Datasheet LTC3565 (Analog Devices) - 10

LTC3565
APPLICATIONS INFORMATION
A general LTC3565 application circuit is shown in inductances, but results in higher output ripple voltage, Figure 4. External component selection is driven by the load greater core loss and lower output capability. requirement, and begins with the selection of the induc- A reasonable starting point for setting ripple current is tor L1. Once L1 is chosen, CIN and COUT can be selected. ΔIL = 0.4 • IOUT(MAX), where IOUT(MAX) is 1.25A. The largest
Operating Frequency
ripple current ΔIL occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified Selection of the operating frequency is a trade-off between maximum, the inductor value should be chosen according efficiency and component size. High frequency operation to the following equation: allows the use of smaller inductor and capacitor values.   Operation at lower frequencies improves efficiency by L = VOUT • 1− VOUT  reducing internal gate charge losses but requires larger   f V O • ΔIL  IN(MAX)  inductance values and/or capacitance to maintain low output ripple voltage. The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins The operating frequency, fO, of the LTC3565 is determined when the peak inductor current falls below a level set by the by an external resistor that is connected between the RT burst clamp. Lower inductor values result in higher ripple pin and ground. The value of the resistor sets the ramp current which causes this to occur at lower load currents. current that is used to charge and discharge an internal This causes a dip in efficiency in the upper range of low timing capacitor within the oscillator and can be calculated current operation. In Burst Mode operation, lower induc- by using the following equation: tance values will cause the burst frequency to increase. RT = 1.21 × 106 (fO)–1.2674 (kΩ) 5000 TA = 25°C where R 4500 T is in kΩ and fO is in kHz or can be selected us- ing Figure 1. 4000 3500 The maximum usable operating frequency is limited by 3000 the minimum on-time and the duty cycle. This can be 2500 calculated as: 2000 FREQUENCY (kHz) 1500 f 1000 O(MAX) ≈ 6.67 • VOUT (MHz) VIN(MAX) 500 0 The minimum frequency is limited by leakage and noise 0 100 200 300 400 500 600 coupling due to the large resistance of R RT (kΩ) T. 3565 F01
Figure 1. Frequency vs R Inductor Selection T
The operating frequency, f
Inductor Core Selection
O, has a direct effect on the inductor value, which in turn influences the inductor ripple Different core materials and shapes will change the current, ΔIL: size/current and price/current relationship of an induc-   tor. Toroid or shielded pot cores in ferrite or permalloy ΔIL = VOUT •1− VOUT  materials are small and don’t radiate much energy, but fO •L  VIN  generally cost more than powdered iron core inductors The inductor ripple current decreases with larger induc- with similar electrical characteristics. The choice of which tance or frequency, and increases with higher VIN or VOUT. style inductor to use often depends more on the price vs Accepting larger values of ΔIL allows the use of lower size requirements and any radiated field/EMI requirements than on what the LTC3565 requires to operate. Table 1 3565fc 10 For more information www.linear.com/LTC3565 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts