Datasheet LTC3411A (Analog Devices) - 10

LTC3411A
applicaTions inForMaTion
A general LTC3411A application circuit is shown in A reasonable starting point for setting ripple current is Figure 4. External component selection is driven by the load ΔIL = 0.4 • IOUT(MAX), where IOUT(MAX) is 1.25A. The largest requirement, and begins with the selection of the inductor ripple current ΔIL occurs at the maximum input voltage. To L1. Once L1 is chosen, CIN and COUT can be selected. guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according
Operating Frequency
to the following equation: Selection of the operating frequency is a trade-off between V ⎛ V ⎞ efficiency and component size. High frequency operation L OUT OUT = • 1 ⎜ − ⎟ allows the use of smaller inductor and capacitor values. f V O • ΔIL ⎝ IN(MAX)⎠ 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 LTC3411A 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 will cause the burst frequency timing capacitor within the oscillator and can be calculated to increase. by using the following equation: R 5000 T = 5 • 107 (fO)–1.6508 (kΩ), TA = 25°C 4500 where fO is in kHz, or can be selected using Figure 1. 4000 The maximum usable operating frequency is limited by 3500 the minimum on-time and the duty cycle. This can be 3000 calculated as: 2500 2000 V FREQUENCY (kHz) 1500 f OUT O(MAX) ≈ 6.67 •  (MHz) 1000 VIN(MAX) 500 0 The minimum frequency is internally set at around 200kHz. 0 400 800 1200 1600 RT (kΩ)
Inductor Selection
3411A F01
Figure 1. Frequency vs RT
The operating frequency, fO, has a direct effect on the in- ductor value, which in turn influences the inductor ripple
Inductor Core Selection
current ΔIL: Different core materials and shapes will change the V ⎛ V ⎞ size/current and price/current relationship of an induc- ΔI OUT OUT L = • 1− tor. Toroid or shielded pot cores in ferrite or permalloy fO •L ⎝⎜ VIN ⎠⎟ materials are small and don’t radiate much energy, but The inductor ripple current decreases with larger induc- generally cost more than powdered iron core inductors tance or frequency, and increases with higher VIN or VOUT. with similar electrical characteristics. The choice of which Accepting larger values of ΔIL allows the use of lower style inductor to use often depends more on the price vs inductances, but results in higher output ripple voltage, size requirements and any radiated field/EMI requirements greater core loss and lower output capability. than on what the LTC3411A requires to operate. Table 1 3411afd 10 For more information www.linear.com/LTC3411A 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 Revision History Typical Application Related Parts