Datasheet LTC3407A-2 (Analog Devices) - 8

LTC3407A-2
U U W U APPLICATIO S I FOR ATIO Inductor Selection Input Capacitor (CIN) Selection
Although the inductor does not influence the operating In continuous mode, the input current of the converter is frequency, the inductor value has a direct effect on ripple a square wave with a duty cycle of approximately VOUT/ current. The inductor ripple current ΔI V L decreases with IN. To prevent large voltage transients, a low equivalent higher inductance and increases with higher V series resistance (ESR) input capacitor sized for the maxi- IN or VOUT: mum RMS current must be used. The maximum RMS V ⎛ V ⎞ OUT OUT capacitor current is given by: ΔIL = • 1– f • L O ⎝⎜ VIN ⎠⎟ V (V – V ) Accepting larger values of ΔIL allows the use of low I ≈I OUT IN OUT RMS MAX V inductances, but results in higher output voltage ripple, IN greater core losses, and lower output current capability. where the maximum average output current IMAX equals A reasonable starting point for setting ripple current is the peak current minus half the peak-to-peak ripple cur- ΔIL = 0.3 • ILIM, where ILIM is the peak switch current limit. rent, IMAX = ILIM – ΔIL/2. The largest ripple current ΔIL occurs at the maximum input voltage. To guarantee that the ripple current stays This formula has a maximum at VIN = 2VOUT, where below a specified maximum, the inductor value should be IRMS = IOUT/2. This simple worst-case is commonly used chosen according to the following equation: to design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple V ⎛ V ⎞ OUT OUT current ratings are often based on only 2000 hours life- L = • ⎜1– ⎟ f • I Δ V time. This makes it advisable to further derate the capaci- O L ⎝ IN MA ( X) ⎠ tor, or choose a capacitor rated at a higher temperature The inductor value will also have an effect on Burst Mode than required. Several capacitors may also be paralleled to operation. The transition from low current operation be- meet the size or height requirements of the design. An gins when the peak inductor current falls below a level set additional 0.1μF to 1μF ceramic capacitor is also recom- by the burst clamp. Lower inductor values result in higher mended on VIN for high frequency decoupling, when not ripple current which causes this transition to occur at using an all ceramic capacitor solution. lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode
Table 1. Representative Surface Mount Inductors
operation, lower inductance values will cause the burst
MANU- MAX DC FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
frequency to increase. Taiyo Yuden CB2016T2R2M 2.2μH 510mA 0.13Ω 1.6mm
Inductor Core Selection
CB2012T2R2M 2.2μH 530mA 0.33Ω 1.25mm Different core materials and shapes will change the size/ CB2016T3R3M 3.3μH 410mA 0.27Ω 1.6mm current and price/current relationship of an inductor. Panasonic ELT5KT4R7M 4.7μH 950mA 0.2Ω 1.2mm Toroid or shielded pot cores in ferrite or permalloy mate- Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm rials are small and don’t radiate much energy, but gener- Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm ally cost more than powdered iron core inductors with Taiyo Yuden NR30102R2M 2.2μH 1100mA 0.1Ω 1mm NR30104R7M 4.7μH 750mA 0.19Ω 1mm similar electrical characterisitics. The choice of which FDK FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm style inductor to use often depends more on the price vs FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm size requirements and any radiated field/EMI require- FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm ments than on what the LTC3407A-2 requires to operate. TDK VLF3010AT4R7- 4.7μH 700mA 0.28Ω 1mm Table 1 shows some typical surface mount inductors that MR70 VLF3010AT3R3- 3.3 work well in LTC3407A-2 applications. μH 870mA 0.17Ω 1mm MR87 VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm M1R0 3407a2f 8