Datasheet LTC3549 (Analog Devices) - 9

LTC3549
OPERATION
be defi ned by the combination of the current needed to waveform, shown in the Typical Performance Character- charge the output capacitance and the current provided istics, shows the output voltage start-up from 0V to 1.2V to the load as the output voltage ramps up. The start-up with a 1kΩ load and VIN = 3.6V.
U U W U APPLICATIO S I FOR ATIO
The basic LTC3549 application circuit is shown on the fi rst materials are small and don’t radiate much energy, but page of this data sheet. External component selection is generally cost more than powdered iron core inductors driven by the load requirement and begins with the selection with similar electrical characteristics. The choice of which of L followed by CIN and COUT. style inductor to use often depends more on the price vs size requirements and any radiated fi eld/EMI requirements
Inductor Selection
than on what the LTC3549 requires to operate. Table 1 For most applications, the value of the inductor will fall shows some typical surface mount inductors that work in the range of 1µH to 10µH. Its value is chosen based well in LTC3549 applications. on the desired ripple current. Large value inductors lower ripple current and small value inductors result in
Table 1. Representative Surface Mount Inductors
higher ripple currents. Higher V
MAX DC
IN or VOUT also increases
MANU- VALUE CURRENT HEIGHT
the ripple current as shown in Equation 1. A reasonable
FACTURER PART NUMBER (µH) (A) DCR (mm)
starting point for setting ripple current is ΔIL = 100mA Taiyo LB2016T2R2M 2.2 315 0.13 1.6 (40% of 250mA). Yuden LB2012T2R2M 2.2 240 0.23 1.25 LB2016T3R3M 3.3 280 0.2 1.6 V LB2016T4R7M 4.7 210 0.25 1.6 OUT ⎛ VOUT ⎞ ∆IL = 1– Panasonic ELT5KT4R7M 4.7 950 0.2 1.2 f • L ⎝⎜ VIN ⎠⎟ (1) Murata LQH32CN4R7M34 4.7 450 0.2 2 The DC current rating of the inductor should be at least TDK VLF3012AT2R2M1R0 2.2 1 0.088 1.2 equal to the maximum load current plus half the ripple VLF3012AT3R3MR87 3.3 0.87 0.11 1.2 VLF3012AT4R7MR74 4.7 0.74 0.16 1.2 current to prevent core saturation. Thus, a 300mA rated VLF3010AT2R2M1R0 2.2 1 0.10 1.0 inductor should be enough for most applications (250mA VLF3010AT3R3MR87 3.3 0.87 0.15 1.0 + 50mA). For better effi ciency, choose a low DC resistance VLF3010AT4R7MR70 4.7 0.74 0.24 1.0 inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation be-
CIN and COUT Selection
gins when the inductor current peaks fall to approximately In continuous mode, the source current of the top MOSFET 100mA. Lower inductor values (higher ΔIL) will cause this is a square wave of duty cycle V to occur at lower load currents, which can cause a dip in OUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the effi ciency in the upper range of low current operation. In maximum RMS current must be used. The maximum RMS Burst Mode operation, lower inductance values will cause capacitor current is given by: the burst frequency to increase. / V V – V [ OUT( IN OUT)]12 C Required I ≅ I
Inductor Core Selection
IN RMS OUT MAX ( ) VIN Different core materials and shapes will change the This formula has a maximum at VIN = 2VOUT, where size/current and price/current relationship of an induc- IRMS = IOUT/2. This simple worst-case condition is com- tor. Toroid or shielded pot cores in ferrite or permalloy monly used for design because even significant devia- 3549f 9