Datasheet LTC3561A (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 1A, 4MHz, Synchronous Step-Down DC/DC Converter |
| Pages / Page | 20 / 9 — APPLICATIONS INFORMATION. Inductor Selection. Operating Frequency. … |
| File Format / Size | PDF / 364 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Inductor Selection. Operating Frequency. Inductor Core Selection. Figure 1. Frequency vs RT
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LTC3561A
APPLICATIONS INFORMATION
A general LTC3561A application circuit is shown in
Inductor Selection
Figure 4. External component selection is driven by the load The operating frequency, f requirement, and begins with the selection of the inductor O, has a direct effect on the inductor value, which in turn influences the inductor ripple L1. Once L1 is chosen, CIN and COUT can be selected. current, ΔIL:
Operating Frequency
V V ΔI OUT • 1− OUT Selection of the operating frequency is a trade-off between L = fO •L VIN efficiency and component size. High frequency operation The inductor ripple current decreases with larger induc- allows the use of smaller inductor and capacitor values. tance or frequency, and increases with higher V Operation at lower frequencies improves efficiency by IN or VOUT. Accepting larger values of ΔI reducing internal gate charge losses but requires larger L allows the use of lower inductances, but results in higher output ripple voltage, inductance values and/or capacitance to maintain low greater core loss and lower output capability. output ripple voltage. A reasonable starting point for setting ripple current is The operating frequency, fO, of the LTC3561A is determined ΔI by an external resistor that is connected between the R L = 0.4 • IOUT(MAX), where IOUT(MAX) is 1A. The largest T ripple current ΔI pin and ground. The value of the resistor sets the ramp L occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified current that is used to charge and discharge an internal maximum, the inductor value should be chosen according timing capacitor within the oscillator and can be calculated to the following equation: by using the following equation: R V V T ≈ 5 × 107 (fO)–1.6508 (kΩ) L = OUT • 1 − OUT f V where f O • ΔIL IN(MAX) O is in kHz, or can be selected using Figure 1. The maximum usable operating frequency is limited by
Inductor Core Selection
the minimum on-time and the duty cycle. This can be Different core materials and shapes will change the calculated as: size/current and price/current relationship of an induc- V f OUT tor. Toroid or shielded pot cores in ferrite or permalloy O(MAX) ≈ 6.67 • (MHz) materials are small and don’t radiate much energy, but V IN(MAX) generally cost more than powdered iron core inductors The minimum frequency is internally set at around with similar electrical characteristics. The choice of which 200kHz style inductor to use often depends more on the price vs 5000 TA = 25°C 4500 4000 3500 3000 2500 2000 FREQUENCY (kHz) 1500 1000 500 0 0 400 800 1200 1600 RT (kΩ) 3561A F01
Figure 1. Frequency vs RT
3561afa For more information www.linear.com/LTC3561A 9
APPLICATIONS INFORMATION
A general LTC3561A application circuit is shown in
Inductor Selection
Figure 4. External component selection is driven by the load The operating frequency, f requirement, and begins with the selection of the inductor O, has a direct effect on the inductor value, which in turn influences the inductor ripple L1. Once L1 is chosen, CIN and COUT can be selected. current, ΔIL:
Operating Frequency
V V ΔI OUT • 1− OUT Selection of the operating frequency is a trade-off between L = fO •L VIN efficiency and component size. High frequency operation The inductor ripple current decreases with larger induc- allows the use of smaller inductor and capacitor values. tance or frequency, and increases with higher V Operation at lower frequencies improves efficiency by IN or VOUT. Accepting larger values of ΔI reducing internal gate charge losses but requires larger L allows the use of lower inductances, but results in higher output ripple voltage, inductance values and/or capacitance to maintain low greater core loss and lower output capability. output ripple voltage. A reasonable starting point for setting ripple current is The operating frequency, fO, of the LTC3561A is determined ΔI by an external resistor that is connected between the R L = 0.4 • IOUT(MAX), where IOUT(MAX) is 1A. The largest T ripple current ΔI pin and ground. The value of the resistor sets the ramp L occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified current that is used to charge and discharge an internal maximum, the inductor value should be chosen according timing capacitor within the oscillator and can be calculated to the following equation: by using the following equation: R V V T ≈ 5 × 107 (fO)–1.6508 (kΩ) L = OUT • 1 − OUT f V where f O • ΔIL IN(MAX) O is in kHz, or can be selected using Figure 1. The maximum usable operating frequency is limited by
Inductor Core Selection
the minimum on-time and the duty cycle. This can be Different core materials and shapes will change the calculated as: size/current and price/current relationship of an induc- V f OUT tor. Toroid or shielded pot cores in ferrite or permalloy O(MAX) ≈ 6.67 • (MHz) materials are small and don’t radiate much energy, but V IN(MAX) generally cost more than powdered iron core inductors The minimum frequency is internally set at around with similar electrical characteristics. The choice of which 200kHz style inductor to use often depends more on the price vs 5000 TA = 25°C 4500 4000 3500 3000 2500 2000 FREQUENCY (kHz) 1500 1000 500 0 0 400 800 1200 1600 RT (kΩ) 3561A F01
Figure 1. Frequency vs RT
3561afa For more information www.linear.com/LTC3561A 9
