Datasheet LTC3548-1 (Analog Devices) - 8

LTC3548-1
APPLICATIONS INFORMATION
A general LTC3548-1 application circuit is shown in higher ripple current which causes this to occur at lower Figure 2. External component selection is driven by the load currents. This causes a dip in effi ciency in the upper load requirement, and begins with the selection of the range of low current operation. In Burst Mode operation, inductor L. Once the inductor is chosen, CIN and COUT lower inductance values will cause the burst frequency can be selected. to increase.
Inductor Core Selection
VIN 2.7V TO 5.5V CIN 10μF Different core materials and shapes will change the V RUN1 RUN2 IN CER size/ current and price/current relationship of an induc- LTC3548-1 V 4.7μH 2.2μH OUT2 VOUT1 tor. Toroid or shielded pot cores in ferrite or permalloy 1.575V SW2 SW1 1.8V 400mA 800mA materials are small and do not radiate much energy, V V C OUT2 OUT1 C OUT2 OUT1 C C FF2 FF1 but generally cost more than powdered iron core induc- 10μF 10μF 330pF 330pF CER V V FB2 FB1 CER tors with similar electrical characteristics. The choice of GND 3548-1 F01 which style inductor to use often depends more on the price vs size requirements and any radiated fi eld/EMI
Figure 2. LTC3548-1 General Schematic
requirements than on what the LTC3548-1 requires to operate. Table 1 shows some typical surface mount
Inductor Selection
inductors that work well in LTC3548-1 applications. Although the inductor does not infl uence the operating fre-
Table 1. Representative Surface Mount Inductors
quency, the inductor value has a direct effect on ripple cur-
PART VALUE DCR MAX DC SIZE
rent. The inductor ripple current ΔI
NUMBER (μH) (
Ω
MAX) CURRENT (A) W
×
L
×
H (mm3)
L decreases with higher inductance and increases with higher V Sumida 2.2 0.075 1.20 3.8 × 3.8 × 1.8 IN or VOUT: CDRH3D16 3.3 0.110 1.10 4.7 0.162 0.90 V I = VOUT OUT L • 1– Sumida 2.2 0.089 0.95 4.1 × 3.2 × 0.8 fO •L VIN CMD4D06 4.7 0.166 0.75 Sumida 2.2 0.116 0.950 4.4 × 5.8 × 1.2 Accepting larger values of ΔIL allows the use of low induc- CMD4D11 3.3 0.174 0.770 tances, but results in higher output voltage ripple, greater Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 LQH32CN 2.2 0.097 0.79 core losses, and lower output current capability. A reasonable starting point for setting ripple current is Toko 2.2 0.060 1.08 2.5 × 3.2 × 2.0 D312F 3.3 0.260 0.92 ΔIL = 0.3 • IOUT(MAX), where IOUT(MAX) is 0.8A for channel 1 Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2 and 400mA for channel 2. The largest ripple current ΔIL ELT5KT 4.7 0.20 0.95 occurs at the maximum input voltage. To guarantee that the ripple current stays below a specifi ed maximum, the
Input Capacitor (C
inductor value should be chosen according to the follow-
IN) Selection
In continuous mode, the input current of the converter is a ing equation: square wave with a duty cycle of approximately VOUT/VIN. V ⎛ V ⎞ OUT OUT To prevent large voltage transients, a low equivalent series L ≥ • ⎜1– ⎟ f • I Δ V resistance (ESR) input capacitor sized for the maximum O L ⎝ IN MA ( X) ⎠ RMS current must be used. The maximum RMS capacitor The inductor value will also have an effect on Burst Mode current is given by: operation. The transition from low current operation begins when the peak inductor current falls below a level V V – V ( ) OUT IN OUT set by the burst clamp. Lower inductor values result in I ≈I RMS MAX V IN 35481fc 8