Datasheet LTC3701 (Analog Devices) - 10

LTC3701
U U W U APPLICATIO S I FOR ATIO
The basic LTC3701 application circuit is shown in Fig- smaller inductor for the same amount of inductor ripple ure␣ 1. External component selection is driven by the load current. However, this is at the expense of efficiency due requirement and begins with the selection of L and RSENSE. to an increase in MOSFET gate charge and switching Next, the power MOSFET M1 and the output diode D1 are losses. selected. Finally CIN (C1) and COUT (C2) are chosen. The inductance value also has a direct effect on ripple current. The ripple current, I
R
RIPPLE, decreases with higher
SENSE Selection for Output Current
inductance or frequency. The inductor’s peak-to-peak RSENSE is chosen based on the required output current. ripple current is: Since the current comparator monitors the voltage devel- oped across RSENSE, the threshold of the comparator V – V  V V  I IN OUT OUT D = + determines the inductor’s peak current. The output cur- RIPPLE f • L  V + V IN D   rent that the LTC3701 can provide is given by: where f is the operating frequency and VD is the forward . I voltage drop of the external Schottky diode. Accepting I RIPPLE OUT = 0 095 – R larger values of IRIPPLE allows the use of low inductances, SENSE 2 but results in higher output voltage ripple and greater core where IRIPPLE is the inductor peak-to-peak ripple current losses. A reasonable starting point for setting ripple cur- (see Inductor Value Calculation). rent is IRIPPLE = 0.4(IOUT(MAX)). The maximum IRIPPLE A reasonable starting point for setting ripple current is occurs at the maximum input voltage. IRIPPLE = (0.4)(IOUT). Rearranging the above equation With Burst Mode operation selected on the LTC3701, the yields: ripple current is normally set such that the inductor current is continuous during the burst periods. Therefore, RSENSE = 1 for Duty Cycle < 20% the peak-to-peak ripple current must not exceed: 12 7 . •IOUT IRIPPLE ≤ (0.03)/RSENSE However, for operation above 20% duty cycle, slope This implies a minimum inductance of: compensation has to be taken into consideration to select the appropriate value of RSENSE to provide the required V – V  V + V IN OUT OUT D  amount of current. Using Figure 2, the value of R L = SENSE is: MIN  . 0 03   V + V  f IN D SF R  R SENSE SENSE = 12 7. I ( )( )( ) OUT 100 (Use VIN = V ( )) IN MAX For noise sensitive applications, a 1nF capacitor placed A smaller value than LMIN could be used in the circuit, between the SENSE+ and SENSE– pins very close to the however, the inductor current will not be continuous chip is suggested. during burst periods.
Inductor Value Calculation Inductor Core Selection
The inductor selection will depend on the operating fre- Once the value of L is known, the type of inductor must be quency of the LTC3701. The internal nominal frequency is selected. High efficiency converters generally cannot af- 550kHz, but can be externally synchronized or set from ford the core loss found in low cost powdered iron cores, approximately 300kHz to 750kHz. forcing the use of more expensive ferrite, molypermalloy or Kool Mµ® cores. Actual core loss is independent of core The operating frequency and inductor selection are inter- related in that higher frequencies permit the use of a Kool Mµ is a registered trademark of Magnetics, Inc. 3701fa 10