Datasheet LT3587 (Analog Devices) - 10

LT3587
APPLICATIONS INFORMATION Inductor Selection
For the inverting channel, the inrush current fl ows from the input through inductor L2, charging the fl ying capacitor A 15μH inductor and a 10μH inductor are recommended C2 and returning through the Schottky diode D for the LT3587 Boost1 channel and Boost3 channel re- S2. spectively. The inverting channel can use 15μH or 22μH The selection of inductor and capacitor values should inductors. Although small size is the major concern for ensure that the peak inrush current is below the rated most applications, for high effi ciency the inductors should momentary maximum current of the Schottky diodes. The have low core losses at 1MHz and low DCR (copper wire peak inrush current can be estimated as follows: resistance). The inductor DCR should be on the order of −1 half of the switch on resistance for its channel: 0.5Ω for tan−1(ϕ) − 0.6)• e ϕ Boost1, 0.4Ω for the inverter and 1Ω for Boost3. For robust I = (VVIN P applications, the inductors should have current ratings L corresponding to their respective peak current during C regulation. Furthermore, with no soft-start, the inductor should also be able to withstand temporary high start-up ϕ = 4L − 1 currents of 1A, 1.1A and 480mA for the Boost1, inverter R2C and Boost3 channels respectively (typ, refer to the Typical where L is the inductance, C is the capacitance and R is Performance Characteristics curves). the total series resistance in the inrush current path, which
Capacitor Selection
includes the resistance of the inductor and the Schottky diode. Note that in this equation, we model the Schottky The small size of ceramic capacitors makes them suitable as having a fi xed 0.6V drop. for LT3587 applications. X5R and X7R types of ceramic capacitors are recommended because they retain their Table 1 gives inrush peak currents for some component capacitance over wider voltage and temperature ranges selections. Note that inrush current is not a concern if the than other types such as Y5V or Z5U. A 1μF input ca- input voltage rises slowly. pacitor is suffi cient for most LT3587 applications. The
Table 1. Inrush Peak Current
output capacitors required for stability depend on the
VVIN (V) R (
Ω
) L (μH) C (μF) IP (A)
application. For most applications, the output capacitor 5 0.68 15 10 2.48 values required are: 10μF for the Boost1 channel, 22μF 5 0.68 22 2.2 1.19 for the inverter channel and 2.2μF for the Boost3 chan- 5 0.68 10 2.2 1.64 nel. The inverter requires a 2.2μF fl ying capacitor. Note 3.6 0.745 15 10 1.64 that this fl ying capacitor needs a voltage rating of at least 3.6 0.745 22 2.2 0.80 VIN + |VNEG|. 3.6 0.745 10 2.2 1.10
Inrush Current Schottky Diode Selection
When a supply voltage is abruptly applied to the VIN pin, the voltage difference between the V For any of the external diode (DS1, DS2 and DS3) selec- IN pin and the CAP pins generates inrush current. For the case of the Boost1 tions, besides having suffi ciently high reverse breakdown channel, the inrush current fl ows from the input through voltage to withstand the output voltage, both forward volt- the inductor L1 and the Schottky D age drop and diode capacitance need to be considered. S1 to charge the Boost1 output capacitor C1. Similarly for the Boost3 channel, the Schottky diodes rated for higher current usually have lower inrush current fl ows from the input through the inductor forward voltage drops and larger capacitance. Although L4 and the Schottky D lower forward voltage drop is good for effi ciency, a large S3 to charge the output capacitor C4. 3587fc 10