Datasheet LTC3548-2 (Analog Devices) - 8

LTC3548-2
APPLICATIONS INFORMATION
A general LTC3548-2 application circuit is shown in The inductor value will also have an effect on Burst Mode Figure 2. External component selection is driven by the operation. The transition from low current operation load requirement, and begins with the selection of the begins when the peak inductor current falls below a level inductor L. Once the inductor is chosen, CIN and COUT set by the burst clamp. Lower inductor values result in can be selected. higher ripple current which causes this to occur at lower load currents. This causes a dip in effi ciency in the upper VIN range of low current operation. In Burst Mode operation, 2.5V TO 5.5V C lower inductance values will cause the burst frequency IN R5 RUN2 VIN RUN1 BM* to increase. MODE/SYNC POR POWER-ON PS* RESET LTC3548-2
Inductor Core Selection
L2 L1 VOUT2 SW2 SW1 VOUT1 Different core materials and shapes will change the size/ C5 current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy V V FB2 OUT1 R2 GND materials are small and do not radiate much energy, C R1 OUT2 COUT1 but generally cost more than powdered iron core induc- 35482 F02 tors with similar electrical characteristics. The choice of *MODE/SYNC = 0V: PULSE SKIPPING MODE/SYNC = VIN: Burst Mode OPERATION which style inductor to use often depends more on the price vs size requirements and any radiated fi eld/EMI
Figure 2. LTC3548-2 General Schematic
requirements than on what the LTC3548-2 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3548-2 applications.
Inductor Selection Table 1. Representative Surface Mount Inductors
Although the inductor does not infl uence the operating fre-
PART VALUE DCR MAX DC SIZE
quency, the inductor value has a direct effect on ripple cur-
NUMBER (μH) (
Ω
MAX) CURRENT (A) W
×
L
×
H (mm3)
rent. The inductor ripple current ΔIL decreases with higher Sumida 2.2 0.075 1.20 3.8 × 3.8 × 1.8 inductance and increases with higher VIN or VOUT: CDRH3D16 3.3 0.110 1.10 4.7 0.162 0.90 V ⎛ V ⎞ I OUT OUT • 1– Sumida 1.5 0.068 0.900 3.2 × 3.2 × 1.2 Δ L = CDRH2D11 2.2 0.170 0.780 f • L O ⎝⎜ VIN ⎠⎟ Sumida 2.2 0.116 0.950 4.4 × 5.8 × 1.2 CMD4D11 3.3 0.174 0.770 Accepting larger values of ΔIL allows the use of low Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 inductances, but results in higher output voltage ripple, LQH32CN 2.2 0.097 0.79 greater core losses, and lower output current capability. Toko 2.2 0.060 1.08 2.5 × 3.2 × 2.0 A reasonable starting point for setting ripple current is D312F 3.3 0.260 0.92 ΔIL = 0.3 • IOUT(MAX), where IOUT(MAX) is 800mA 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 (CIN) Selection
inductor value should be chosen according to the follow- 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 ⎞ L OUT OUT • 1– To prevent large voltage transients, a low equivalent series ≥ ⎜ ⎟ f • ΔI V O L ⎝ IN MA ( X) ⎠ resistance (ESR) input capacitor sized for the maximum 35482fb 8