Datasheet

LT3650-8.2/LT3650-8.4
12
36508284fd
R
SENSE
: Charge Current Programming
The LT3650 charger is configurable 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:
R
SENSE
=
0.1
I
MAX(AVG)
where I
MAX(AVG)
is the maximum average charge current.
A 2A charger, for example, would use a 0.05Ω sense
resistor.
to 35% of I
MAX
, so an inductor value can be determined
by setting 0.25 < ∆I
MAX
< 0.35.
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+∆I
MAX
/2) • I
MAX
, and an RMS rating above I
MAX
. In-
ductors must also meet a maximum volt-second product
requirement. If this specification 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:
V
BAT
• 1 −
V
BAT
V
IN(MAX)






(V • µs)
applicaTions inForMaTion
Figure 1. Programming Maximum Charge Current Using R
SENSE
365082 F01
SW
BOOST
SENSE
BAT
R
SENSE
LT3650
Figure 2. 2A Charger Switched Inductor Value
(R
SENSE
= 0.05Ω) 25% to 35% I
MAX
Ripple Current
Figure 3. 1.3A Charger Switched Inductor Value
(R
SENSE
= 0.075Ω) 25% to 35% I
MAX
Ripple Current
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 (∆I
MAX
)
can be approximated using the relation:
L =
10
∆I
MAX






•R
SENSE
•(V
BAT
+ V
F
)
• 1−
V
BAT
+ V
F
V
IN(MAX)
+ V
F














(µH)
In the previous relation, ∆I
MAX
is the normalized ripple
current, V
IN(MAX)
as the maximum operational voltage,
and V
F
is the forward voltage of the rectifying Schottky
diode. Ripple current is typically set within a range of 25%
MAXIMUM OPERATIONAL V
IN
VOLTAGE (V)
9
0
SWITCHED INDUCTOR VALUE (µH)
2
6
8
10
14
15
21
24
4
12
12
18
27
3230
365082 F02
9 15
21
24
12
18
27
3230
MAXIMUM OPERATIONAL V
IN
VOLTAGE (V)
0
SWITCHED INDUCTOR VALUE (µH)
2
6
8
10
20
14
4
16
18
12
365082 F03