Datasheet LTC3417A-2 (Analog Devices) - 9

LTC3417A-2
APPLICATIONS INFORMATION
A general LTC3417A-2 application circuit is shown in 160 Figure 4. External component selection is driven by the 140 load requirement, and begins with the selection of the 120 inductors L1 and L2. Once L1 and L2 are chosen, CIN, 100 COUT1 and COUT2 can be selected. (kΩ) 80 TR
Operating Frequency
60 Selection of the operating frequency is a tradeoff between 40 effi ciency and component size. High frequency operation 20 allows the use of smaller inductor and capacitor values. 0 Operation at lower frequencies improves effi ciency by 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 FREQUENCY (MHz) reducing internal gate charge losses but requires larger 3417A-2 F01 inductance values and/or capacitance to maintain low
Figure 1. Frequency vs R
output ripple voltage.
T
The operating frequency, fO, of the LTC3417A-2 is deter- mined by pulling the FREQ pin to V
Inductor Selection
IN for 1.5MHz opera- tion or by connecting an external resistor from FREQ to Although the inductor does not infl uence the operating ground. The value of the resistor sets the ramp current frequency, the inductor value has a direct effect on ripple that is used to charge and discharge an internal timing current. The inductor ripple current, ΔIL, decreases with capacitor within the oscillator and can be calculated by higher inductance and increases with higher VIN or VOUT. using the following equation: V I OUT L = VOUT 1– 1.61• 1011 fO •L VIN RT ≈ (Ω) – 16.586kΩ fO Accepting larger values of ΔIL allows the use of low induc- for 0.6MHz ≤ fO ≤ 4MHz. Alternatively, use Figure 1 to tances, but results in higher output voltage ripple, greater select the value for RT. core losses and lower output current capability. The maximum operating frequency is also constrained A reasonable starting point for setting ripple current is by the minimum on-time and duty cycle. This can be ΔIL = 0.35ILOAD(MAX), where ILOAD(MAX) is the maximum calculated as: current output. The largest ripple, ΔIL, occurs at the maxi- mum input voltage. To guarantee that the ripple current V f OUT O(MAX) 6.67 MHz ( ) stays below a specifi ed maximum, the inductor value VIN(MAX) should be chosen according to the following equation: The minimum frequency is limited by leakage and noise coupling due to the large resistance of R T. V L = VOUT 1– OUT f _ O • IL VIN(MAX) 3417a2fa 9