Datasheet

LT3487
9
3487f
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER PHONE URL
Taiyo Yuden [phone] www.t-yuden.com
Murata [phone] www.murata.com
Kemet [phone] www.kemet.com
Inrush Current
The LT3487 uses internal Schottky diodes. When a sup-
ply voltage is abruptly applied to the V
IN
pin, the voltage
difference between V
IN
and V
CAP
generates inrush current
fl owing from the input through the inductor L1 and the
internal Schottky diode DP to charge the boost output
capacitor C4. For the inverting channel, there is a similar
inrush current fl owing from the input through the inductor
L2 path, charging the fl ying capacitor C2 and returning
through the internal Schottky diode DN. The maximum
current the Schottky diodes in the LT3487 can sustain
is
2A
. The selection of inductor and capacitor values should
ensure that the peak inrush current is below 2A. The peak
inrush current can be calculated as follows:
I
V
L
eSIN
P
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
–.
•
• • arctan
– •arctan
06
ω
α
ω
ω
α
ωω
α
α
ω
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎤
⎦
⎥
=
+
=
r
L
LC
r
L
15
2
1
4
2
.
•
•
–
•
where L is the inductance, r is the resistance of the inductor
and C is the output capacitance. For low DCR inductors,
which is usually the case for this application, the peak
inrush current can be simplifi ed as follows:
I
V
L
e
P
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
–.
•
•
–•
06
2
ω
α
ω
π
Table 3 gives inrush peak currents for some component
selections. Note that inrush current is not a concern if the
input voltage rises slowly.
Table 3. Inrush Peak Current
V
IN
(V) R (Ω) L (μH) C (μF) I
P
(A)
5 0.18 10 4.7
1.44
5 0.235 15 2.2
1.06
3.6 0.18 10 4.7
0.979
3.6 0.245 10 4.7
0.958
3.6 0.345 15 2.2
0.704
External Diode Selection
As stated previously, the LT3487 has internal Schottky
diodes. The Schottky diode, DP, is suffi cient for most
step-up applications. However, for high current inverter
applications, a properly selected external Schottky diode in
parallel with DN can improve effi ciency. For external diode
selection, both forward voltage drop and diode capacitance
need to be considered. Schottky diodes rated for higher
current usually have lower forward voltage drops and
larger capacitance, which can cause signifi cant switching
losses at a 2MHz switching frequency. Some recommended
Schottky diodes are listed in Table 4.
APPLICATIO S I FOR ATIO
WUU
U
Table 4. Recommended Schottky Diodes
PART NUMBER FORWARD CURRENT (mA) FORWARD VOLTAGE DROP (V)
DIODE CAPACITANCE
(pF at 10V) MANUFACTURER
PMEG2010AEB 1000 0.51 7.5 Philips
www.semiconductors.
philips.com
CMDSH2-3 200 0.49 15 Central Semiconductor
www.centralsemi.com
RSX051VA-30 500 0.35 30 ROHM
www.rohm.com
ZHCS400 400 0.425 18 Zetex
www.zetex.com