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LTC3565EDDPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3565EDDPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 22 page  LTC3565 3565fa applications inForMation A general LTC3565 application circuit is shown in Figure 4.Externalcomponentselectionisdrivenbytheload 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, of the LTC3565 is determined 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 timingcapacitorwithintheoscillatorandcanbecalculated by using the following equation: RT = 1.21 × 106 (fO)–1.2674 (kΩ) where RT is in kΩ and 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: fO(MAX) ≈ 6.67 • VOUT VIN(MAX) (MHz) The minimum frequency is limited by leakage and noise coupling due to the large resistance of RT. Inductor Selection The operating frequency, fO, has a direct effect on the inductor value, which in turn influences the inductor ripple current, ΔIL: ΔIL = VOUT fO •L • 1− VOUT VIN ⎛ ⎝⎜ ⎞ ⎠⎟ 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),whereIOUT(MAX)is1.25A.Thelargest ripple current ΔILoccursatthemaximuminputvoltage.To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L = VOUT fO • ΔIL • 1− VOUT VIN(MAX) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Figure 1. Frequency vs RT 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 sizerequirementsandanyradiatedfield/EMIrequirements than on what the LTC3565 requires to operate. Table 1 RT (k) 0 0 500 1500 2000 2500 5000 4500 3565 F01 1000 100 200 300 400 600 500 3000 3500 4000 TA = 25°C |
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