Datasheet LTC3405A-1.5, LTC3405A-1.8 (Analog Devices) - 8

LTC3405A-1.5/LTC3405A-1.8
U U W U APPLICATIO S I FOR ATIO
The basic LTC3405A series parts application circuit is
Table 1. Representative Surface Mount Inductors
shown in Figure 1. External component selection is driven
MAX DC MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
by the load requirement and begins with the selection of L Taiyo Yuden LB2016T2R2M 2.2µH 315mA 0.13Ω 1.6mm followed by CIN and COUT. LB2012T2R2M 2.2µH 240mA 0.23Ω 1.25mm LB2016T3R3M 3.3µH 280mA 0.2Ω 1.6mm
Inductor Selection
Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm For most applications, the inductor value will fall in the Murata LQH32CN2R2M33 4.7µH 450mA 0.2Ω 2mm range of 2.2µH to 10µH. Its value is determined by the Taiyo Yuden LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm desired ripple current. Large value inductors lower ripple Panasonic ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm current and small value inductors result in higher ripple Panasonic ELT5KT100M 10µH 680mA 0.36Ω 1.2mm currents. Higher VIN or VOUT also increases the ripple Sumida CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm current as shown in equation 1. A reasonable starting point for setting ripple current is ∆IL = 120mA (40% of 300mA).
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET 1 ⎛ VOUT ⎞ ∆IL = VOUT⎜1− is a square wave of duty cycle V (f)(L) ⎟ OUT/VIN. To prevent large ⎝ V (1) IN ⎠ voltage transients, a low ESR input capacitor sized for the The DC current rating of the inductor should be at least maximum RMS current must be used. The maximum equal to the maximum load current plus half the ripple RMS capacitor current is given by: current to prevent core saturation. Thus, a 360mA rated / V V [ ( − V )]1 2 inductor should be enough for most applications (300mA OUT IN OUT C required I I IN RMS ≅ OMAX + 60mA). For better efficiency, choose a low DC-resistance VIN inductor. This formula has a maximum at VIN = 2VOUT, where The inductor value also has an effect on Burst Mode IRMS = IOUT/2. This simple worst-case condition is com- operation. The transition to low current operation begins monly used for design because even significant deviations when the inductor current peaks fall to approximately do not offer much relief. Note that the capacitor 100mA. Lower inductor values (higher ∆IL) will cause this manufacturer’s ripple current ratings are often based on to occur at lower load currents, which can cause a dip in 2000 hours of life. This makes it advisable to further derate efficiency in the upper range of low current operation. In the capacitor, or choose a capacitor rated at a higher Burst Mode operation, lower inductance values will cause temperature than required. Always consult the manufac- the burst frequency to increase. turer if there is any question. The selection of C
Inductor Core Selection
OUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- Different core materials and shapes will change the size/ ment for COUT has been met, the RMS current rating current and price/current relationship of an inductor. Tor- generally far exceeds the IRIPPLE(P-P) requirement. The oid or shielded pot cores in ferrite or permalloy materials output ripple ∆VOUT is determined by: are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar ⎛ 1 ⎞ V I ESR electrical characteristics. The choice of which style induc- ∆ OUT ≅ ∆ L⎜ + ⎟ ⎝ 8fCOUT ⎠ tor to use often depends more on the price vs size require- ments and any radiated field/EMI requirements than on where f = operating frequency, COUT = output capacitance what the LTC3405A series parts require to operate. Table and ∆IL = ripple current in the inductor. For a fixed output 1 shows some typical surface mount inductors that work voltage, the output ripple is highest at maximum input well in LTC3405A series parts applications. voltage since ∆IL increases with input voltage. 3405a1518fa 8