Datasheet LTC3568 (Analog Devices) - 8

LTC3568 applicaTions inForMaTion A general LTC3568 application circuit is shown in A reasonable starting point for setting ripple current is Figure 5. External component selection is driven by the load ΔIL = 0.4 • IOUT, where IOUT is the maximum output cur- requirement, and begins with the selection of the inductor rent. The largest ripple current ∆IL occurs at the maximum L1. Once L1 is chosen, CIN and COUT can be selected. input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be
Operating Frequency
chosen according to the following equation: Selection of the operating frequency is a tradeoff between V  V  efficiency and component size. High frequency operation L OUT OUT = •  − 1  allows the use of smaller inductor and capacitor values. f • I Δ V O L  IN M ( AX)  Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger The inductor value will also have an effect on Burst Mode inductance values and/or capacitance to maintain low operation. The transition from low current operation output ripple voltage. begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in The operating frequency, fO, of the LTC3568 is determined higher ripple current which causes this to occur at lower by an external resistor that is connected between the RT load currents. This causes a dip in efficiency in the upper pin and ground. The value of the resistor sets the ramp range of low current operation. In Burst Mode operation, current that is used to charge and discharge an internal lower inductance values wil cause the burst frequency timing capacitor within the oscillator and can be calculated to increase. by using the following equation: 4.5 TA = 25°C = − 9 78 1011 1 08 . • . (Ω) 4.0 ( ) R f T O 3.5 or can be selected using Figure 2. 3.0 2.5 The maximum usable operating frequency is limited by 2.0 the minimum on-time and the duty cycle. This can be calculated as: FREQUENCY (MHz) 1.5 1.0 fO(MAX) ≈ 6.67 • (VOUT / VIN(MAX)) (MHz) 0.5 The minimum frequency is limited by leakage and noise 0 0 500 1000 1500 coupling due to the large resistance of RT. RT (kΩ) 3568 F02
Inductor Selection Figure 2. Frequency vs RT
Although the inductor does not influence the operat-
Inductor Core Selection
ing frequency, the inductor value has a direct effect on ripple current. The inductor ripple current ΔI Different core materials and shapes wil change the size/cur- L decreases with higher inductance and increases with higher V rent and price/current relationship of an inductor. Toroid or IN or V shielded pot cores in ferrite or permal oy materials are small OUT: and don’t radiate much energy, but general y cost more than V  V  ΔI OUT OUT powdered iron core inductors with similar electrical char- L = • 1− f •L acteristics. The choice of which style inductor to use often O  VIN  depends more on the price vs size requirements and any Accepting larger values of ΔIL allows the use of low induc- radiated field/EMI requirements than on what the LTC3568 tances, but results in higher output voltage ripple, greater requires to operate. Table 1 shows some typical surface core losses, and lower output current capability. mount inductors that work wel in LTC3568 applications. 3568fa 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 Applications Package Description Revision History Typical Application Related Parts