lt3651-4.2 Linear Technology Corporation, lt3651-4.2 Datasheet - Page 12

• Manufacturer Part Number: lt3651-4.2
• Description: Monolithic 4a High Voltage Li-ion Battery Charger Features
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
LT3651-4.2
However in order to facilitate start-up, the LT3651 enables
the switch if V
ing of the BOOST capacitor which then permits the high
side switch to saturate and efficiently operate. The boost
capacitor charges to full potential after a few cycles.
The design should consider that as the switcher turns on
and input current increases, input voltage drops due to
source input impedance and input capacitance. This po-
tentially allows the input voltage to drop below the internal
V
potentially stall start-up. If an input current sense resis-
tor is used, its drop must be considered as well. These
problems are worsened because input current is largest
at low input voltage. Pay careful attention to drops in the
power path. Adding a soft-start capacitor to the RNG/SS
pin and setting UVLO with the SHDN pin is required at
low V
BAT Output Decoupling
It is recommended that the LT3651 charger output have
a decoupling capacitor. If the battery can be disconnected
from the charger output this capacitor is required. The
value of this capacitor (C
operational V
The voltage rating on C
tery float voltage.
R
The LT3651 charger is configurable to charge at average
currents as high as 4A (see Figure 1). If RNG/SS maximum
voltage is not limited, the inductor sense resistor, R
has 95mV across it at maximum charge current so:
where I
R

IN
SENSE
SENSE
R
C
UVLO turn-on and thus disrupt normal behavior and
BAT
SENSE
IN
: Charge Current Programming
is 24mΩ for a 4A charger.
MAX(AVG)
.
≈
20
=
BOOST
IN
µF
I
MAX AVG
0 095
voltage such that:
.
is the maximum average charge current.
+
(



voltage is low. This allows initial charg-
V
350
V
IN MIN
)
(
BAT
µF
BAT
must meet or exceed the bat-
)



) is related to the minimum
SENSE
,
Inductor Selection
The primary criteria for inductor value selection in the
LT3651 charger is the ripple current created during switch-
ing. Ripple current, ∆I
25% to 35% of the maximum charge current, I
percentage typically gives a good compromise between
losses due to ripple and inductor size. An approximate
formula for inductance is:
Worse-case ripple is at high V
forward voltage of the synchronous switch (approximately
0.14V at 4A). Figure 2 shows inductance for the case of a
4A charger. The inductor must have a saturation current
equal to or exceeding the maximum peak current in the
inductor. Peak current is I
Magnetics vendors typically specify inductors with maxi-
mum RMS and saturation current ratings. Select an induc-
tor that has a saturation current rating at or above peak
current, and an RMS rating above I
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 approximately:
Figure 1. Programming Maximum Charge Current Using R
L
f
OSC MHz
=
V
BAT
∆
(
I
MAX
)
V
•
BAT
•



LT3651-4.2
f
1
OSC
–
+
BOOST
V
V
(
IN MAX
BAT
SW
F
MHz
V
CS
MAX
(
BAT
, is typically set within a range of
MAX
)
)
•




 
+ ∆I
(
– 1
IN
V µs
and high V
•
V
MAX
+
V
BAT
MAX
IN
)
365142 F01
R
/2.
+
SENSE
+
. Inductors must
V
V
F
F
BAT
 
 
( )
µH
. V
MAX
F
is the
. This
SENSE
365142f