Datasheet LTC3407A (Analog Devices) - 8

LTC3407A
APPLICATIONS INFORMATION
VIN = 2.5V TO 5.5V
Inductor Core Selection
CIN R7 V Different core materials and shapes will change the size/ IN R6 BURST* R5 MODE/SYNC POR POWER-ON PULSESKIP* RESET current and price/current relationship of an inductor. Toroid LTC3407A or shielded pot cores in ferrite or permalloy materials are RUN/SS2 RUN/SS1 V L2 L1 V small and don’t radiate much energy, but generally cost OUT2 OUT1 SW2 SW1 C2 C1 more than powdered iron core inductors with similar elec- C4 C3 trical characterisitics. The choice of which style inductor V V FB2 FB1 R4 R2 to use often depends more on the price vs size require- GND COUT2 R3 R1 COUT1 ments and any radiated fi eld/EMI requirements than on what the LTC3407A requires to operate. Table 1 shows *MODE/SYNC = 0V: PULSE SKIP 3407A F01 MODE/SYNC = VIN: Burst Mode some typical surface mount inductors that work well in LTC3407A applications.
Figure 1. LTC3407A General Schematic Table 1. Representative Surface Mount Inductors MANUF- MAX DC Inductor Selection ACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Although the inductor does not infl uence the operat- Taiyo CB2016T2R2M 2.2μH 510mA 0.13Ω 1.6mm Yuden CB2012T2R2M 2.2μH 530mA 0.33Ω 1.25mm ing frequency, the inductor value has a direct effect on CB2016T3R3M 3.3μH 410mA 0.27Ω 1.6mm ripple current. The inductor ripple current ΔIL decreases Panasonic ELT5KT4R7M 4.7μH 950mA 0.2Ω 1.2mm with higher inductance and increases with higher VIN or Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm VOUT: Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm V I OUT Taiyo NR30102R2M 2.2μH 1100mA 0.1Ω 1mm L = VOUT • 1– f V Yuden NR30104R7M 4.7μH 750mA 0.19Ω 1mm O • L IN FDK FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm Accepting larger values of ΔIL allows the use of low FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm inductances, but results in higher output voltage ripple, FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm greater core losses, and lower output current capability. A TDK VLF3010AT4R7- 4.7μH 700mA 0.28Ω 1mm MR70 reasonable starting point for setting ripple current is ΔIL = VLF3010AT3R3- 3.3μH 870mA 0.17Ω 1mm 0.3 • I MR87 LIM, where ILIM is the peak switch current limit. The VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm largest ripple current ΔIL occurs at the maximum input M1R0 voltage. To guarantee that the ripple current stays below a specifi ed maximum, the inductor value should be chosen
Input Capacitor (CIN) Selection
according to the following equation: In continuous mode, the input current of the converter is a V square wave with a duty cycle of approximately VOUT/VIN. L = VOUT • 1– OUT f To prevent large voltage transients, a low equivalent series O • IL VIN(MAX) resistance (ESR) input capacitor sized for the maximum The inductor value will also have an effect on Burst Mode RMS current must be used. The maximum RMS capacitor operation. The transition from low current operation current is given by: begins when the peak inductor current falls below a level VOUT(VIN – VOUT) set by the burst clamp. Lower inductor values result in IRMS ≈IMAX V higher ripple current which causes this transition to occur IN at lower load currents. This causes a dip in effi ciency in where the maximum average output current IMAX equals the upper range of low current operation. In Burst Mode the peak current minus half the peak-to-peak ripple cur- operation, lower inductance values will cause the burst rent, IMAX = ILIM – ΔIL/2. frequency to increase. 3407afa 8