LT3462 LINER [Linear Technology], LT3462 Datasheet - Page 5

• Manufacturer Part Number: LT3462
• Description: Inverting 1.2MHz/2.7MHz DC/DC Converters with Integrated Schottky in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Inrush Current
The LT3462 has a built-in Schottky diode. When supply
voltage is applied to the V
between V
input through the inductor and the Schottky diode to
charge the flying capacitor to V
nonrepetitive surge current the Schottky diode in the
LT3462 can sustain is 1.5A. The selection of inductor and
capacitor value should ensure the peak of the inrush
current to be below 1.5A. The peak inrush current can be
calculated as follows:
where L is the inductance between supply and SW, and C
is the capacitance between SW and D.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
Inductor Selection
Each of the two inductors used with LT3462 should have
a saturation current rating (where inductance is approxi-
mately 70% of zero current inductance ) of approximately
0.25A or greater. If the device is used in the charge pump
mode, where there is only one inductor, then its rating
should be 0.35A or greater. DCR of the inductors should
be less than 1 . For LT3462, a value of 22 H is suitable
if using a coupled inductor such as Sumida CLS62-220. If
using two separate inductors, increasing the value to
47 H will result in the same ripple current. For LT3462A,
a value of 10 H for the coupled inductor and 22 H for two
inductors will be acceptable for most applications.
I
P
V
IN
12
5
5
(V)
V
IN
IN
L
C
– .
and V
–
O
1
6
exp –
D
L ( H)
generates inrush current flowing from
U
47
22
33
2
U
IN
L
C
pin, the voltage difference
–
1
C ( F)
1
1
1
W
IN
. The maximum
U
I
0.70
0.60
1.40
P
(A)
Capacitor Selection
Ceramic capacitors are recommended. An X7R or X5R
dielectric should be used to avoid capacitance decreasing
severely with applied voltage and at temperature limits.
The “flying” capacitor between the SW and D pins should
be a ceramic type of value 1 F or more. When used in the
dual inductor or coupled inductor topologies the flying
capacitor should have a voltage rating that is more than the
difference between the input and output voltages. For the
charge pump inverter topology, the voltage rating should
be more than the output voltage. The output capacitor
should be a ceramic type. Acceptable output capacitance
varies from 1 F for high V
(–5V). The input capacitor should be a 1 F ceramic type
and be placed as close as possible to the LT3462/LT3462A.
Layout Hints
The high speed operation of the LT3462 demands careful
attention to board layout. You will not get advertised
performance with careless layout. Figure 2 shows the
recommended component placement. A ceramic capaci-
tor of 1 F or more must be placed close to the IC for input
supply bypassing.
GND
Figure 2. Suggested Layout
C3
R1
C2
LT3462/LT3462A
OUT
1
2
3
C1
(–36V), to 10 F for low V
C4
R2
L1
6
5
4
L2
3462 F03
V
V
IN
OUT
3462af
OUT
5