lt3650-4.1 Linear Technology Corporation, lt3650-4.1 Datasheet - Page 12

• Manufacturer Part Number: lt3650-4.1
• Description: High Voltage 2 Amp Monolithic Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

Inductor Selection
The primary criteria for inductor value selection in an
LT3650 charger is the ripple current created in that inductor.
Once the inductance value is determined, an inductor must
also have a saturation current equal to or exceeding the
maximum peak current in the inductor. An inductor value
(L), given the desired amount of ripple current (ΔI
can be approximated using the relation:
In the previous relation, ΔI
current, V
and V
diode. Ripple current is typically set within a range of 25%
LT3650-4.1/LT3650-4.2
APPLICATIONS INFORMATION
R
The LT3650 charger is confi gurable to charge at average
currents as high as 2A. Maximum charge current is set by
choosing an inductor sense resistor such that the desired
maximum average current through that sense resistor
creates a 100mV drop, or:
where I
A 2A charger, for example, would use a 0.05Ω sense
resistor.
12
Figure 1. Programming Maximum Charge Current Using R
SENSE
L
R
•
⎛
⎜
⎝
SENSE
=
1
F
⎛
⎝ ⎜
: Charge Current Programming
MAX(AVG)
−
is the forward voltage of the rectifying Schottky
Δ
⎡
⎢
⎢
⎣
IN(MAX)
I
10
V
MAX
=
I I N MAX
V
LT3650
I
BAT
(
MAX AVG
⎞
⎠ ⎟
is the maximum average charge current.
0 1 .
•
as the maximum operational voltage,
+
(
)
R
+
BOOST
V
SENSE
SENSE
365042 F01
F
V
BAT
SW
)
F
⎤
⎥
⎥
⎦
⎞
⎟
⎠
MAX
•
( )
μ
(
V
H
BAT
is the normalized ripple
+
V
F
)
R
SENSE
SENSE
MAX
)
to 35% of I
by setting 0.25 < ΔI
Magnetics vendors typically specify inductors with
maximum RMS and saturation current ratings. Select an
inductor that has a saturation current rating at or above (1+
ΔI
must also meet a maximum volt-second product require-
ment. If this specifi cation is not in the data sheet of an
inductor, consult the vendor to make sure the maximum
volt-second product is not being exceeded by your design.
The minimum required volt-second product is:
MAX
V
BAT
Figure 3. 1.3A Charger Switched Inductor Value
(R
/2) • I
Figure 2. 2A Charger Switched Inductor Value
(R
SENSE
SENSE
•
⎛
⎜
⎝
16
12
10
MAX
4
MAX
8
1−
8
6
4
2
0
= 0.075
0
0
= 0.05
MAXIMUM OPERATIONAL V
MAXIMUM OPERATIONAL V
, and an RMS rating above I
, so an inductor value can be determined
V
IN MAX
V
(
BAT
6
6
Ω
Ω
MAX
) 25% to 35% I
) 25% to 35% I
)
12
< 0.35.
12
⎞
⎟
⎠
(
V
18
18
•
IN
IN
μ
VOLTAGE (V)
MAX
VOLTAGE (V)
MAX
s
)
24
24
Ripple Current
Ripple Current
365042 F02
365042 F03
30
MAX
30
. Inductors
36504142fb