Datasheet LTC3548A (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator |
| Pages / Page | 20 / 9 — OPERATION. Figure 1. LTC3548A General Schematic. APPLICATIONS … |
| File Format / Size | PDF / 255 Kb |
| Document Language | English |
OPERATION. Figure 1. LTC3548A General Schematic. APPLICATIONS INFORMATION. Inductor Selection. Inductor Core Selection
Model Line for this Datasheet
Text Version of Document
LTC3548A
OPERATION
VIN = 2.5V TO 5.5V CIN VIN R7 R6 BURST* R5 POWER-ON MODE/SYNC POR PULSE SKIP* RESET RUN/SS2 RUN/SS1 C4 C3 LTC3548A L2 L1 VOUT2 SW2 SW1 VOUT1 C2 C1 V V FB2 FB1 R4 R2 C GND OUT2 R3 R1 COUT1 3548A F01 *MODE/SYNC = 0V: PULSE SKIP MODE/SYNC = VIN: Burst Mode OPERATION
Figure 1. LTC3548A General Schematic APPLICATIONS INFORMATION Inductor Selection
The inductor value will also have an effect on Burst Mode operation. The transition from low current operation Although the inductor does not influence the operat- begins when the peak inductor current falls below a level ing frequency, the inductor value has a direct effect on set by the burst clamp. Lower inductor values result in ripple current. The inductor ripple current ΔIL decreases higher ripple current which causes this transition to occur with higher inductance and increases with higher VIN or at lower load currents. This causes a dip in efficiency in VOUT : the upper range of low current operation. In Burst Mode V ⎛ V ⎞ ΔI OUT OUT operation, lower inductance values will cause the burst L = • 1– f ⎝⎜ V ⎠⎟ frequency to increase. O •L IN Accepting larger values of ΔI
Inductor Core Selection
L allows the use of low inductances, but results in higher output voltage ripple, Different core materials and shapes will change the size/ greater core losses, and lower output current capability. A current and price/current relationship of an inductor. Toroid reasonable starting point for setting ripple current is ΔI or shielded pot cores in ferrite or permalloy materials are L = 0.3 • I small and don’t radiate much energy, but generally cost LIM, where ILIM is the peak switch current limit. The largest ripple current ΔI more than powdered iron core inductors with similar elec- L occurs at the maximum input trical characteristics. The choice of which style inductor voltage. To guarantee that the ripple current stays below a to use often depends more on the price vs size require- specified maximum, the inductor value should be chosen ments and any radiated field/EMI requirements than on according to the following equation: what the LTC3548A requires to operate. Table 1 shows V ⎛ V ⎞ some typical surface mount inductors that work well in L ≥ OUT • ⎜1– OUT f ⎟ ⎝ V LTC3548A applications. IN(MAX) ⎠ O • ΔIL 3548afa 9
OPERATION
VIN = 2.5V TO 5.5V CIN VIN R7 R6 BURST* R5 POWER-ON MODE/SYNC POR PULSE SKIP* RESET RUN/SS2 RUN/SS1 C4 C3 LTC3548A L2 L1 VOUT2 SW2 SW1 VOUT1 C2 C1 V V FB2 FB1 R4 R2 C GND OUT2 R3 R1 COUT1 3548A F01 *MODE/SYNC = 0V: PULSE SKIP MODE/SYNC = VIN: Burst Mode OPERATION
Figure 1. LTC3548A General Schematic APPLICATIONS INFORMATION Inductor Selection
The inductor value will also have an effect on Burst Mode operation. The transition from low current operation Although the inductor does not influence the operat- begins when the peak inductor current falls below a level ing frequency, the inductor value has a direct effect on set by the burst clamp. Lower inductor values result in ripple current. The inductor ripple current ΔIL decreases higher ripple current which causes this transition to occur with higher inductance and increases with higher VIN or at lower load currents. This causes a dip in efficiency in VOUT : the upper range of low current operation. In Burst Mode V ⎛ V ⎞ ΔI OUT OUT operation, lower inductance values will cause the burst L = • 1– f ⎝⎜ V ⎠⎟ frequency to increase. O •L IN Accepting larger values of ΔI
Inductor Core Selection
L allows the use of low inductances, but results in higher output voltage ripple, Different core materials and shapes will change the size/ greater core losses, and lower output current capability. A current and price/current relationship of an inductor. Toroid reasonable starting point for setting ripple current is ΔI or shielded pot cores in ferrite or permalloy materials are L = 0.3 • I small and don’t radiate much energy, but generally cost LIM, where ILIM is the peak switch current limit. The largest ripple current ΔI more than powdered iron core inductors with similar elec- L occurs at the maximum input trical characteristics. The choice of which style inductor voltage. To guarantee that the ripple current stays below a to use often depends more on the price vs size require- specified maximum, the inductor value should be chosen ments and any radiated field/EMI requirements than on according to the following equation: what the LTC3548A requires to operate. Table 1 shows V ⎛ V ⎞ some typical surface mount inductors that work well in L ≥ OUT • ⎜1– OUT f ⎟ ⎝ V LTC3548A applications. IN(MAX) ⎠ O • ΔIL 3548afa 9
