Datasheet LTC1772B (Analog Devices) - 8

LTC1772B
APPLICATIONS INFORMATION
The basic LTC1772B application circuit is shown in Figure 1. The inductance value also has a direct effect on ripple External component selection is driven by the load require- current. The ripple current, IRIPPLE, decreases with higher ment and begins with the selection of L1 and RSENSE (= R1). inductance or frequency and increases with higher VIN or Next, the power MOSFET, M1 and the output diode D1 is VOUT. The inductor’s peak-to-peak ripple current is given selected followed by CIN (= C1)and COUT(= C2). by: V IN VOUT VOUT + VD
R
I
SENSE Selection for Output Current
RIPPLE = f L ( ) V IN + VD RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage de- where f is the operating frequency. Accepting larger values veloped across RSENSE, the threshold of the comparator of IRIPPLE allows the use of low inductances, but results determines the inductor’s peak current. The output current in higher output voltage ripple and greater core losses. the LTC1772B can provide is given by: A reasonable starting point for setting ripple current is 0 105 . I IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE I RIPPLE OUT = − occurs at the maximum input voltage. RSENSE 2 The ripple current is normally set such that the induc- where IRIPPLE is the inductor peak-to-peak ripple current tor current is continuous down to approximately 1/4 of (see Inductor Value Calculation section). maximum load current. This results in: A reasonable starting point for setting ripple current is 0 0 . 3 I I ≤ RIPPLE = (0.4)(IOUT). Rearranging the above equation, it RIPPLE RSENSE becomes: This implies a minimum inductance of: 0 0875 . RSENSE = for Duty Cycle < 40% V V IOUT L IN VOUT OUT + VD MIN = 0.03 V IN + VD However, for operation that is above 40% duty cycle, slope f R SENSE compensation effect has to be taken into consideration to select the appropriate value to provide the required amount (Use VIN(MAX) = VIN) of current. Using Figure 3, the value of RSENSE is: A smaller value than LMIN could be used in the circuit; ( . 0 0875 SF ) however, the inductor current transitioning from continuous RSENSE = I ( ) 100 to discontinuous will occur at a higher load current. OUT
Power MOSFET Selection Inductor Value Calculation
An external P-channel power MOSFET must be selected The operating frequency and inductor selection are inter- for use with the LTC1772B. The main selection criteria for related in that higher operating frequencies permit the use the power MOSFET are the threshold voltage VGS(TH) and of a smaller inductor for the same amount of inductor ripple the “on” resistance RDS(ON), reverse transfer capacitance current. However, this is at the expense of effi ciency due CRSS and total gate charge. to an increase in MOSFET gate charge losses. 1772bfa 8