Datasheet LTC3419 (Analog Devices) - 9

LTC3419
APPLICATIONS INFORMATION
A general LTC3419 application circuit is shown in Figure 1. or shielded pot cores in ferrite or permalloy materials are External component selection is driven by the load small and do not radiate much energy, but generally cost requirement, and begins with the selection of the more than powdered iron core inductors with similar inductor L. Once the inductor is chosen, CIN and COUT electrical characteristics. The choice of which style can be selected. inductor to use often depends more on the price versus size requirements, and any radiated fi eld/EMI requirements,
Inductor Selection
than on what the LTC3419 requires to operate. Table 1 Although the inductor does not infl uence the operating shows some typical surface mount inductors that work frequency, the inductor value has a direct effect on ripple well in LTC3419 applications. current. The inductor ripple current ΔIL decreases with
Table 1. Representative Surface Mount Inductors
higher inductance and increases with higher VIN or VOUT:
MANU- MAX DC FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
V ⎛ V ⎞ ΔI OUT OUT L = • ⎜1− ⎟ ( ) 1 Taiyo Yuden CB2016T2R2M 2.2μH 510mA 0.13Ω 1.6mm f • L V ⎝ ⎠ CB2012T2R2M 2.2μH 530mA 1.25mm O IN 0.33Ω CB2016T3R3M 3.3μH 410mA 0.27Ω 1.6mm Accepting larger values of ΔIL allows the use of low Panasonic ELT5KT4R7M 4.7μH 950mA 0.2Ω 1.2mm inductances, but results in higher output voltage ripple, Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm greater core losses, and lower output current capability. Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm A reasonable starting point for setting ripple current is Taiyo Yuden NR30102R2M 2.2μH 1100mA 0.1Ω 1mm 40% of the maximum output load current. So, for a 600mA NR30104R7M 4.7μH 750mA 0.19Ω 1mm regulator, ΔIL = 240mA (40% of 600mA). FDK FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm The inductor value will also have an effect on Burst Mode FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm operation. The transition to low current operation begins TDK VLF3010AT4R7- 4.7μH 700mA 0.28Ω 1mm when the peak inductor current falls below a level set by MR70 VLF3010AT3R3- 3.3μH 870mA 0.17Ω 1mm the internal burst clamp. Lower inductor values result in MR87 higher ripple current which causes the transition to occur VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm M1R0 at lower load currents. This causes a dip in effi ciency in the upper range of low current operation. Furthermore,
Input Capacitor (C
lower inductance values will cause the bursts to occur
IN) Selection
with increased frequency. In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT/ VIN.
Inductor Core Selection
To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum Different core materials and shapes will change the size/ RMS current must be used. The maximum RMS capacitor current and price/current relationship of an inductor. Toroid current is given by: VIN 2.5V TO 5.5V V (V − V OUT IN OUT ) C1 I ≈ I RUN2 VIN RUN1 RMS MAX MODE V IN LTC3419 L2 L1 VOUT2 SW2 SW1 VOUT1 Where the maximum average output current I CF2 CF1 MAX equals the peak current minus half the peak-to-peak ripple cur- VFB2 VFB1 rent, I R4 GND R2 MAX = ILIM – ΔIL /2. This formula has a maximum at COUT2 R3 R1 COUT1 VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case 3419 F01 is commonly used to design because even signifi cant
Figure 1. LTC3419 General Schematic
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