Datasheet LTC3560 (Analog Devices) - 8

LTC3560
OPERATION (Refer to Functional Diagram) Slope Compensation and Inductor Peak Current
signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current Slope compensation provides stability in constant for duty cycles > 40%. However, the LTC3560 uses a frequency architectures by preventing subharmonic patented scheme that counteracts this compensating oscillations at high duty cycles. It is accomplished internally ramp, which allows the maximum inductor peak current by adding a compensating ramp to the inductor current to remain unaffected throughout all duty cycles.
APPLICATIONS INFORMATION
The basic LTC3560 application circuit is shown in Figure 1. current to prevent core saturation. Thus, a 960mA rated External component selection is driven by the load re- inductor should be enough for most applications (800mA + quirement and begins with the selection of L followed by 160mA). For better effi ciency, choose a low DC-resistance CIN and COUT. inductor. The inductor value also has an effect on Burst Mode
Inductor Selection
operation. The transition to low current operation begins For most applications, the value of the inductor will fall when the inductor current peaks fall to approximately in the range of 1μH to 3.3μH. Its value is chosen based 200mA. Lower inductor values (higher ΔIL) will cause this on the desired ripple current. Large value inductors to occur at lower load currents, which can cause a dip in lower ripple current and small value inductors result in effi ciency in the upper range of low current operation. In higher ripple currents. Higher VIN or VOUT also increases Burst Mode operation, lower inductance values will cause the ripple current as shown in equation 1. A reasonable the burst frequency to increase. starting point for setting ripple current is ΔIL = 320mA (40% of 800mA).
Inductor Core Selection
I = 1 Different core materials and shapes will change the size/ L (f) L() VOUT 1 VOUT V (1) current and price/current relationship of an inductor. Toroid IN or shielded pot cores in ferrite or permalloy materials are The DC current rating of the inductor should be at least small and don’t radiate much energy, but generally cost equal to the maximum load current plus half the ripple
Table 1. Representative Surface Mount Inductors MAX DC MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Toko A960AW-1R2M-518LC 1.2μH 1.8A 46mΩ 1.8mm A960AW-2R3M-518LC 2.3μH 1.5A 63mΩ 1.8mm A997AS-3R3M-DB318L 3.3μH 1.2A 70mΩ 1.8mm Sumida CDRH2D11/HP-1R5 1.5μH 1.35A 64mΩ 1.2mm CDRH3D11/HP-1R5 1.5μH 2A 80mΩ 1.2mm CDRH2D18/HP-2R2 2.2μH 1.6A 48mΩ 2.0mm CDRH2D14-3R3 3.3μH 1.2A 100mΩ 1.55mm TDK VLF3010AT-1R5M1R2 1.5μH 1.2A 68mΩ 1.0mm VLF3010AT-2R2M1R0 2.2μH 1.0A 100mΩ 1.0mm Coilcraft D01608C-222 2.2μH 2.3A 70mΩ 3.0mm LP01704-222M 2.2μH 2.4A 120mΩ 1.0mm Cooper SD3112-2R2 2.2μH 1.1A 140mΩ 1.2mm EPCO B82470A1222M 2.2μH 1.6A 90mΩ 1.2mm 3560fb 8