LT3587
10
3587fc
Inductor Selection
A 15µH inductor and a 10µH inductor are recommended
for the LT3587 Boost1 channel and Boost3 channel re-
spectively. The inverting channel can use 15µH or 22µH
inductors. Although small size is the major concern for
most applications, for high effi ciency the inductors should
have low core losses at 1MHz and low DCR (copper wire
resistance). The inductor DCR should be on the order of
half of the switch on resistance for its channel: 0.5Ω for
Boost1, 0.4Ω for the inverter and 1Ω for Boost3. For robust
applications, the inductors should have current ratings
corresponding to their respective peak current during
regulation. Furthermore, with no soft-start, the inductor
should also be able to withstand temporary high start-up
currents of 1A, 1.1A and 480mA for the Boost1, inverter
and Boost3 channels respectively (typ, refer to the Typical
Performance Characteristics curves).
Capacitor Selection
The small size of ceramic capacitors makes them suitable
for LT3587 applications. X5R and X7R types of ceramic
capacitors are recommended because they retain their
capacitance over wider voltage and temperature ranges
than other types such as Y5V or Z5U. A 1µF input ca-
pacitor is suffi cient for most LT3587 applications. The
output capacitors required for stability depend on the
application. For most applications, the output capacitor
values required are: 10µF for the Boost1 channel, 22µF
for the inverter channel and 2.2µF for the Boost3 chan-
nel. The inverter requires a 2.2µF fl ying capacitor. Note
that this fl ying capacitor needs a voltage rating of at least
V
IN
+ |V
NEG
|.
Inrush Current
When a supply voltage is abruptly applied to the V
IN
pin,
the voltage difference between the V
IN
pin and the CAP
pins generates inrush current. For the case of the Boost1
channel, the inrush current fl ows from the input through
the inductor L1 and the Schottky D
S1
to charge the Boost1
output capacitor C1. Similarly for the Boost3 channel, the
inrush current fl ows from the input through the inductor
L4 and the Schottky D
S3
to charge the output capacitor C4.
APPLICATIONS INFORMATION
For the inverting channel, the inrush current fl ows from the
input through inductor L2, charging the fl ying capacitor
C2 and returning through the Schottky diode D
S2
.
The selection of inductor and capacitor values should
ensure that the peak inrush current is below the rated
momentary maximum current of the Schottky diodes. The
peak inrush current can be estimated as follows:
I
P
=
(V
VIN
− 0.6) • e
−1
ϕ
tan
−1
(ϕ)
L
C
ϕ=
4L
R
2
C
− 1
where L is the inductance, C is the capacitance and R is
the total series resistance in the inrush current path, which
includes the resistance of the inductor and the Schottky
diode. Note that in this equation, we model the Schottky
as having a fi xed 0.6V drop.
Table 1 gives inrush peak currents for some component
selections. Note that inrush current is not a concern if the
input voltage rises slowly.
Table 1. Inrush Peak Current
V
VIN
(V) R () L (μH) C (μF) I
P
(A)
5 0.68 15 10 2.48
5 0.68 22 2.2 1.19
5 0.68 10 2.2 1.64
3.6 0.745 15 10 1.64
3.6 0.745 22 2.2 0.80
3.6 0.745 10 2.2 1.10
Schottky Diode Selection
For any of the external diode (D
S1
, D
S2
and D
S3
) selec-
tions, besides having suffi ciently high reverse breakdown
voltage to withstand the output voltage, both forward volt-
age drop and diode capacitance need to be considered.
Schottky diodes rated for higher current usually have lower
forward voltage drops and larger capacitance. Although
lower forward voltage drop is good for effi ciency, a large
