller inductor for the same amount of inductor size for a fixed inductor value, but it is very dependent on ripple current. However, this is at the expense of efficiency inductance selected. As inductance increases, core losses due to an increase in MOSFET gate charge losses. go down. Unfortunately, increased inductance requires The inductance value also has a direct effect on ripple more turns of wire and therefore copper losses will current. The ripple current, I increase. Ferrite designs have very low core losses and are RIPPLE, decreases with higher inductance or frequency and increases with higher VIN or Kool Mu is a registered trademark of Magnetics, Inc. 7