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LTC3561AIDD-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3561AIDD-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3561A 10 3561af APPLICATIONS INFORMATION A general LTC3561A application circuit is shown in Figure 4. External component selection is driven by the load requirement, and begins with the selection of the inductor L1. Once L1 is chosen, CIN and COUT can be selected. Operating Frequency Selection of the operating frequency is a trade-off between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency, fO,oftheLTC3561Aisdetermined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by using the following equation: RT ≈ 5 × 107 (fO)–1.6508 (kΩ) where fO is in kHz, or can be selected using Figure 1. The maximum usable operating frequency is limited by the minimum on-time and the duty cycle. This can be calculated as: f O(MAX) ≈ 6.67 • V OUT V IN(MAX) (MHz) The minimum frequency is internally set at around 200kHz Inductor Selection The operating frequency, fO, has a direct effect on the inductor value, which in turn influences the inductor ripple current, ΔIL: ΔI L = V OUT f O •L •1 − V OUT V IN ⎛ ⎝⎜ ⎞ ⎠⎟ The inductor ripple current decreases with larger induc- tance or frequency, and increases with higher VIN or VOUT. Accepting larger values of ΔIL allows the use of lower inductances, but results in higher output ripple voltage, greater core loss and lower output capability. A reasonable starting point for setting ripple current is ΔIL = 0.4 • IOUT(MAX), where IOUT(MAX) is 1A. The largest ripple current ΔIL occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L = V OUT f O • ΔIL •1 − V OUT V IN(MAX) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Inductor Core Selection Different core materials and shapes will change the size/current and price/current relationship of an induc- tor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs Figure 1. Frequency vs RT RT (kΩ) 0 0 500 1500 2000 2500 5000 4500 3561A F01 1000 400 800 1200 1600 3000 3500 4000 TA = 25°C |
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